Radio Boulevard
Western Historic Radio Museum


WWII Radio Equipment - Part 2
U.S. Navy and U.S. Army Air Forces

WWII Airborne Radio Communications
 Airborne Radio Navigation Equipment
Basic Pre-WWII & WWII Airway Navigation


PBY Catalina with Bendix DU-type Direction Finder Loop Antenna installation (1938 photo)

Airborne Radio Communications Gear & Airborne Radio Direction Finding-Navigation Equipment

  U. S. Navy

The navigation methods and the equipment described in this section are both extremely dated. All air navigation methods and the equipment in use today have evolved and advanced tremendously compared to those methods and equipment used over 80+ years ago. Satellites and computers are driving all navigation today. Pre-WWII, the only satellite orbiting the Earth was the Moon and most computers were slide rules. I've included some basic information about pre-WWII (and WWII) air navigation methods to show how this radio equipment was used and what the operators were trying to accomplish at that time. While the following equipment can still perform their design functions and demonstrate the principles of Radio DF navigation using a "Radio Compass," all of the various Radio Range Beacon signals and other Air-Nav signals referenced in the following write-ups are no longer being transmitted (and haven't been for well-over half-a-century.) The only "old-time" Air-Nav that's still around (barely) are Non-directional Beacons and these can easily be received by this equipment. However nowadays, NDBs are also considered antiquated and they are seldom, if ever, used for air navigation. Many airports still keep their NDB in operation as "aviation tradition" and its signal is provided mainly as airport radio identification (at least in the USA.)

Navy Department - RCA Manufacturing Co., Inc. - Contractor

 MODEL  DZ-2 - Aircraft Radio Direction Finder
NAVY Designation: Type CRV/CEX-46152  

(CEX example shown was manufactured by Emerson Radio & Phonograph Co., CRV was used for RCA Mfg. Co.,Inc. builds)

RCA Manufacturing Co., Inc. built the earliest versions of this Radio Direction Finding receiver for the Navy for use in air navigation and search-rescue. The DZ-2 dates from 1939 but there was an earlier DZ-1 (almost identical but with upper frequency end of 1500kc, the DZ-A was similar to DZ-1.) The subsequent contracts were built by other companies during WWII (up into 1942.)

DZ-2 General Description - Using 8 tubes in a superhet circuit and tuning from 15 to 70 kc and from 100 to 1750 kc, the DZ-2 used a rotatable dual loop antenna (the VLF/LF loop is used on Bands 1 & 2 while the LF/MW loop is used on Bands 3, 4, 5 & 6) and a fixed vertical "sense" antenna (usually a "T" wire from the cockpit to the tail) to determine "true" direction (called Uni-lateral Reception/Sense.) Non-directional and Bi-lateral (bi-directional) options were also provided. The DZ-2 also featured a BFO (CW/MCW toggle switch in CW) and a switched audio filter ("SHARP" switches in a bandpass filter with a CF of 1020hz for CW reception.) The receiver used a cushioned shock mount.

POWER was provided by the aircraft battery/charger system buss running at approximately +28vdc cable-routed to the DZ-2 POWER connector and internally to the front panel POWER switch. When in the "ON" position, +28vdc was routed to the tube heaters and to pin B of the DYNAMOTOR connector which then routed the voltage down the three conductor DYNAMOTOR cable to the external dynamotor box. The dynamotor box had only one three-pin connector. The DZ-2 "ON" position turns the motor section of the dynamotor on and then the generator output section routed the +230vdc B+ back up the DYNAMOTOR cable via pin A to power up the DZ-2 B+ requirements. Pin C provided the common chassis ground return.

photo above: DZ-2 top of the chassis

The loop antenna (CRV-69065) was installed in an aerodynamic housing externally mounted on the aircraft fuselage with cables for signals and loop rotation routed to the aircraft interior for operator control. A dual-scale azimuth compass was provided and the compass was fitted with a relay-operated articulated mask that allowed only the correct scale to be viewed depending on the selection of BI-LAT (bi-lateral or bi-directional) or UNI-LAT (uni-lateral or single null cardioid pattern) functions and also on the frequency band selected. The two loops were mounted one loop within the other and offset at a 90 angle to one another (with the VLF/LF loop being the outer loop and the LF/MW loop being the inner loop.) The two loops were center-tapped and also shielded from each other. The corresponding two azimuth compass scales were also offset 90. The relay-operated compass mask would show the correct scale for the loop in use (dependent on the FREQUENCY RANGE selected since the VLF/LF loop is oriented 90º from the MW loop) and then would also change again when UNI-LAT was selected since that required a 90 repositioning of the loop from the Bi-directional null position. (more details in "DFing with the DZ-2" below.)

photo above: DZ-2 bottom of the chassis. The long rectangular gray box at the lower right is the optional line filter for the +24vdc input voltage (aircraft batt-gen buss.)

Tubes used in the DZ-2 were mid-thirties glass envelope types with 6 volt heater requirements. RF-6D6, LO-76, Mixer-6C6, IF1-6D6, IF2-6D6, Det-6C6, BFO-76, AF Out-41 were the tube used. V-101 is actually a neon overload protection bulb on the RF amplifier grid. Noticeable was the tuning range gap from 70kc up to 100kc. This was to allow for the IF amplifier stages that operated at 89kc. The first IF amplifier stage input transformer had a tertiary winding that was switched in above 200kc. The tuned frequencies below 200kc employed somewhat loose coupling in the IF to provide sharp selectivity (also, this transformer was tuned to 88kc.) Above 200kc, the tertiary winding was switched in and that increased the coupling and broadened the IF bandwidth. The two remaining IF transformers were tuned to 89kc and were of standard configuration. The DZ-2 BFO was set to produce a 1000hz beat note on Bands 1 and 2 but to zero beat the IF on Bands 3, 4, 5 and 6.

No AVC was employed in the DZ-2 since the receiver was strictly for DF and the use of AVC could affect the operator's ability to find the nulls in signal levels. Audio output impedance was 600Z ohms. Dual phone jacks allowed the operator not only use 'phones but to also insert an audio output meter to monitor the receiver output level to aid in tuning in nulls. The audio output transformer T105 had an internal and separate choke in series with the transformer plate winding to provide isolation for the screen voltage to the type-41 tube and the plate winding of the audio transformer. 

DFing with the DZ-2 - NON-DIR (non-directional) reception used just the sense antenna since it was omni-directional and usually allowed finding and tuning-in the desired signal frequency easily. BI-LAT (bi-directional) used just the loop antenna since it provided a "figure 8" pattern. UNI-LAT (uni-lateral) used both the loop and the sense antenna combined within the receiver's RF amplifier input section to provide a "cardioid pattern" that indicated true direction. DFing with the DZ-2 involved first using BI-LAT to determine approximate bi-directional signal minimum levels or nulls with one of the two nulls then selected and the bearing noted. The DZ-2 was then switched to UNI-LAT which resulted in the selection of the uni-lateral scale on the compass (the scale mask was shifted to show 90º offset.) The loop was then rotated to the same bearing that had been noted in BI-DIR on the formerly showing azimuth scale (essentially, rotating the loop 90º.) This orients the loop so that the minimum response of the cardioid pattern is either pointing directly towards the signal source or pointing 180º in the opposite direction away from the signal source. The UNILATERAL ADJUSTMENT control was then rotated while noting the signal level listening for a noticeable drop or null in the signal level. If a minimum null wasn't found, then the loop was rotated 180 and the UNILATERAL ADJUSTMENT again rotated listening for a minimum null response. IF the minimum null response heard was when the loop was at the first compass bearing (same noted Bi-directional bearing) then that compass reading was the correct bearing direction (the null towards the signal source.) IF, however, the minimum response was found when the 180 rotation was selected, then the actual correct bearing was due to the 180 rotation resulting in the loop null then pointing towards the signal source at that bearing indication on the compass

photo above
: The data plates on top of the DZ-2 cabinet. Note that the "contractor" is RCA Mfg. Co., Inc. but this receiver was actually built by Emerson Radio & Phonograph Corp.
The DZ-2 Loop Compass (Loop Drive Assembly) CRV-69064 - Photo left - The dual scale consists of two 0 to 36 indexed scales offset by 90º, one or the other of which is covered by an articulated mask (in the photo the upper scale is showing 90º and the masked lower scale would be showing 0º.) The box connector (behind the extension tube collar) is for the relay cable that connects to the base of the aerodynamic housing of the loop. That two pin receptacle is connected internally to the seven pin loop cable connector which has the five wires necessary for the dual loop connections and the two wires that provide the "switched" +24vdc to operate the Loop Compass' internal solenoid that positions the scale mask depending on the DZ-2 frequency range selected or whether UNILATERAL is selected.

What is shown in the photo is only the Loop Compass/Drive Assembly. Not shown are the extension tube and the aerodynamic housing containing the dual loop assembly (mainly because I don't have those pieces.)

The Loop Compass/Drive Assembly is shown as actually setting on its "hand wheel." When installed in the aircraft, the aerodynamic housing and loop antennae were mounted externally on top of the fuselage and mounted to the aircraft's "skin" with a reinforcing plate. The extension tube was secured to the bottom-plate of the loop mount and, if a long extension tube was required, it was also secured with support brackets to the airframe. The Loop Compass/Drive Assembly was mounted to the extension tube using the collar nut, internal collett and secured with set screws. The extension tube was the proper length for the final compass position to be  at "eye level" for the radio operator to easily see the scales and manipulate the hand wheel, the brake and the vernier control knob. When installed, the compass housing didn't rotate but was kept in a stationary position by its mounting to the extension tube. Inside the extension tube is the loop shaft that is mounted to the loop antenna assembly at the top end and then "keyed" to the compass drive at the lower end. The "keyed" loop shaft always maintains its correct relationship to the compass scales. The loop shaft is secured at the lower end of the shaft with a collett and nut underneath the housing.

Once the Loop Compass/Drive Assembly and the Loop Antenna Assembly were completely installed on the aircraft, operation was as follows,...if the brake was in the OFF position, then the vernier drive was disengaged and hand wheel could be used to turn the loop to any position quickly and the compass would show the direction bearing of the loop relative to the aircraft fuselage. If the brake was placed in the ON position, then the hand wheel was locked and the vernier drive was engaged and the vernier knob could be turned to allow the operator to slowly and accurately rotate the loop and observe the compass scale while listening for a null.

DZ-2 SN:1486 - Retrofitting Dynamotor Operation and Building Proper Cables

DZ-2 SN: 1486 - I got this DZ-2 in 1995. It came from my old radio collector friend from years ago, Fred Winkler (1926-2002.) Fred had changed the tube heater wiring from the original series-parallel to parallel to operate the tube heaters on six volts AC using a filament transformer as the heater voltage source. He had a regulated and adjustable +200vdc power pack that was vacuum tube based. However, at one time Fred had been collecting rejected Ni-Cad batteries from old rechargeable electric shavers. He got the old batteries from someone he knew that did the warranty servicing on the electric shavers (this was in the mid-1980s.) About half of the batteries he collected were defective but the other half were still in good operating condition. Fred made up a battery pack that contained enough of the Nicads to provide +200vdc. At one time, he operated this DZ-2 with a six volt DC battery for the tube heaters and the +200vdc Nicad battery for the B+. I remember that Fred operated the DZ-2 was out of its case most of the time. When doing that sort of operation, Fred apparently would write the call letters and dial setting of various AM-BC stations he heard in water-based ink on the tops of the aluminum shields (lucky it was water-based ink that cleaned off easily without damaging the aluminum matte finish.)

DZ-2 Rework Needed - I used the DZ-2 a few times in 1995. I even installed the AC PS components into a metal cabinet and cleaned up the power connections but that was as far as I went at that time. Fast forward 25+ years, interest in pre-WWII air navigation had me thinking about putting this DZ-2 back to its original configuration to operate on +24vdc for the tube heaters, to have the B+ provided by a dynamotor as original and to utilize the proper connectors for correctly built cables. A replica MW loop would be built to allow DF testing.

DZ-2 Chassis Rewiring - Some of the original tube heater wiring was still present but two original wires from the harness had been cut too short to be reconnected to the proper tube socket. Several of the original inter-connecting series-parallel wires were entirely missing. Luckily, I had a "wire junk box" that had many pieces of the exact type of vintage wire, even with the correct insulation color and tracer color. It was pretty easy to remove the non-original parallel wiring because it was a "twisted pair" with yellow and green rubber insulation on the wires. The retrofit back to series-parallel heater wiring was made very easy because the DZ-2 manual has the detailed wiring diagram with numbered identification of each wire. This isn't a schematic but a pictorial representation of how the wires are connected and routed within the receiver. For the two harness wires that were too short I used the exact type of wire for the extensions and covered the soldered splice with vintage black sleeving that was then "tucked" out of sight (under the harness.) There were two shunt resistors that were originally IRC brand 68 ohm 1W resistors on V-105 and V-106. Of course, these resistors had been removed when the DZ-2 was wired for parallel heaters. I couldn't find two IRC brand 68 ohm resistors but I found two matching 68 ohm BED resistors that looked convincingly original when installed. I did a DCR test to see if everything measured correctly and after that test I connected up +28vdc to pin A of the POWER receptacle and negative chassis ground to pin B. The tubes didn't light-up. It seemed that wiring for the change to 6.3vac parallel filaments had bypassed the fuse block but my return to original DC series-parallel wiring now had the fuse block properly connected. It turned out that the old fuse was blown (probably a long time ago.) The fuse was the larger type 4AG found in some older military equipment. A standard 3AG fuse won't "snap in" the holder clips. I had a selection of the correct type 4G in the old parts bins so with a good fuse installed, the tubes illuminated and looked a correct orange color (I know,...I should measure the heater voltage on each tube but the "color test" is okay for now.) For the final test, I connected up +200vdc to the DYNAMOTOR pin A. With a short antenna on the Sense antenna terminal I picked up several AM-BC stations indicating that the DZ-2 was functioning well enough considering I had it connected up using test clip leads. Proper cables should provide an improvement. Building Proper Cables - The correct types of connectors were purchased off of eBay but the proper type of cables had to be built. The reason for "custom" building is that the cables need to be shielded and require specific gauge wires depending on the current the wire has to carry. Specific DZ-2, multi-conductor, shielded cables are obviously not being manufactured by anyone anymore. Even in WWII, these cables were "custom" manufactured and then supplied in bulk lengths with the DZ-2 equipment for "custom" installation within the aircraft with the "installers" building the interconnection radio cables. So "custom" building the cables is the only option for correct operation of the DZ-2 in its original configuration.

POWER cable - The manual is vague about the wire gauge used in the cables. The most common spec shown in the cable drawings is 41 strands of .010" diameter wire (30 gauge) which describes 14 gauge wire. I built the POWER cable using two 14 gauge wires with a rubber sleeve covering both wires. The rubber sleeve was covered with braided copper shielding harvested from old RG-8 coaxial cable. The entire cable was wrapped with black electrician's tape. The proper MS3106E16S, a 2 pin connector, was installed on one end with the shield drain wire connected to the chassis ground pin. The opposite end had large spade lugs installed (soldered) and a shield drain wire also with a small spade lug. I made this cable 50" long.

DYNAMOTOR cable - This cable was built from three conductor 14 gauge rubber insulated power cable. Again, braided copper shielded was installed with drain wires on each end. The shield and cable were wrapped with black electrician's tape. Although the original cable used two 14 gauge wires for the +24vdc to the Dynamotor +LV and the Chassis Ground along with an 18 gauge wire for B+ from the dynamotor, it was much easier to just use the 14-3 power cable (obviously the 14 gauge wire will not present much of an IR drop but the B+ current is so low IR drop isn't ever a problem anyway.) Connector type MS3106R14S was used and the cable length is 40."

LOOP cable - This cable is built from five 18 gauge wires that connect to the two loops and the common loop ground. Two 16 gauge wires are used for connecting the LP-21 compass scale mask relay which is actuated by the band switch position or when switching to uni-lateral. The connector is AN3106A16S-1S and the cable is shielded.

Dynamotor Box, DM-28 Mount, Filter Board - To operate the DZ-2 "as original" I was going to build a replica of the dynamotor-filter (identified as CEX-21562.) I had a DM-28 dynamotor but that was all,...just the dynamotor. No filter box and no filter components,...not even the mounting base for the DM-28. While the DM-28 was originally for the BC-348 receiver, it does have the same specifications as the dynamotor used with the DZ-2. Finding a DM-28, especially one that's missing the filters and mount, is fairly easy. These were spares and were for replacing a defective dynamotor and reusing the original filter-mount. Using a "spares" doesn't deprive a BC-348 restorer of a complete dynamotor unit.

This was going to be a build-project that would function as the original did. It would have the correct value filter components inside the case. But it would only be a replica, that is, I'll make it look close to the original (complete and original DZ-2 Dynamotors are difficult to find.) I needed to identify the four wires that were exiting the bottom hole of the DM-28. I had to remove the end bells to verify which wires went where. I reinstalled the end bells and applied +27vdc to the +LV side. On the +HV side I had +244vdc showing on the DDM (that's with no load.)  >>>

>>>   Finding the capacitors for the filter wasn't a problem. I needed one 3.75uf with at least 50wvdc, I had several 4uf 400vdc capacitors. I also needed 1uf at 300wvdc, I had 1uf at 630wvdc. Also, two .01uf at 400wvdc, I used .01uf at 600wvdc. The inductor, L301, had to be built. The original L301 was an air core inductor wound on a .375" diameter form with 17 gauge wire. I had to use 16 gauge wire (much easier to find.) All of the inductor parameters are in the manual which makes building L301 easy, sort of. I suspected that a homebrew attempt would probably result in the L being little low so I used a .375" diameter powered-iron rod for a core to increase the L. I wound five layers each wrap insulated with one thin layer of blue masking tape. Each layer was 50 turns. When completed I wrapped the choke with black electrician's tape. Measured L was 1.7h and the DCR was 0.4 ohms. The L was about 25% higher than the original choke and the DCR was about 0.15 ohms more than the original choke. I installed the filter components on a vintage garolite terminal board and tested the operation of the dynamotor with the filter and with the dynamotor output running the DZ-2. All operations were as expected. With no antenna connected and the gain at maximum, some brush noise could be heard but the entire test operation was done using clip leads with no shielding and, of course, the DZ-2 wasn't in its cabinet either.
The dynamotor aluminum box was built using a six inch cube aluminum project box. I cut the top side down by 1.5" so the box was then 6" x 6" x 4.5" with removable top and bottom pieces. The original DZ-2 dynamotor box was 7.5" x 6.5" x 4.5" so my replica is fairly close in overall size. The DM-28 dynamotor didn't have its original mounting base so I made one out of .063" aluminum. It's mounted to the dynamotor with two spacers and two 10-32 screws. The four wires coming out of the bottom of the dynamotor pass through a .375" hole in the new base. Each corner of the base has a rubber grommet for cushioning. The dynamotor mount is mounted to four 8-32 studs with washers and nuts to allow elevating the dynamotor mount slightly above the aluminum bottom and thus allowing it to only be suspended by the rubber grommets. This also allows room for the four wires to exit the bottom of the dynamotor for connection to the filter and the box connector.

The only connection to and from the dynamotor/filter is through the three pin MS box connector that mates with the connector plug at the end of the DYNAMOTOR cable. The filter circuit mounts on two small garolite terminal boards that are mounted to the bottom screws that mount the box connector. After all of the mounting holes were drilled and a "dry run" of assembly to make sure everything did fit together, the box was painted black wrinkle finish. After a few days of curing time for the wrinkle finish, the entire dynamotor box and dynamotor unit could be reassembled and the wiring completed.

Operation is quite an improvement from the clip lead connections. Now, with the proper size shielded cables built with the correct gauge wires, voltages are at spec with minimal IR drop. With +27vdc input operating the dynamotor and the DZ-2 tube heaters and with the dynamotor providing about +220vdc B+, the DZ-2 is performing better than ever. With the antenna disconnected and the VOLUME fully advanced virtually no dynamotor noise can be heard indicating that the filter and the shielded cables dramatically reduce noise within the receiver.

However, the dynamotor RFI noise was radiating and with an antenna connected to the DZ-2 input the noise "picked up" was significant. In fact, the noise could even be received by any other near by receiver. I was relying on the cable shield and DZ-2 chassis connection to ground the case but the entire DZ-2/dynamotor system wasn't connected to a substantial ground. The solution was very simple, the aircraft the battery-charger system negative was tied directly to the airframe and all shields, chassis, etc. are also tied to the airframe. I didn't have the AC power supply negative (-24vdc) tied to the power supply chassis which then connected to the house ground. A small jumper from -24vdc terminal to the power supply chassis ground reduced the RFI noise to a whisper.

I think another improvement might come from replicating the way the equipment was originally installed in the aircraft. All of the cabinets, shock mounts and other pieces were mechanically mounted to the aircraft framework and the battery/charger system had the negative tied to the airframe. By using a 0.50" wide copper braid cable to interconnect all of the cabinets together and then tie those to the -24vdc and that then tied to the house ground might come close.

photo left
: The completed replica CEX-21562, DZ-2 Dynamotor-Filter

photo right: Inside the box showing the DM-28 and filter


Basic Pre-WWII and WWII Airway Navigation

The Radio Compass and Directional Antennae - The discovery of the directional characteristics of antennas is generally credited to two individuals, Heinrich Hertz and Karl F. Braun, both making their discoveries between 1878 to 1888. Marconi also noted directional properties of antennas in his experiments (by about 1906.) In 1909, Bellini and Tosi discovered that two relatively large, 90º opposing coils with a smaller rotatable coil within the opposing coils' field would show signal direction indication without moving the larger opposing coils. This eventually developed into the goniometer that allowed the directional indicator part of the Bellini-Tosi antennas to be located near the receiving equipment and allowed the larger stationary opposing coils of the antenna to be much larger and work at much longer wavelengths. Pursuing the navigational uses of loop antennae, Engineer, Fredrich Kolster (Federal Telegraph Company) invented the "Radio Compass" which allowed accurate direction to be determined by combination of the loop working with a separate sense antenna. Another directional antenna system was the Adcock arrays which were developed in response to the ambiguity of direction indication when skywave propagation was involved. The Adcock antenna system used four relatively large vertical antennas arranged in a square formation with buried feed lines to prevent skywave pickup. The use of verticals favored ground wave for direction indications which was more accurate than skywave propagation. A goniometer was installed at the four feedline ends which were internally connected to the stator coils of the goniometer while the rotor of the goniometer was mechanically connected to the signal direction indicator. Adcock antennas were used as early as WWI and were later used extensively in aviation navigation and by the military in semi-portable DF equipment. 

As radio circuits improved so did the radio compass. By the time the Navy had accepted the vacuum tube (around WWI) the capabilities of the receivers improved dramatically over the earlier mineral detector receivers. Use of the radio compass for navigation was demonstrated by the US Navy on July 6, 1920 using a Radio Compass equipped Curtis F-5-L flying boat.

The First Use of the Airborne Radio Compass - The idea of using direction finding techniques for radio signal location was already being used for maritime navigation and the military had been experimenting with utilizing the same ideas for aviation navigation. On July 6, 1920, the Navy had a Curtis-Felixstowe F-5-L flying boat, with "Radio Compass" equipment installed, fly out of the Hampton Roads area of Virginia (includes the city of Norfolk.) The F-5-L was to navigate entirely by Radio Compass to a rendezvous with the battleship USS OHIO that was at an unknown location at sea somewhere within a 100 mile semi-circular radius east of Norfolk and then to return, by Radio Compass navigation, to Norfolk. The USS OHIO was equipped with radio transmitting equipment and was sending out a "beacon" signal that the F-5-L received within minutes of becoming airborne. Once the bearing was established, the flying boat then flew in that direction for 90 minutes before sighting the USS OHIO at just under 100 miles off shore. The F-5-L crew circled the USS OHIO several times and then proceeded to use a land-based Navy radio beacon to navigate back to Norfolk. The demonstration proved that air navigation by "radio compass" was practical and needed to be developed further.

The Curtis-Felixstowe F-5-L was a large flying boat powered by two Liberty V-12 engines and piloted by a four man crew. The navigator was usually up front since at that time most aerial navigation was visual. Further back was a double-cockpit for the pilot and co-pilot. Behind the wings was the radio operator's position. The radio gear was mounted inside the cockpit in a recessed area protected from the open cockpit environment but still accessible for the radio operator. It's unknown where the loop antenna was mounted.

The battleship USS OHIO had been decommissioned in 1918 but was put back into active service at the beginning of 1919 and sent to Philadelphia. It was used by the Navy for several experiments involving radio signals for uses other than communications. Besides the Radio Compass experiment, the USS OHIO also was part of an experiment to use radio to remotely control the operation of a radio-fitted target ship (using the old USS IOWA.) The experiment was to have the USS OHIO use radio signals to remotely operate and thus remotely "tow" the USS IOWA from Philadelphia to Hampton Roads, a task that was performed successfully. Due to its smaller size and age, along with its slow maneuverability, the USS OHIO was again decommissioned in 1923 and sold for scrap.  

Shown to the right is an artist's depiction of the Radio Compass event titled "First Use of The Radio Compass - 1920." Charles H. Hubbell was a famous "aviation" artist who supplied the artwork for a series titled "Electronics in Flight" that was done for Thompson-Rand-Woolridge, Inc. in 1961 as part of a group of aviation-theme calendars. The calendar write-up that is below the artwork erroneously states the USS OHIO was 15 miles off shore. Had that been true, the crew of the F-5-L could have easily visually spotted the USS OHIO once they were aloft. The misprint probably was intended to read "95 miles." The entire write-up is below the artwork and the image to the right.

First Use of The Radio Compass - 1920
"On July 6, 1920, a Navy F-5-L Flying Boat left Norfolk, Virginia and, guided entirely by Radio Compass, made a rendezvous with the battleship U.S.S. OHIO 15(sic) miles off the coast, then returned to base. This marked the first use of the Radio Compass in Air Navigation."

Department of Commerce - Bureau of Airways - Before 1925, nearly all air navigation was visual. Well-known landmarks were used, rivers followed, the few major roads were sometimes used as navigation landmarks. Airmail pilots were sometimes delayed when weather conditions obscured sighting landmarks that they used to visually follow their flying route. Some early air navigation aids were the large white arrows (usually made out of concrete) that were ground-mounted and placed in remote areas pointing the way to a specific airport. Later, large rotating beacon lamps were installed on towers usually in the same locations as the white arrows as navigation aids for night flying or poor visibility conditions.

Early Airborne Navigation Radio Receiver Indicators - In 1926, the Department of Commerce, in charge of commercial flying through the newly created Bureau of Airways and Navigation, began to implement methods and equipment to utilize radio stations at airports to provide navigation information for pilots. The initial system was a non-directional radio beacon at the airport that provided a radio signal that allowed a pilot to use a radio to listen to the signal strength, try to determine the strongest signal response (by changing the airplane course.) The strongest response would generally indicate the correct direction of the airport. This system wasn't very accurate, it was difficult to use and it was thought that a visual indicator would be an improvement.

By the late-twenties, the airport radio beacon system had improved by using a directional beacon that employed an Adcock directional antenna that was comprised of four vertical towers in a 425' square with antennas in each corner. One transmitter would feed two diagonal antennas transmitting a signal modulated with 65hz and a second transmitter used the remaining two diagonal antennas sending an 85hz modulated signal. The antenna radiation pattern was a large "four-leaf clover" with the four main lobes providing the strongest signal and the minimum signal was between the lobes. The aircraft had to fly in the direction of the airport and on the correct minimal signal null (between the lobes.) The aircraft receiver-indicator was a system that used vibrating reeds within the pilot's instruments to indicate direction. The visual indicator would show equal vibration height of both reeds if the airplane was flying directly "on the beam" since both transmitted signals were at minimum. But, if the airplane drifted to the right then the 65hz reed increased in height because that radiation pattern lobe was being flown into. If the airplane drifted left then the 85hz reed with increase in height for the same reason. Which ever reed was showing increased vibration indicated to the pilot if he had drifted to the right or the left of the beam. The pilot had to fly in the proper direction of the airport and maintain the reeds equal in height to successfully navigate to the desired airport. The reed tips were painted white for increased visibility within the indicator.

By the mid-thirties, the vibrating reeds were being replaced with more modern systems of navigation that allowed the airports to use the same basic type of directional antenna system for beam navigation but the improvements were made to the transmitters used and also to the airborne DF equipment being used.

Radio Range Beacon Signals, Airport Range Beacon Antenna SystemsBy the mid-to-late 1930s, navigation from one airport to another airport involved flying the aircraft on a specific course at a specific altitude that was called an "Airway." The Airway was defined by Radio Range Beacons that were located at major airports and sometimes by Remote Radio Range Beacons that were in areas that were out of the range of any Airport Radio Range Beacons. Most Radio Range Beacons were able to be reliably received by aircraft out to a distance of about 50 miles. A major airport Radio Range Beacon would have more powerful transmitters and could be received out to 100 miles or more. The intersection of two different Range Beacon beams from two different airports could span a distance of up to 200 miles although most intersections were somewhat less than the maximum with 100 miles being average. Where the distance between two airports exceeded 200 miles then a Remote Radio Range Beacon station was sometimes installed to provide more consistent coverage. Where no Remote Range Beacon was installed the pilot had to know where the next Airway Range Beacon "beam" was using the navigation chart and then plot a heading to that point to intersect the next beam along the course. Well-travelled Airways, mainly in the Eastern part of the USA, would generally have consistent Radio Range Beacon coverage but, in the Western USA, consistent coverage was provided only on the busiest Airways.
Each Radio Range Beacon sent out a specific type of signal radiation pattern that was created by the type of antenna system used. The two types of antenna systems were the "Loop" which consisted of two large 300' long rectangular loops that center-crossed each other at right angles and were mounted about 30 feet high. The other type was the Adcock Tower antenna system that consisted of four 125' tall towers arranged in a 425' square with a tower in each corner. Although the Adcock four-square tower system can be used for DFing when combined with a goniometer, if the antennas are used as phased transmitting towers they create a specific type of radiation pattern (four-leaf clover pattern.) These tower antenna systems were sometimes called Modified Adcock Tower Antenna Systems or sometimes Range Tower Antenna Systems. The feedlines to these antennae were buried to help prevent skywave radiation and assure the the radiated patterns were accurate and consistent. The Radio Range transmitter was located in a building at the center of the square. An additional communications transmitter used a fifth tower in the center of the square that operated a different ground-to-air frequency than the Range Beacon used and was for weather announcements or voice communications to incoming airplanes. The Radio Range transmitter energized the Adcock "beam" antenna with a transmitter that operated in the 200kc to 400kc frequency range and was automatically keyed to produce a continuous string of dots and dashes. The output of the transmitter went to an automatically timed electronic switch that would allow a dot and a dash to energize one loop or one diagonal pair of towers to produce an "A" and then would switch to the other loop or pair of towers to allow a dash and dot ("N") to be sent.
photo above: Maspeth approach to LaGuardia Field, NY showing the five tower Range Range Antenna system in 1943. Installed by Radio Receptor Co. - photo from "Highways of the Air"

The orientation of the two loops or the two diagonal pair of towers caused a "four-leaf clover" radiation pattern to be produced which resulted in four "nulls" in which the signal strength of the "A" and the "N" were equal and resulted in a continuous tone at the aircraft receiver. Each null or "beam" was 3 wide and, as mentioned, the signal could extend out an average of 50 miles and to over 100 miles for major airports. If the incoming airplane drifted off the "beam" one of the four-leaf clover antenna pattern lobes would become stronger and the pilot would begin to hear that letter much stronger and the other letter much weaker. As the airplane drifted more off the beam, the stronger letter would dominate. The navigation charts would indicate the orientation of the A or N relationship to the four beams and that would indicate to the pilot or navigator which way the airplane had drifted allowing him to correct his course back onto the beam.

Many installations required adjustment to the radiated pattern due to the local terrain (to avoid mountain ranges or nearby hills along the course, for instance.) Sometimes in order to intersect with other Range Beacon beams, the radiated pattern required some directional adjustment. Usually a goniometer or a vario-coupler was used to alter the radiated pattern from one lobe or the other to adjust the pattern by "bending" the radiated "beams" as needed. Sometimes when only one beam needed to be adjusted, an excited parasitic antenna was placed within the antenna field to alter the radiated pattern of the desired lobe.

Position Markers, Fan-Markers, Cone-of-Silence Markers, Rotating Lamp Beacons - Since following the "beam" could result in the aircraft traveling perhaps as far as 100 miles, Position Markers were sometimes installed at various distances along each beam. Most position markers by 1940 were upward radiating VHF signals (most were on 75mc.) When received as the airplane passed overhead, the signal would be of very short duration so usually the marker signal would be modulated in a manner to actuate a switch in the radio gear that turned on an indicator lamp. Earlier position markers were designated as "M" markers and usually were on an adjacent frequency to the Radio Range Beacon and usually sent the same Morse ID call that the Range Beacon did. Because of the slightly different frequency and the lower power of "M" markers, they were easily identified by pilots. "M" markers were usually shown on nav-charts indicated by a circle with a "M" inside and the frequency of operation.

Fan-Markers were 75mc VHF position markers that were generally located mid-way along the airway beam or sometimes at locations where a different Radio Range "beam" intersected the beam being flown with the object being to specifically identify the beam that the pilot should fly on to stay with in the Range Approach Channel (the flight corridor) to the airport runway. Sometimes Fan-Markers were placed to warn about obstructions ahead or to indicate the airport approach. Some Fan-Marker installations also had rotating lamp beacons and course lighting to visually aid the pilot as he approached the airport. Fan-Markers antennas produced a radiated pattern that resembled an "open fan" inline with the beam which allowed the pilot enough time to receive the information being transmitted as the airplane flew over the marker. Fan-Markers were usually shown on nav-charts as medium-size ellipses along the beam indication with the marker ID (usually a single letter sent in Morse) shown along side the ellipse.

Cone-of-Silence markers were located at the airport. As the airplane approached the airport, the "beam" signal would begin to "break up" at about one thousand feet out and could not be received at all directly over the airport. Cone-of-Silence markers were VHF transmitters (75mc) with an antenna that radiated a cone-shaped signal upwards with a modulated 3000hz tone that usually triggered an indicator lamp on the radio gear. The Cone-of Silence signal range depended on the aircraft altitude but at 5000 feet altitude the signal could be received out a little over 2000 feet. At 1000 feet altitude the range was about 1200 feet. The Cone-of-Silence generally indicated that the pilot was going to "over-shoot" the runway and should prepare for a turn after passing over the airport.

Usually, along the Airway course there were several rotating beacon lamps to aid in visual navigation at night. As mentioned, many times a rotating beacon lamp was also located at a Fan-Marker position. Some beacon lamps blinked in Morse to identify themselves and these types would have their Morse ID indicated on the Nav-chart. Sometimes there were also Inner and Outer Marker Beacons that were VHF types of position markers that indicated the distance to the runway of the airport mainly to help the pilot with the landing approach. Also, most airplane to tower communications were beginning to use VHF by about 1940 (128mc to 132mc at the time) but also had MW and HF capability (tower frequencies were indicated on the nav-chart.)

Right-Left Indicators - Some Radio Compasses utilized a visual indicator to show direction of drift off of a beacon signal that was being used for homing. Generally, the Right-Left Indicator was a center-zero meter that would be driven by the radio circuitry to move down scale (to the left) or up scale (to the right) in response to signal changes from the loop and sense antenna. The design used the omni-directional sense antenna's phase relationship to the variable phase relationship of the loop. At a loop null, the phase difference between the sense antenna and the loop is 90º but switches rapidly to 180º off the null. The induced signal voltage to the loop is at maximum when the loop axis is inline with the signal. When the loop is perpendicular to the signal the induced voltage is at minimum on either side of the loop. Within the Radio Compass circuitry, the loop input goes through an RF amplifier with a 90º phase-shifter circuit and into a dual balanced modulator (a dual triode circuit) that also has a 48hz audio oscillator driving one side of the balanced modulator. The output of the balanced modulator is inductively mixed and connected to the sense antenna where it is then fed into the RF amplifier of the receiver. From the detector/AVC/1st AF circuit of the receiver the signal is routed to a 48hz AVC amplifier and a Compass Output stage and these two signals are inductively coupled to the R-L meter. The R-L meter is a special dual coil unit (dynamometer) with a field coil and a moving coil. Since the field coil is driven by the 48hz audio oscillator and the phase shifter/balanced modulator is also driven by the 48hz audio oscillator driving the moving coil, when the loop is at a null the phase difference is zero and the meter stays at center. If the airplane drifts "off the beam" it appears to the Radio Compass circuitry that the loop has changed position and thus the phase changes with the result that the R-L Indicator moves to either the right or the left depending on where the beacon is in relationship to the airplane's new course. By keeping the R-L Indicator needle centered, the pilot or navigator was assured that the airplane was flying "on the beam." NOTE: This is a description of how the Bendix Radio Compasses functioned.

Finding the Aircraft's Position - If the airplane was not flying "on the beam" but was at an unknown position for some reason it was relatively easy for the pilot or navigator to quickly get a "fix" on the airplane's position. Using the Navigation Chart, a nearby beacon was selected. The nav-chart showed the location, frequency and call of the beacon so tuning it in on the DF receiver was easily accomplished. Using the Homing loop a bearing was taken on the null (it didn't matter which null was selected) and a line drawn on the chart referenced from the beacon's location. Then another nearby beacon from a moderately different location was selected, tuned in and a bearing determined. When the second line was drawn on the chart, the point of intersection of the two lines indicated the airplane's position. This was relatively accurate but since the airplane was traveling in a specific direction at flight speed, the faster the triangulation was performed, the more accurately the position could determined. AM-BC stations were usually shown on nav-charts because their strong carrier signal could easily be used as a beacon for Homing or for triangulation.
Aeronautical Navigation Charts and Computers - There were different charts for daytime or nighttime flying with different map projections, e.g., Mercantor for nighttime flying and Lambert for daytime flying. Mercantor projections are purposely distorted to present square map grids but as the distance increases the distortion errors increase and a correction factor had to be added or subtracted to the course. The nighttime charts were simplified with no landmarks shown other than those that would be visible at night. Additional information shown on navigation charts included marine beacon calls and frequency, weather station calls and frequency, AM Broadcast station calls and frequency, general terrain and elevations, latitude and longitude, major visual navigation points (roads, railways, lakes and rivers,) other visual indicators such as tower lights that included rotating lights, lights that blinked Morse, or other types of lighted beacons. By the early 1940s many nav-charts would show a Range Approach Channel that was a corridor that pilot was to stay inside of while on the "beam" as the airplane approached the airport and runway. Some Range Approach Channels would intersect with different beams and channels if the course required the pilot to make a turn onto another beam to follow to a specific approach to the airport. Generally, if the chart was intended for in-flight use it would be an Air Navigation Sectional Chart that covered about a 300 mile "section" of a specific area of land with many details about airports, beacons, topography, etc., to aid flight navigation. If the chart was intended for ground use or reference for flight planning it was a Radio Direction Finding Map which covered a much, much larger area (like the entire Southwest or the entire Northeast of the USA) and the details were more general, usually showing Radio Range Beacons but not showing the A or N quadrants. Markers, course lights, lamp beacons, etc. likewise, were not shown since the scale of the map covered such a large area.  

Navigators usually were equipped with several types of tools that were usually kept in a kit and included many types of scales or articulated pointers, usually made of transparent plastic, for placement on top of charts. Also included were various types of calculators (sometimes called "computers" but most were like circular slide rules and able to perform several calculations or conversions simultaneously.) Some tools had combinations of transparent articulated pointers and sliding scales for calculating variables or position. These calculators, scales and conversion tables helped to speed-up the process and hopefully increase navigation accuracy.

photo right: A "Time-Distance" computer, Type D-4, from WWII, for the USAAF.
 Diameter of this device is 4 inches. There are more computation scales
 on the backside of this Type D-4 computer.

Aeronautical Charts - 1938 to 1947

1938 Air Navigation Chart


1938 BUFFALO, NY area. The Range Beacon "beams" are shown as four light-red spokes emanating from the airport location. Note how the Buffalo Airport's Range Beacon antenna pattern has been "bent" to match other Range Beacons in the area. Also, note that the course approach bearing is shown in degrees inside each spoke and each quadrant has the A or N identification shown. Each airport beacon also has the frequency shown in kilocycles and the call shown in Morse.

Note that Buffalo AP Range Beacon is on 266kc and the call is BJ shown in Morse. Also, IT at Dunkirk is a Position Marker that indicates another beam's intersection on the BJ 49 approach beam. At this time in 1938, the Position Marker (if even present) would send its call on both 371kc and 266kc. VHF was used for Markers by about 1941.

Also, note that weather broadcasts are on WWAB on 236kc. WKBW 1480kc is an AM-BC station shown north of Buffalo.

Magnetic Variation is shown as the dotted arced red vertical lines with degrees and direction indicated. Magnetic Variation is also shown in red within the latitude and longitude markings on the map's edges. 

Stars, solid circles and spoked wheels indicate visual beacons such as rotating lights, etc. Star within a circle indicates a marine beacon with call and frequency shown within a rectangular box. Most nav-charts contained a wealth of information for the pilot and navigator. 

1943 Air Navigation Sectional Charts

1943 HELENA, MONTANA area. AP Radio Range Beacon is HL on 371kc. The ellipses indicate a Fan Marker on 75mc indicating either airport approach or other information. Almost all of the Fan Markers will also have a rotating lamp beacon at the marker location and course lights. North on 165 beam note rotating lamp beacon 42 with the Fan Marker "U" and also East on 279 beam a similar lamp beacon with the Fan Marker "W" identified as 1. These lamp beacons associated with Fan Markers also had course lights showing the pilot the runway direction. Note the Radio Approach Channels are identified by colors, e.g. GREEN 2, AMBER 2, etc. Each beam has the A or N quadrant identified. Most of the beams intersect with beams from other Radio Range Beacons to provide the pilot with almost constant navigation direction signals. The dotted line in the upper left indicates Magnetic Variation which in this area was 20ºE. Stars indicate a lamp beacon. Stars with arrows indicate a rotating lamp beacon with course lights. Gear-shaped circle indicates an airport. Note that TOWNSEND "V" is a lamp beacon with course lights but not a Fan Marker. The course lights directed the pilot to the intersection from 319º from Bozeman to 279º from Helena. Airport elevation is the italic number. Various shades of brown colors indicate elevation between 3000 and 5000 feet. Charts had to be carefully studied before the flight began since so much information is presented on each chart. This is just a small area of the particular sectional Air-Nav chart which is 42" x 24" in size.


1943 SALINA, KANSAS area showing Salina AP, Smokey Hill Army Air Field, Hutchinson AP and the Hutchinson Navy Air Base. The shaded borders marked CAUTION AREA indicate that the pilot should avoid this area, if possible. Note that near Salina is a marked DANGER AREA southwest of the Smoky Hill Army Air Field indicating this area is not to be flown over. The Hutchinson AP Tower is on 272kc and on 126.18mc while the Radio Range Beacon is on 284kc with the call HS. Note that west of Salina is an AM-BC KSAL station tower operating on 1150kc. Since the elevation in Kansas is between 1000 feet and 2000 feet the map color is white. This Aeronautical Navigation Section Map measures 42" x 24" and covers the central portion of Kansas. These Section Maps might seem large for single seater airplanes but the pilots had their routes planned and generally folded the map up with just the necessary areas showing. In larger planes with navigators it might have been possible to have the map fully unfolded. 

1947 Aeronautical Map for Radio Direction Finding

1947 RENO, NEVADA area. Reno AP Radio Range Beacon is RNO on 254kc. Note that the AP is "Hubbard Airfield" which was at sometime purchased by United Airlines and then sold to the City of Reno in the 1950s.

KOH is shown on 630kc - on that frequency from 1940 up to about 1990 (now on 780kc.)

LOL on 209KC was a Remote Radio Range Beacon located east of Lovelock, Nevada.

FFN on 230kc was the Radio Range Beacon SE of Fallon, Nevada

Of interest is the Remote Radio Range beacon located on Donner Summit DOS 385kc.

Radio Range Beacon patterns show the four beams within a 360º compass rose with 0º indicating magnetic North. Beams could be the referenced to the compass rose shown to indicate the beam heading. Note that both Magnetic N and True N are usually shown on the compass rose with True N plotted for the magnetic deviation. The dotted red line indicates Magnetic Variation which is 18º E in this area.

Circles with stars inside indicate a military air base, e.g., McClellan AFB or Mather AFB near Sacramento. Gear-shaped circles indicate a municipal or commercial AP.

This map is for the entire Southwest US so only a small portion is shown in the photo. Much larger areas are shown in Radio Direction Finding Maps compared to Navigation Sectional Charts since the DF Map isn't necessarily for in-flight navigation. Also, DF Maps won't have the A or N quadrants indicated on the compass rose of each Radio Range Beacon for the same reason. The elevation is indicated by color with browns being above 5000 feet. Note that below 3000 feet is green (Sacramento.) The entire map measures 36" x 26" and the map grid is shown in the Lambert projection.

Radio Range Beacons started to be replaced with the VOR navigation system by the early-1950s and very few Radio Range Beacons were left by the 1960s.

Only the largest and busiest Airports had all of these types of radio navigation installations. Small airports might only have a radio beacon like a non-directional beacon that allowed a pilot to find the airport using a "Homing Loop" type of navigation along with visual navigation. Usually there was some type of ground communication although it might not be with an airport tower. During WWII, a lot of air traffic in the USA used Airways navigation but in other countries or perhaps small islands only small temporary or "make-shift" runways might be used. This type of navigation would rely more on crude beacons or other methods to determine the runway's location from some distance out. Other types of loop DFing might be necessary to find an exact bearing to an unknown small runway located in a remote area that had no radio beacons. With small airports without beacons, the pilot or navigator could plot a course to that airport and then use "dead reckoning" to fly there. Dead reckoning used the plotted bearing, the calculated distance, the air speed, the altitude, any crosswinds and the aircraft's magnetic compass to estimate a fairly accurate course to the small airport. Once on course, then visual navigation could also be used and if any landmarks were known those could be used to verify the course accuracy.

Most Airways navigation involved flying "on the beam" which was done using the Homing Loop position with the loop set athwartship and steering the airplane towards the Radio Range Beacon with listening and watching for Position Markers as the course was flown.

Most "Search and Rescue" operations involved using the UNI-LATERAL setup with the loop in combination with a sense antenna to produce a cardioid pattern that indicated "true" direction of an unknown location signal source. Calculations were then performed to allow setting an accurate flying course to accomplish the rescue.

Search and Rescue Operations, Determining True Direction of Signals from an Unknown Location

For "search and rescue" operations, true direction had to be measured and then calculated to allow a proper course to be flown to find the unknown signal source, usually a downed aircraft's life raft with the pilot or crew operating an emergency transmitter. With knowing what types of signals were going to be received and knowing what the directional loop response was showing, next came the more difficult part of determining the DF "true" bearing,...the calculations. Figuring a true bearing was more involved than just reading the loop compass scale. The loop/sense antenna was normally tuned for a null response since this was much more accurate than trying to determine the "antenna position to maximum signal" response (the null on a cardioid pattern is very deep and very apparent compared to the very broad signal peak, therefore the null was easier to find and much more accurate for direction finding.) However, once the correct "null" was determined, the resulting loop compass reading was relative to the airplane fuselage (the nose of the airplane was usually considered 0) so the navigator also had to know exactly the bearing of the course that the airplane was flying (using the aircraft's magnetic compass and compensating for magnetic deviation.) The aircraft flight bearing (and any deviations) were added or subtracted as necessary from the loop compass scale reading to arrive at true bearing and direction of the received signal.

Loop Response Patterns - What happens when setting the various loop positions is that the bi-directional and the uni-lateral/cardioid patterns rotate along with the loop rotation. When rotating the loop 90 after finding one of the bi-lateral nulls, the resulting loop position may have the cardioid null pointing towards the signal source but it could also be pointing away from the signal source at a 180º opposite bearing. The loop response with the peak signal amplitude pointing at the signal source is just about the same signal amplitude strength as the signal response off of the sides. Comparing the two uni-lateral loop positions, one being the first bearing and then 180º opposite the first bearing and then noting which position shows the minimum signal response then indicates the desired "true" direction with the cardioid null pointing at the signal source in that loop position. See drawings below showing the relationship of the loop patterns.




Deviations, Quadrantal Errors and Magnetic Variation - Some types of deviations could occur from the aircraft structure in relation to the location of the loop antenna on the aircraft fuselage and could come from the wings, vertical stabilizers, other antennae or engines and prop wash. These were called quadrantal errors and some compass instruments could mechanically adjust out these types of errors since they were essentially caused by the aircraft structure itself and didn't change (with the exception that some structures might be resonant to the frequency of operation.) Quadrantal errors tended to become more of an issue as the frequency of operation increased. At LF or even MW, quadrantal errors were stable and minimal.

A specific aircraft's quadrantal errors were first determined upon completing the installation of the DF equipment and with the aircraft on the ground. A technician would position himself and a portable transmitter out 1000 feet from the nose of the airplane (usually referenced as 0º.) Test signals at various frequencies were transmitted and the results at the airplane equipment logged. Then the technician moved to 30º still 1000 feet out and another series of test signals were transmitted and readings taken at the airplane. The technician then moved to 60º at 1000 feet, so on and so on, until an entire circle had been made around the airplane with signals transmitted and readings logged.

Once the ground testing was completed and the results logged then further testing was performed with the aircraft aloft at 5000 feet elevation. Since there couldn't be a technician with a portable transmitter at that altitude, a fixed ground beacon signal was used and the airplane flew in a circle out at a specific distance (at least 1 mile) around the beacon antenna. Readings were logged every 30 until all twelve positions were logged. At this point all quadrantal errors were known and either logged or if the DF compass allowed, the errors were adjusted out.  NOTE: This is how the RCA DZ-2 installation was performed. Bendix Aviation had a different procedure for using a fixed beacon at altitude testing that involved flying a course made up of several inline half-turns (90º turns) and taking readings at specific places along the course with a full-turn (180º turn) at the end of the course and then repeated readings taken along the the return course back to the starting point. Bendix had a prepared form that was filled-out and had all of the readings taken during the test. These readings were then used to mechanically adjust and compensate the DF azimuth compass for the quadrantal errors.

The most important bearing deviation was from "magnetic variation," which is the deviation of magnetic N from true N, and it depended where on Earth the aircraft was flying. When calculating magnetic variation it was important to know whether the deviation was to the West or to the East and the important rule was "always add Westerly deviation and subtract Easterly deviation." As an example, Dayton, Nevada has a magnetic deviation of about 18 E, while a location like New York City has a magnetic deviation of about 12 W. Magnetic variation was always shown on the navigation charts being used.

All of the deviations were generally known in advance since magnetic deviation is charted for physical locations on Earth and the aircraft deviations or quadrantal errors were measured at the installation of the DF equipment in the airplane and either logged or adjusted out. In addition to these calculations, the navigator had to also factor in the air temperature, barometric pressure (dependent on altitude,) the aircraft's speed and wind drift.

Uni-lateral Directional Uses - Uni-lateral was used when an unknown signal from an unknown location needed to be DF'd. Most likely the signal was from a downed aircraft's life raft with the raft's occupant(s) running the emergency transmitter (a Gibson Girl, for instance - covered in another section further down. Even though it was basically for the USAAF they were installed in most types of aircraft.) An exact bearing/direction was needed to find the emergency transmitter and rescue the downed pilot (and crew, depending on the type of downed aircraft.) Time was usually critical, so with "true" direction known, the airplane could fly that course and eventually find the raft and occupants. Sometimes, if there was enough time, a second bearing from a different position might be taken to provide "triangulation" information for a more exact location of the emergency transmitter. If there was a second airplane involved in the search, the second bearing could be performed almost simultaneously and the bearings mutually shared via radio for the quickest rescue which, if there were "high seas," could be performed by a nearby ship since landing a PBY in rough ocean conditions was fairly difficult and consumed a lot of fuel.

Bi-lateral Directional Uses - When the aircraft was flying towards a "known location" beacon, then Bi-lateral was used and the loop locked in position perpendicular to the fuselage (athwartship) and the airplane course determined by steering the airplane in the direction of the bearing of the minimum signal response. This was called "Homing." An Audio Output Meter could be plugged into one of the aircraft receiver's phone jacks to use as a visual indicator of minimum signal response. Since the Range Beacon's call and location were shown on the navigation charts, Bi-lateral allowed "beam navigation" and kept the airplane on course within a defined "Airway" to the airport or city that the beacon was transmitting from. The "Airway beams" locations and bearing directions were also shown on the navigation charts along with the "between the beams" signals of "A" and "N" Morse (MCW) identifiers to indicate via the radio signals and by the navigation charts where the airplane was in relation to the four fairly narrow navigation "beams" from the Airport Range Beacon transmitter/antenna system. The "A" and "N" beam loop IDs would combine when the airplane was "on the beam" and a continuous tone was heard although this information was only transmitted for around 30 seconds, then the Range Beacon callsign was sent on the A loop and then on the N loop, then a long pause and then all of the information sent again (this format was continuously repeated.)

"Basic Pre-WWII and WWII Airway Navigation"  and  "Search & Rescue Operations, Determining True Direction of Signals from an Unknown Location"


1. Manuals/Handbooks for RCA DZ-2 Direction Finding Receiver, Bendix MN-26A Radio Compass, RCA ARB Radio Receiver, USN/Bendix DU-1 Directional Loop Antenna

2. Aeronautic Radio - U.S. Department of Commerce - Aeronautics Bulletin No. 27 - publication from July 1, 1937

3. Practical Air Navigation by Thoburn C. Lyon - Special Publication No. 197 - 1938 edition - U.S. Department of Commerce

4. The Radio Manual by George Sterling - 2nd and 3rd Editions have chapters on Air Navigation and Equipment - 2nd Edition latest printing 1931. 3rd Edition latest printing 1943

5. Highways of the Air by Radio Receptor Company - 16 page brochure on Radio Range Beacons and Position Markers - 1943 publication


Navy Department - Western Electric Company

RU-GF Series of Aircraft Radio Receivers & Transmitters

Model RU-16  Type CW-46051A    -    Model GF-11  Type CW-52063A


The earliest RU Series of aircraft receivers date from about 1930 and the earliest GF Series of aircraft transmitters date from about 1932. The early models were built by Aircraft Radio Corporation. Both the receiver and the transmitter evolved throughout thirties and, although the design was certainly showing its age by WWII, the last contracts are from 1941 (for the RU-19 and GF-12.) Like all pre-WWII equipment, contracts were for very small quantities so the early versions are very rare. The most commonly seen RU/GF versions are the RU-16 and the GF-11 which were produced in fairly large quantities in the very early part of WWII (but apparently not used extensively in actual service compared to the contract quantities produced.) The contracts actually date from before WWII began for the USA, April 21, 1941 with Western Electric Company as the contractor. The RU-16 and the GF-11 both operated on +12vdc implying that the installation would be in earlier types of aircraft. By 1941, +12vdc aircraft power was quickly being replaced with the more efficient +24vdc power.

The intended use for the RU/GF equipment was in single-seater or two-seater airplanes (radio op/observer seated behind the pilot) but the manual also mentions "flying boats" as another possible user in the installation instructions. Each installation into an particular airplane was "custom fitted" with each of the connecting cables custom-built from supplied "bulk cable." Additionally, flex control cables were also custom-fitted and were built from supplied "bulk" flex cable material. In some single-seater airplanes, the only place to install the radio gear was behind the pilot's seat so remote controls using flex control cables and spline drive flex cables along with remote switch boxes and tuning heads were installed to allow the pilot to have the essential radio controls in front of him. There is also evidence that some RU-16/GF-11 gear was installed into a few small Navy boats and that some vehicular installations may have occurred from time to time. The USMC is said to have had some vehicles equipped with RU/GF gear. Not all RU-Series receivers were paired with the GF-Series transmitters. The RU-18 was usually paired with the much larger GO-Series transmitters. Some RU-Series receivers were setup in "receive only" stations while others might just be used for DF purposes. 

photo above
: RU-16 Receiver - serial number 8989

There was also a U.S. Army version of the RU/GF equipment, the SCR-AL-183. The receiver was designated as BC-AL-229 and the transmitter was BC-AL-230. The contracts are from the late-thirties up into 1940 with Western Electric as the contractor. This equipment is very similar in appearance to the RU/GF equipment but internally both the receiver and transmitter abound with minor differences. The SCR-AL-183 was the 12 volt version and the SCR-AL-283 was the 24 volt version. The Army versions were also intended for one and two-seater aircraft installations and are found in both black wrinkle finish and in bare aluminum.

The overall use of the later equipment was very low by mid-WWII. This non-use resulted in many complete RU-16/GF-11 equipment packages being sold on the post-WWII surplus market "new in the box" which accounts for the "fairly common" status of the RU-16/GF-11. The RU-17/GF-12 were the 24 volt versions and apparently this equipment was used much more extensively during WWII and isn't encountered as often as the RU-16/GF-11. For quick identification the data plates on the 12 volt units had a black field while the data plates on the 24 volt units used a blue field.


RU-16 Receiver Circuit - The earliest RU receivers used triode tubes in a TRF circuit with tracking BFO. A tracking BFO utilized an identical section of the main ganged tuning capacitor along with coils that allowed adjustment of an oscillator to "track" or "tune along with" the tuned RF frequency accurately. Usually, a tracking BFO would be set one kilocycle higher than the RF tuned frequency to allow a heterodyne to be audible, allowing demodulation of a CW signal. All early RU receivers were built by Aircraft Radio Corporation. The early RU versions didn't have an AGC circuit. Additionally, tuning range was limited by the few available coil sets. By early 1941, the RU-16 had been designed. It was the first version of the RU to use an AGC circuit. Six tubes are used in the RU-16 circuit which is still a TRF (tuned radio frequency) receiver with tracking BFO. The tubes used are 1RF - 78, 2RF - 78, 3RF - 78, AGC - 77, Detector - 77, AF Out/BFO - 38233 (aka 1642.) The last tube, type 38233/1642, is a dual triode that provides the tracking BFO with one triode and the Audio Output stage with the other triode. The plug-in coil assemblies each contain five shielded coil units - four units that determine the RF tuning range of the assembly and one unit for the tracking BFO coil required. The "dual frequency range" coil assemblies contained an internal switch that was operated by lever located on the front of the assembly. The single range coils had a metal handle-type strap for removing the coil from the receiver. 

Antennas - There are two antenna inputs, A and L - L. The L - L terminals are for a "homing loop" antenna. The Antenna or Loop switch could be set up to operate locally at the receiver or remotely via a flexible cable. The A terminal could be connected to any of the typical aircraft antennae available and depended mainly on what type of airplane was involved. Most single-seater airplanes had a wire antenna from the cockpit to the tail. Two-seaters usually had an aerodynamic mast near the airplane nose with a wire running to the tail. A central wire dropped down beside the rear-seat part of the cockpit and entered the side of the fuselage for the radio gear connection (a "T" antenna.) Some installations used a trailing wire (depended on the aircraft.) It was also possible to use the DU, DU-1 or DW-1 Amplified Direction Finding Loop which worked with an external sense antenna to provide a "true direction" cardioid pattern that allowed determining a correct bearing towards an unknown location signal. The output of the DU/DW Loop was connected to A on the RU receiver. The complete RU/GF setup provided power to operate a DU-type loop.   The DW-1 Loop is profiled further down this page.

The AGC Circuit -  Homing Loops provided a "figure-8" pattern with two deep nulls off of each side of the loop. The loop would be set athwartship and then the airplane steered toward the null. The general direction was known and the "homing loop" provided accurate navigation to a specific airport or Radio Range Beacon along a defined airway. A "Test Meter" could be used as a signal carrier level indicating device. Since "Homing" assumed a modulated carrier beacon was to be tuned (the constant tone of the A and N modulation plus the carrier when on the beam,) the receiver must be in AUTO for the meter to indicate RF amplifier cathode current that varied because the AGC tube was controlling the RF Amplifier's grid bias when in AUTO. The AGC tube rectifies the modulated wave envelope from the third RF amplifier (before the detector tube) and develops the AGC control voltage based on amplitude of the carrier wave. The modulation level doesn't significantly affect the AGC bias voltage due to filtering within the RU circuit. The meter is inserted in the RF cathode circuit to ground. Since the pilot would be flying in the direction indicated by the loop's null, he would be looking for the weakest signal which is indicated by the highest reading of RF amp cathode current. If the airplane drifted off course, the loop would not be pointing at the null and the signal carrier amplitude would increase which would cause an increase in the AGC bias, increasing the RF amp grid bias and reducing the RF gain and reducing the RF amplifier cathode current resulting in a lower meter reading. Once the pilot was on course he watched the meter and if it began to show reduced current, he knew the airplane had drifted "off the beam" and required some course correction. Since the AGC tube control is before the detector and has significant TC loading, AUTO (AGC) can also be used with CW operation. Since the RF amplifier tubes' cathodes are grounded in AUTO, the output level (INCREASE OUTPUT dual potentiometer) is controlled with a variable resistance on the audio output line. In MANUAL, a variable resistance (dual pot INCREASE OUTPUT) is connected into the RF amplifier tubes' cathode circuit to control the RF Gain of the receiver and the audio output level is fixed at maximum output. The switching of the dual pot's functions is accomplished by the Receiver Output cable connection to the Junction Box and the Receiver Switch Box cable connection to the Junction Box.

GF-11 Transmitter Circuit - The GF-Series started in 1932 and had a number of changes that run up to the GF-12 version. The GF-11 transmitter differs substantially from the early-1930s versions of the circuit that employed two type-10 tubes as modulators, a type-10 PA and a type-45 as an oscillator. The early GF transmitters only had one coil set so frequency options were limited. As the GF evolved, more coil sets were provided and the power output was increased from 3 watts up to about 15 watts. The GF-11 dates from 1941 and uses two type 89 tubes and two type 837 tubes. One of the 89 tubes was the master oscillator tube while the second 89 could be a MCW audio oscillator, an audio sidetone generator on CW/MCW or a Voice modulator depending the the mode selected. The two 837 tubes were operated in Push-Pull as the power amplifier. The 837 screens and suppressor grids were tied together and modulated by the type 89 AF tube in the MCW and Voice modes. The two 89 tube filaments are connected in series for 12vdc operation and the two 837 tubes are connected in parallel (837 tube uses 12 volt filaments.) Low voltage (+12 to +14vdc) was supplied by the aircraft battery/charging system buss and B+ was supplied by the shared dynamotor, that is, both the RU and the GF obtained their B+ from the same dynamotor (CW-21109A) and various resistor dividers within the circuits of each unit. Tuning ranges are determined by plug-in coil sets that provide a frequency range of 2000kc to 3200kc and from 3000kc to 9050kc. Eight plug-in coil sets were supplied with the GF-11. 

RF power output for the GF-11 was about 2 to 7 watts for all modes in the 2-3mc range and 12 to 15 watts in all modes in the 3-9mc range. The meter is an RF amp meter (the radio op tuned for maximum RF current to the antenna.) There were two PL-68 phone jacks on the side of the GF-11 that provided meter access to measure Modulator current and also the PA plate current. The RU-series Test Meter (optional for the RU-16) could be used to measure these points if desired (MOD I was direct but PLATE I had an internal shunt to scale the meter to 5x I.) Absolutely necessary for operation was the Transmitter Control Box CW-23097. This box had the switch for CW, MCW or VOICE modes of operation, RADIO-ICS switch, a neon +HV indicator, input jacks for an external key or mike and a connector for the cable to interface with the RU-16 Junction Box for transmitter operation. On top of the Transmitter Control Box was a hand key button for CW/MCW.


photo left: GF-11 Transmitter - serial number 8888

The Dynamotor-Filter Box - CW-21109A -  The power source for both the RU-16 and the GF-11 is the dynamotor. The dynamotor is mounted on top of the Filter Box which contains the various circuitry components. The voltage input is +12vdc up to +14vdc with better efficiency of operation at the higher input voltage. Running current is between 8 amps and 10 amps depending on the load but initial surge current is quite high (probably >35 amps.) A two conductor cable connected the dynamotor to the aircraft battery-charger buss. To be able to "turn on" the dynamotor from a remote switch box (the CW-23096A) required a relay inside the Filter Box that was operated by battery-charger voltage and the remote switch. The relay had very large contacts for conducting the fairly high current to the "motor" side of the dynamotor. The operating output voltage depended on the battery input voltage and also on the output load but generally was between +350vdc and +400vdc when operating both the RU-16 and the GF-11. Negative bias voltages were required for the GF-11 and for the RU-16 AGC tube. By elevating the output negative wire from the "generator" above chassis using a 1000 ohm WW resistor and a 140 ohm WW resistor in series to chassis, a voltage divider network allowed about -95vdc and -80vdc to be available for bias requirements. The dynamotor itself was built by Eclipse Aviation for Western Electric.


The Ancillary Pieces - The RU-16/GF-11 (actually the entire RU/GF series) required a considerable collection of peripheral ancillary equipment and interconnecting cables to actually operate the receiver and the transmitter with the dynamotor.

Cables and Plugs - In addition to several peripheral boxes there was an array of specifically "identified by number" interconnection cables with special connector plugs with unique pin patterns or different diameters that interconnected the RU-16/GF-11, the Dynamotor and the various switch boxes through the Junction Box. Originally, bulk cable was supplied with the equipment and each interconnection cable had to be custom-built using the correct type bulk cable with the correct connector plugs installed. Additionally, the bulk cables were "un-jacketed" to allow the cable shields to be easily bonded to the aircraft frame for lowest noise pickup. Cables were supposed to have a metal identification tag installed during construction. Since each installation was "custom-fitted" to the aircraft most of the original RU-GF cables remained in the aircraft. What is found today are mostly unused RU-GF connector plugs that have to be used to build new cables. Most plugs weren't identified except perhaps with a single number stamped on the shell. Lack of specific identification complicates finding some of the plugs. The connector pin numbering is unique to each type of plug, that, and the pin patterns have to be used to identify an unmarked plug. A typical plug is shown installed in the dynamotor photo to the right (it's a #134 plug.) 

photo above
: RU-16/GF-11 Dynamotor CW-21109A - serial number 6142.

photo above
: CW-62008A Junction Box with many of the required connector plugs partially inserted. Note that some of the plugs do have identification numbers on the shell. Plugs 37 and 80 are substitute plugs that are required for "receiver only" operation (both plugs) or for "single-seater" aircraft installation (80 only.) Box receptacles 74 and 76 are provided to power accessories like the LM-type Frequency Meter, the DU-1-type amplified loop antenna or the ZB-type Homing Adapter.

Junction Box, Switch Boxes and Substitute Plugs -  The Junction Box is essential for the proper interconnecting and operation of the entire RU-GF system. Likewise, the Dynamotor is necessary to provide B+ voltage to both transmitter and receiver by way of its connection to the Junction Box. To actually operate the RU-16 receiver required a large 11 wire cable from the RU-16 to the Junction Box. The RU-16 Remote Switch Box (photo below) provided the switching for CW/MCW, for selecting AUTO/MANUAL along with ON/OFF function for the entire system, a Gain control for the receiver output, two phone jacks for the audio output, a three-circuit phone jack for the Test Meter. The Receiver Switch Box was cable-connected to the Junction Box.

To operate the GF-11 transmitter required its nine wire cable to connect to the Junction Box and the Transmitter Control Box was also cable-connected to the Junction Box. There was also a GF-11 Extension Control Box that connected to the Junction Box (use was optional and intended for two-seater airplanes) and a Remote Transmitter Control switch (was user supplied and connected to the phone jack on Junction Box - operated the PTT/T-R line.) For single antenna T-R operation the Antenna Relay box was necessary (it also connected to the Junction Box using a small two pin plug and cable.)

There was also an optional and externally connected "RU Test Meter" that could be added to the setup (PL-68 plug inserted into the RU-16 switch box for Homing or into jacks on the side of the GF-11.) There were two connectors (74 and 76) on the Junction Box that could be used to provide voltage to operate a LM-type CFI (Crystal Frequency Indicator, aka: heterodyne frequency meter) or an amplified loop antenna like the DU-series. It was also possible to power a "Homing Adapter" like the ZB Series from either connector 74 or 76. (ZB-3 "Homing Adapter" profiled further down this page.)

Two "substitute plugs" were supplied and must be installed in the Transmitter Control Box connector and the Extension Control Box connector (37 and 80) of the Junction Box if a "receive only" setup was intended. If the Extension Control Box wasn't needed such as in a single-seater airplane then only sub-plug 80 is required. These "substitute plugs" each had an internal jumper to route the circuitry as necessary for this type of operation. Sub-plug 37 grounds the 38233 tube cathode so the receiver will function without the Transmitter Control Box's "RADIO/ICS-1/ICS-2" switch and sub-plug 80 completes the circuit so the Remote Transmitter Switch will function without the Extension Control Box.

Flex Cables, Extension Key, Microphone & ICS - Remote Loop-Antenna Switch flex cables and Remote Tuning Range Switch flex cables were included in the package but their use depended on the ultimate installation requirements. These flex cable connected controls were necessary in some single-seater installations where the radio gear was located behind the pilot and only the remote controls and switch boxes for the radio operation were up front. The flex cable was also supplied in bulk and each remote flex cable had to be custom-built for the installation requirements. An external telegraph key (called an "extension key") could be used, a carbon mike like the RS-38 was needed if Voice operation was desired and a set of Lo-Z 'phones (600Z ohms) was also necessary. The ICS (Inter-Communication System) provided a method for the radio op to talk to the pilot through the use of the Transmitter Control Box by switching to ICS-1 or ICS-2 and working into the pilot's Extension Control Box. ICS-1 allowed intercommunication with the radio signals still audible and ICS-2 removed the radio signals and only allowed the intercommunication (this position was "spring loaded" so the receiver signals couldn't be "locked out.") The ICS connection to the RU-16 audio output grid provided the sidetone through the 'phones when in "RADIO" on the Transmitter Control Box and when in the CW or MCW modes.

photo above: A RU-16 Single Range plug-in coil assembly for tuning Range E. Note the "extraction" handle. Also, five shielded coil units that function as (3) RF coils, (1) Detector-Coupler coil and (1) Tracking BFO coil. Tunes from approximately 1200kc up to 2100kc.

Plug-in Coils - All plug-in coils for both the RU-16 and the GF-11 are somewhat difficult to extract out of either piece of equipment. It seems likely that the coil sets needed were installed during pre-flight setup. The dual range RU coils allowed the pilot or radio op two ranges that could be easily selected during flight as required (a HF range for comms and a MW range for 500kc for navs, for example.) Single range coils were used when only one operating frequency was going to be needed. The GF coils were also single range and the required coil set was installed in the transmitter during the pre-flight setup for the intended operating frequency.

photo right: A RU-16 Dual Range plug-in coil assembly. This is the O-P Range coil set with a total coverage using both ranges of 187kc up to 455kc. The switch lever is located on the front-facing part of the coil assembly and is marked "FREQ - HIGH - LOW."

photo above: GF-11 coil set for 3675-4525kc in storage container

The RU used four "dual frequency range" coil packs and five single range coil packs. There were several other coil assemblies listed in the manual but only four duals and five singles were normally supplied with the receiver. Each coil assembly originally had a specific metal case to store it in. When all of the normally supplied coil assemblies were available the RU-16 could tune from 190kc up to 13.575mc. The coil assemblies not supplied were different combinations of frequency ranges and slightly different frequency ranges on the single coils. Ultimately, even if all coil assemblies were available, the same 190kc to 13.575mc was complete frequency coverage of the coil sets. The GF-11 also had its own plug-in tuning modules and each of those also originally had a metal box for protection during storage. Eight GF-11 tuning modules were supplied allowing the transmitter to operate from 2.0mc up to 9.05mc.

Interchangeability - Many of the ancillary pieces are interchangeable to the RU-16/GF-11 from the RU-17/GF-12 and RU-18 receivers and possibly earlier versions. It depends on what the piece's function was. The Receiver Tuning Head, for example, since it's entirely mechanical and the tuning dial scale is 0 to 100, is interchangeable within many of the RU-series receivers. Uniquely necessary for 24 volt operation is the proper Dynamotor, Junction Box and Antenna Relay Unit. The 24 volt receivers just have a voltage divider in the circuit to basically operate a 12 volt receiver on 24 volts. The NAVAER 08-5Q-100 manual is very specific with two pages of possible interchangeability of the various pieces for both the RU and GF equipment.

photo above: Receiver Switch Box. INCREASE OUTPUT control actually is a dual pot that controls the RF gain in MANUAL and Audio output level in AUTO.

More RU/GF Ancillary Pieces

1. Receiver Tuning Head - CW-23012 - Depending on the installation, the RU-16 might have required a Receiver Tuning Head that was connected to the tuning gear box by way of a flexible spline cable similar to old car speedometer cables. The spline flex cable was also supplied in bulk lengths and custom fitted for the installation requirements. Shown in photo 1 is the Type CW-23012, in this case, for the RU-17 (blue tag) but most ancillary components were interchangeable between the RU-16 and RU-17. Since the tuning dial on the receiver and on the remote tuning head were both scaled "0 to 100," a tuned frequency versus dial readout chart was attached to the top of the receiver's tube cover plate. The fiducial could be mounted in several positions around the dial perimeter to allow mounting the Tuning Head in the best position for pilot or radio op visibility in the particular installation.

2. Direct Coupler - MC-127 - For "local" operation of the receiver tuning there was a small direct coupler tuning adapter that could be installed in lieu of the Receiver Tuning Head. Where the installation allowed the radio op to have the receiver in front of him, the MC-127 coupler eliminated the flex drive cable and remote tuning head. Often times, the MC-127 greatly reduced the backlash that was inherent in the flex drive cable operation. However, the miniscule size of the MC-127 hampers easy fast tuning with the crank and most tuning is done slowly by using the "fingers on the wheel-rim perimeter" approach.

3. Antenna Relay Unit - CW-23049 - If a single antenna is used for both receive and transmit then the Antenna Relay Unit must be used. It is driven by the GF-11 PTT through the Junction Box and the Antenna Relay Unit is connected by a two conductor cable to the Junction Box. Push-terminals for Antenna - ANT, Receiver - REC and Transmitter - TR. Ground is achieved through the case being mounted to the airframe.

4. RU/GF Test Meter - CBY-22266 - The Test Meter was optional for the RU-16/GF-11 gear. The Test Meter is mentioned in the manual as desirable for a visual indicator for Homing. For RU interfacing, a PL-68 plug is inserted into the Meter Jack on the Receiver Switch Box. The Test Meter is also mentioned in the manual for use with the GF-11 transmitter. The PL-68 plug can be inserted into the MOD jack or into the PLATE jack that are on the left side of the transmitter. In PLATE, a built-in shunt reduces the current so there is a factor of five times the meter scale reading for actual PLATE current. The GF-Meter shown was built by Aircraft Radio Corp. for the GF-8 but it can be used with the GF-11 since all of the meter specifications and requirements didn't change.

5. & 6. Telephonics Corp. - RS-38 CTE-51004-C and CTE-26003A - The RS-38 is a handheld carbon microphone with push-to-talk button located on the top of the mike and features a "noise cancelling" mouth piece. The RS-38 is specified as the mike to use in the manual although a "noise cancelling" T-17 would also work. The mike uses a PL-68 plug. The Transmitter Control Box had a jack for an extension telegraph key using a PL-55 plug. There was a large button-type key on top of the Transmitter Control Box but it's awkward to use and limited in its ability to send decent Morse. For Morse proficient radio ops, the 26003A was a well-built "flame proof" key to use with the GF-11 and in the hands of a good radio op could allow excellent Morse to be transmitted. The "mushroom head" knob is standard for the 23006 key. Both the RS-38 and the 23006A are from Telephonics Corp.












For the ultimate in detailed information about the RU-16 Receiver and GF-11 Transmitter plus restoration and rebuilding information on the RU-16/GF-11 operation station,...go to "RU16-GF11 Restoration"
use home-index below for navigation.

Navy Department - RCA Manufacturing Co., Inc.

Aircraft Radio Receiver - Model ARB - Type CRV-46151

The ARB receiver was a six tube superheterodyne receiver intended for use in USN aircraft. It was an updated version of the earlier, mid-thirties RU receiver series that required several sets of plug-in coil assemblies to change tuning ranges in addition to a baffling array of remote boxes, remote cables and a junction box for interconnection of all of the extra pieces. The ARB receiver simplified the hook-up and dramatically improved the overall performance by replacing the RU's TRF with Tracking BFO circuit with a superheterodyne circuit. The ARB receiver tuned from 195kc up to 9.05mc in four bands. Two dual-frequency IF amplifiers are utilized with 135kc used in the 195kc to 1600kc range and 915kc used from 1.6mc to 9.05mc (a dual frequency BFO was also required.) The receiver used 12 volt heater tubes (in series-parallel for 24-28 volt operation) and had one RF amplifier 12SF7, a Converter 12SA7, two IF amplifiers, both 12SF7, Det-AVC-1AF stage 12SF7 and an audio output stage 12A6. A neon bulb was used as a voltage regulator for the LO part of the Converter stage. The lower two bands could be set up for loop operation, specifically for homing DF purposes. All four bands could be used for Communications and operated with the various types of aircraft antennae available. To simplify the external power hookup, the aircraft +28vdc buss was connected to POWER for tube heaters and inside the ARB was a dynamotor that provided the +230vdc B+.

The ARB was designed for either single-seater (pilot only) or two-seater aircraft (pilot and radio op) but could also be found in larger aircraft with a crew that included radio op/navigator, pilot and co-pilot. Modes of reception were CW, MCW and Voice with the options of AVC or MVC (Manual gain control.) When in "AVC," the Volume control operated as an AF gain control with RF/IF sensitivity controlled by the AVC line. When in "MVC," the Volume control operated as a RF/IF gain control with the audio gain set to maximum. Selectivity options when in AVC were either Sharp or Broad. The "Broad" position was intended to ease tuning in the 1600kc up to 9.05mc range where signals tended to be more difficult to "tune in" due to the wide span of frequency coverage in each of the two bands (additionally, the remote receiver tuning head was particularly difficult to "fine tune" due to flexing of the cable drive which resulted in significant "backlash" if not installed correctly.)

The initial ARB receivers are pre-WWII and were used for Communications and for Homing DF. For DF, the aircraft had to be equipped with a rotatable, non-center-tapped, loop antenna that could be connected to the ARB receiver terminals marked L1 and L2 (L2 is chassis ground.) When connected in this way, the loop would be operational only on the two lower frequency bands only (195kc to 1600kc.) This type of loop antenna would have a bi-directional "figure-8" pattern and was generally set "athwartship" and the airplane steered towards the minimum signal response (homing.) The two other antenna terminals are marked AT and AF. AT indicated a "Trailing Antenna" which was normally installed in larger aircraft and consisted of a clad stranded steel cable with flight weight that could be reeled out to about 200 feet behind the aircraft when in flight. AF indicated "Fixed Antenna" which was a smaller antenna consisting of an off-center fed wire between the cockpit and the tail of the airplane or it could be a short vertical installed on larger aircraft (usually 4 to 5 feet tall maximum.)

photo above
: ARB Receiver on its shock mount (hard to see) with ZB-3 Homing Adapter on top, Operator's Control Box and Receiver Tuning Head to the right. On the Operator's Control Box, the unused connector plug marked "Control Box" would be used for the wire cable running to the Pilot's Control Box. The opening with the threaded barrel is for the Bowden cable that ran to the cockpit to allow the pilot to switch control of the receiver from "REMOTE" back to the radio operator, "LOCAL." Dual phone jacks could be used for two sets of 'phones or one jack could be used to operate an Audio Output Meter for DF purposes.

The Control Boxes - To actually operate the ARB receiver required the Operator's Control Box (Type CRV-23256) or the Pilot's Control Box (CRV-23254.) The receiver could be operated by either of the control boxes when the aircraft installation was for "dual control setup" required for radio-navigator or pilot control. There wasn't a volume control on the receiver but each control box had a volume control. Likewise, there wasn't a phone jack on the receiver for audio output and, again, the control boxes had dual phone jacks for audio output. Normally, the audio output impedance was set for LOW which was 600Z ohms. By moving a pair of jumpers in either control box the audio output impedance could be set to HI or 4000Z ohms. Either control box could also switch bands on the receiver remotely when the receiver's "MOTOR" switch was ON as this enabled the receiver's motor-driven band switch. This function could be disabled with the receiver front panel "MOTOR" switch and the band switch on the receiver then operated manually.

The Operator's Control Box had a port with a threaded barrel for installing a bowden cable that mechanically connected internally to the "LOCAL-REMOTE" switch. If the radio op wanted to pass control of the receiver to the pilot, he switched the lever to "REMOTE." This operated the bowden cable and pushed the bowden cable knob at the pilot's location to the "up" position. Through the inter-connecting wire cable the Pilot's Control Box electrically was activated and the Operator's Control Box was deactivated. The Pilot's Control Box now allowed the pilot to have control of receiver operation. Additional to this "dual control setup" was a second (flex cable connected) Receiver Tuning Head that was installed in the cockpit near the pilot. This setup allowed the pilot to tune the receiver, switch bands, control gain and select reception mode. To pass control back to the radio op, the pilot would push down the bowden cable knob and that operated the Operator's Control Box "LOCAL-REMOTE" switch via the bowden cable returning control back to "LOCAL" and activating the Operator's Control Box and deactivating the Pilot's Control Box. Where the installation only required the pilot to operate the receiver then just the Pilot's Control Box could be installed and it would provide normal operational control of the receiver by a single operator.

 photos: The Pilot's Control Box CRV-23254 (left) and the Operator's Control Box Type CRV-23256 (right.) The Operator's Control Box adds the LOCAL-REMOTE switch.

The Receiver Tuning Head -  Receiver tuning was accomplished by using a Receiver Tuning Head (Type CV-23253) that had a conical tuning dial scale viewed behind an index window and a hand crank type of tuning control. Coupling to the receiver was via a metal flexible spline-ended cable in a metal flexible housing (similar to the speedometer cable used in older cars.) There was also a direct coupler that could be attached to the receiver tuning gearbox that allowed direct "at the receiver" tuning utilizing the receiver dial for frequency readout but the dial scale was minuscule so a magnifying lens was built into the dial bezel.

Additionally, the direct coupler had a "feed-thru" connection that allowed the direct coupler and the receiver tuning head to be connected together and to simultaneously operate the receiver tuning (this setup tends to compromise the otherwise smooth operation of the Receiver Tuning Head.) Hint: For smooth tuning operation don't over-tighten the collar nuts on any of the flex cables or couplers. Also, avoid "tight" bends in the routing of the flex cable with no radius tighter than 6" for best results. Securely mounting the flex cables and tuning heads will also help significantly by keeping all parts in a stable, fixed position.

Ancillary Parts
- The ARB used one power cable, one control box cable, one flexible tuning (spline-ends) cable, one "Pilot's" control box, one remote tuning head, a shock mount and phones. It was also possible to setup for "dual operation" by adding an "Operator's" control box and control box cable (connected to the first control box) along with a second flexible tuning cable, "T" coupler and second tuning control head for larger aircraft where either a navigator or radioman was aboard. A bowden cable was also necessary to allow switching from "Remote" to "Local" control from the pilot's position.

Operational Notes - A very selective IF bandwidth was not really desirable in a WWII aircraft receiver that might be operated solely by the pilot. Quick location of the desired signal (a homing beacon, for example,) with a minimum of operational movements was necessary. Also, interference was practically nil in normal operations at the time. Today, when trying to use the ARB as a station receiver, one will immediately find that the IF passband is extremely wide, even in the SHARP position (at least 10kc bandwidth in Narrow.) The ARB is certainly sensitive enough but adjacent frequency signals will be heard and sometimes these adjacent signals seem to overwhelm the desired signal. A small separate receive antenna might help reduce some QRM. Also, a loop antenna might allow "nulling" the offending QRM. The use of preselectors or converters only seems to complicate the set up for a receiver that wasn't designed for and never intended for ham band operations.

photo left: Close-up of the Receiver Tuning Head Type CRV-23253

Initial Concept of Homing DF with the ARB - The ARB was originally intended for use with a non-center-tapped loop antenna that allowed one end of the loop to be grounded. These loops responded in a "figure-8" or bi-directional pattern and could provide a bearing by DFing a known beacon signal to allow "homing-in" on the signal. This provided the pilot or navigator with a very accurate bearing towards the origin of the "known" signal. The bi-directional pattern allowed for two "deep" nulls off each side of the loop. The nulls were much easier to detect and much more accurate for determining the "minimum response" at specific loop positions. Since the Operator's Control Box had two phone jacks, an Audio Output Meter could be plugged in to allow an accurate visual indicator for minimum signal response. Though "true" direction might be ambiguous since there were two nulls, generally the pilot and navigator knew the approximate direction of the airfield they were flying to and the loop's null gave them a precise bearing toward the airport's Range Beacon. Pre-WWII, there were many Remote Airways Radio Range beacons along with regular Airport Range Beacons that provided navigation radio DFing signals to allow the pilot or navigator to determine the correct course (called an Airway) to fly to a desired airport. Once the airplane was "on course" using one of the four "beams" from the beacon, then the "A" and "N" signals in combination (a constant tone) along with the navigation chart indications allowed following an accurate course to the airport. In cases where only a non-directional beacon was available for navigation, the pilot or navigator would set up the loop athwartship to allow navigating the aircraft to a desired airport or other location. Sometimes an AM-BC station would be used for a beacon since their signals were usually strong and consistent and the transmitting location (city) was known. All beacons and most strong AM-BC stations were shown on the navigation charts. AM-BC stations were supposed to identify themselves with their call and city location at least once an hour to assist any aircraft that were using the AM-BC station as a beacon. It was also possible, if an accurate position of the aircraft wasn't known, for the pilot or navigator to take bearings on two known beacons (shown on nav-chart.) The bearings could be transferred to the nav-chart as lines (although navigators usually had plastic scales and "computers" for this function.) Where the two lines intersected on the chart would indicate the aircraft's location (a form of triangulation.)  


photo above:
ZB-3 Homing Adapter
. Installing the ZB Homing Adapter on the RU receiver required removing the RU vacuum tube cover. The ZB shock mount holes and slide-clips align with the mounting pins that were for the RU vacuum tube cover. The ZB shock mount plate doubles as a RU vacuum tube cover when installed. There were two "slide latch" pins on top of the ARB cabinet to allow for the installation of a VHF homing receiver such as one of the ZB-Series (ZB, ZB-1, 2 or 3.)

 Navy Dept.- Contractor: Western Electric - Mfg by: Zenith Radio Corp.

Model ZB-3 Homing Adapter  - Type CZR-69076

The ZB-Series was actually designed in the late-thirties for use with the RU receivers but were later also installed on ARB receivers. These Homing receivers used four 954 acorn-type tubes and received direction-location signals transmitted by aircraft carriers. The carrier transmitted a homing signal using a small rotating VHF beam antenna mounted high up on the carrier superstructure that sent out a signal that was "timed" to send specific but differently coded signals every 30 of rotation (speed of rotation was fairly slow at about 3 rpm.) This would allow identification of each 30 sector by a pilot using a "homing adapter" and radio receiver to ascertain an accurate and specific direction to the aircraft carrier. VHF was used since the radio wave was highly directional with the antenna used producing a very narrow beam width that wasn't affected by propagation. The actual signal was comprised of a VHF carrier wave that contained a modulated subcarrier of 700kc. The sub-carrier was modulated with MCW signals consisting of various Morse letters that identified 12 sectors separated by 30º increments of a 360º degree circle surrounding the carrier. The ZB receiver would initially receive the VHF signal which was then down-converted so it could be received on the RU or ARB receiver in the airplane when tuned to 700kc. The pilot had to know which letter identified which 30º sector around the carrier relative to his flying position (for security, these codes changed daily.) The strongest signal indicated in which sector the airplane was flying and this allowed the pilot to calculate where the carrier was and how to approach the landing deck. Depending on the aircraft's altitude, the homing signal could be received out as far as 275 miles away. Use of the ZB Homing Adapters allowed pilots to find their way back to their carrier from long distances, or at night or in otherwise very poor visual conditions.

The ZB-3 Homing Adapters would receive the VHF "homing signal" from the carrier transmitter that usually operated around 242mc. The converter of the ZB receiver would then subtract the VHF carrier leaving the sub-carrier and modulation. The MW sub-carrier wave could then be tuned in on the ARB receiver (or the RU receiver.) If the VHF carrier wave was intercepted by the enemy, no information could be detected without the ZB-type conversion taking place. There were other ZB Homing Adapters at other frequencies, for example the ZB-2 utilized frequencies in the 34mc to 58mc range while the ZB-3 utilized frequencies in the 234mc to 258mc range with the normal operating frequency being 242mc.

The ZB Homing Adapters required a specific ZB-Control Box that routed power from the ARB receiver or from the Junction Box in the case of RU receivers. A specific ZB-Antenna Relay Box was also required and that allowed switching RF signals to the ARB (either from the Homing Adaptor or from the MW or HF aircraft antenna.)

photo above
: The ZB-3 chassis showing the voltage selector switch on the right (some RU receivers operated on 12vdc.) Also the antenna input coaxial cable going to the 1st RF amplifier stage. The next two stages to the left are the second and third RF amplifiers. The left side last tube is the converter/LP filter circuit.

The rear panel ZB connectors are routed as follows: The three-pin connector cable is routed to the ARB accessories socket or the RU Junction Box. The four pin connector cable is routed to the ZB-Antenna control box. The eight pin connector cable is routed to the ZB-control box. These connections allowed the ARB or RU to be used for "Homing" or for communications. Also included in the ZB accessories was a test oscillator with VHF output. The "snap pins" on top of the ZB are for the canvas cover (called a "skirt") that could be installed when the ZB wasn't in use.

Later versions were given the designations of ARR-1 and ARR-2. The ARR-1 is very similar to the ZB-3 while the very late versions of the ARR-2 used an on-board dynamotor and flexible cable remote tuning control.


photo right: Rear of the ZB-3 showing the various receptacles.


photo above: The DW-1 version of the Amplified Direction Finding Loop

Navy Department - Bendix Radio (Div. of Bendix Aviation Corp.)

Model DW-1

 Type CRR-50061 Coupler  &  CRR-69052 Plug-in Loop

TRF Amplified Direction Finding Loop


As radio navigation evolved, it was obvious that smaller, light-weight equipment would be necessary for airborne installations. The radio compass was especially suited for air search and rescue operations that involved finding the unknown bearing to a received signal from an unknown location. By combining the loop bi-directional pattern with a "sense" antenna (usually a small vertical or short wire antenna,) a cardioid pattern would result, giving the user the ability to determine "true direction" of a signal because of its single null response. The use of a "sense" antenna in combination was a loop began during WWI and developed during the 1920s. The Bellini-Tosi goniometer-type of loop (1909) had shown that a very small loop was usable at low frequencies and actually provided some advantages over very large loops at the same frequency of operation. The small loops were needed for aircraft navigation and, by the 1930s, loops in combination with an azimuth compass and an dual RF amplifier box that would provide RF amplification and RF tuning of the responses of both the loop and the sense antenna had been developed. 

The DU, the DU-1 and the DW-1 loop antennas were designed for use with the RU-series of receivers (1935 to 1941) and most of the DU-type loop contracts for the Navy date from around 1940, however, this type of loop dates back to the mid-thirties (Amelia Earhart had a Bendix loop installed on her Lockheed Electra 10E - see photo below.) In the case of the DU-style loop, power for the RF amplifier box came from the RU-Junction Box. The DW-1 and DU-1 can also be operated much more conveniently with the ARB receiver. The DW-1 can be powered directly by the ARB accessories connector that provides +28vdc and +230vdc. Inside the DW-1 housing is a dual RF amplifier arrangement with one 12SK7 tube amplifying the loop signal and the other 12SK7 tube amplifying the sense antenna signal. The plates of the two tubes are connected together and that performs the "mixing" function that creates the cardioid pattern allowing the ARB receiver operator to determine "true direction" by the loop's position relative to the aircraft fuselage and course. The DW-1 loop is colored gray for 180º of its circumference and black for the other 180º of circumference. Gray is "inline" with 90º on the azimuth compass and is considered the "front" of the loop and is generally set up on the aircraft in that orientation. The DW-1 has a three-position switch marked R-B-D that changes the antennae as follows: R is the Sense antenna only which is omni-directional. B is the loop only which is bi-directional. D is the loop and sense antennae combined giving a cardioid pattern. The RF tuner on the DW-1 provides coverage from 190kc up to 1900kc with the intended use being DFing navigation beacons or AM-BC stations (for homing) in the LF and MW part of the spectrum.

There isn't any specific information that the DU-1 or the DW-1 were ever used in combination with the ARB - this equipment set up isn't mentioned in either manual. However, electronically and physically, it's very easy to operate them together and to determine "true direction" of an unknown signal. The output of the DW-1 is connected to the AT antenna terminal with the control box set to COMMUN (for operation on all bands for AT input.) Since the combining of sense antenna and loop for a cardioid pattern is already accomplished by the DW-1 circuit only a connection to the ARB RF amplifier is necessary. Actually, if the DW-1 is powered separately, its output can be connected to any radio receiver antenna input for DFing. Additionally, two jumpers can be moved internally to change the series filament connection on the 12SK7 tubes to parallel connections for +12vdc operation of the DW-1 (along with +230vdc B+.) The parallel filaments +12vdc connections would be necessary if I used the DW-1 with the RU-16 receiver.

This DW-1 required a little bit of mechanical rework to get it function correctly. All components were checked and found to be within tolerance. Once the mechanical problems were corrected, the DW-1 was ready to test. I have to admit that I wasn't expecting too much from the DW-1. A few years earlier I had access to a DU-1 that I was testing for a fellow LW enthusiast but the results were less-than impressive (I wasn't using the loop with an ARB receiver however.) I had also tried out this very same DW-1 at a local mil-radio collector's shack with it connected to his ARB receiver but, again, the results were not impressive. A different story with the rebuilt DW-1 in operation with my recently serviced and aligned ARB receiver. The signal levels available with either the DW-1 loop or just a small sense antenna are very strong. It's very easy to set up the DW-1 with the ARB and the DF results are very apparent making it easy to find nulls and take bearings. The ARB and DW-1 loop combination is excellent for demonstrating WWII DF procedure (if anyone is interested.)

photo right: Amelia Earhart holds an earlier version loop and compass while a probable sales rep for Bendix holds the RF tuner. ca: March 1937
This was obviously a well-attended "photo shoot" as there are many, many shots of Earhart with the Bendix Loop well-represented on the Internet.
This particular photo happens to be one that shows both the loop-compass and the RF amplifier-tuner box (and the interconnecting cable.)

DW-1 with ARB Combination - Operation - Connect the DW-1 to the ARB Accessories connector with a proper cable. Connect a Sense Antenna to the A terminal on the DW-1. This can be any relatively short wire, e.g. 10 feet, oriented vertically (if possible.) Connect the DW-1 output to the AT input on the ARB. Set the Control Box to COMMUN and select the AM-BC range of frequencies. Set the DW-1 to R (Sense Antenna.) Switch ON the ARB and the DW-1. With the Sense antenna selected (R) you can tune in a relatively weak AM-BC station (don't tune in a local AM station as the signal will be way too strong for DFing.) Adjust the VOLUME on the Control Box for an average listening level. Next, switch the DW-1 to B (Loop only) and rotate the loop for the strongest signal response and then "tune-in" the DW-1 frequency dial for the strongest signal. AM-BC will be on Band 2 or Band 3 depending on where with the AM-BC band you are tuning. Highest frequency that can be tuned is 1900kc (on the DW-1.) Once the desired signal is tuned to maximum, next rotate the loop (approximately 90º) and you should find one of the two nulls. Increase the VOLUME to be sure you have the loop exactly at the null and observe the compass bearing. Now, switch the DW-1 to D (Loop+Sense) and rotate the loop exactly +90 from the observed compass bearing. If the signal remains strong, then carefully rotate the loop another +180 and you should find the null. The null is very sharp and when the null is found the loop axis will be "pointing" at the signal and the compass gives the bearing. If you actually set-up the DW-1 oriented N-0º & S-180º then you would have a pretty close bearing to the signal (if you adjust for magnetic deviation, you'd be even more accurate.) You might have to adjust the DW-1 Gain pot for best null response. Adjust the Gain pot by switching between R and B and adjust the pot for the best balance of the two responses. Be sure to have the loop pointing at the signal source for this adjustment. This will assure that a deep null will result when in the D position. There is a moveable mask on the DW-1 compass that only allows a little over 120 of the compass to be viewed. When the "loop-only" null is determined, then the mask opening can be moved to have the "zero" index at the observed bearing and then about +90 of the observed bearing will be at about two-thirds towards the opposite end of the mask opening.


Since the "Gibson Girl" has been mentioned in the write-ups on the Air-Nav receivers above and, even though it's sort of "out of place" with Navy gear (since it was USAAF,) I'm putting the "Gibson Girl" write-up here with the Navy Air-Nav gear. By the end of WWII, both the USAAF and the Navy were using the SCR-578 and post-war the designation changed to AN/CRT-3 with the AN indicating "Army-Navy."

SCR-578 "Gibson Girl" Emergency Transmitter
BC-778 Transmitter plus Accessories, Initial Contractor: Bendix Radio

AN/CRT-3 "Gibson Girl"
T-74 Emergency Transmitter plus Accessories

By the late-thirties there had been various types of emergency radio transmitters developed to aid in rescue at sea of downed aircraft crews. Most early attempts were crude and not very effective. In 1941, the Germans had come out with their "Not Sende Gerat 2" aka NS2. It actually was a compact, more effective version of their earlier fairly large and heavy NS1. The NS2 was similar to the yet-to-be developed "Gibson Girl" with a 500kc transmitter operated by a hand-crank generator, a slightly curved housing and a yellow paint job. Similar "Gibson Girl-type" accessories went with the NS2.

In 1941, the British captured a German NS2 and copied it with it becoming the British T-1333 emergency transmitter used by the RAF. Again, the T-1333 is very similar to the yet-to-be developed "Gibson Girl." When a second NS2 was captured, the British gave it to the USA with the idea that the USA had the manufacturing resources and capability to produce the huge number of emergency transmitters that would be required for the war effort. The USA engineers further refined the design concept and built a more robust mechanical configuration. The USA version became the BC-778 transmitter that, along with all of its accessories, was designated the SCR-578. Production started in 1942 with Bendix Radio (a division of Bendix Aviation Corporation) being the initial contractor.

The BC-778 was a two tube transmitter that operated MCW on 500kc. Power was provided by a dual voltage generator that was gear driven when turned with a hand crank on top of the transmitter. Voltages were +24vdc and +330vdc when the generator was driven with the hand crank turned at about 80 RPM (the German NS2 generator required 120 RPM.) The tubes were a 12A6 oscillator that was grid modulated by a 12SC7 Tone Oscillator. Power output was about 5 watts. Turned with the hand crank simultaneously with the generator was the code wheel that keyed the transmitter automatically. The code wheel sent ten "SOS" signals and then about ten seconds of "key down" to allow for DFing the position of the transmitter. The BC-778 had to be tuned for maximum brightness of the neon lamp indicator.

It was also possible to have one operator crank the generator and another operator press the KEY button and send Morse (it was very difficult for one operator to both crank and send Morse at the same time.) The transmitter had a large canvas strap the was used to secure the BC-778 between the operator's legs which then allowed for easier manipulation of the hand crank (both hands could be then used.) Under good conditions on the open sea with the search aircraft at a 2000' elevation, the signal could be picked-up out to about 200 miles. The BC-778 was packed in a weather-proof flexible case that contained many accessories to allow the proper set-up of the emergency transmitter for best signal results.

photo above: The WWII Gibson Girl BC-778-D

Accessories - The BC-778 was equipped with a reel out antenna wire that was about 250' long,...but how to get the antenna aloft for proper operation? There were two options available. If it was windy enough, a box kite could be deployed to lift the antenna into position. If there was no wind then the balloon was used. To fill the balloon with a lighter-than-air gas required using the hydrogen generator provided in the kit. The hydrogen generator was a cylinder that contained a grain-type material that when exposed to salt water (or sea water) would produce hydrogen gas and heat. The generator cylinder got too hot to hold during the production of H2 gas. The H2 gas would be used to fill the balloon using a rubber tube provided. The balloon would rise and pull the antenna up with it. Another device in the kit was a signal lamp that plugged into a socket in front of the BC-778. The signal lamp was mounted to the operator's head with a canvas strap. The lamp could send SOS simultaneously with the transmitter keying or it could be switched on "Continuous." The signal lamp was for nighttime rescue operations. Good antenna operation would depend on a proper ground and in the front panel of the BC-778 is a compartment that has a metal cable with a "sinker-threaded plug" attached to the end. This ground wire was dropped over the side of the life raft into the sea water to provide the ground. There were other tools and extra antenna wire included in the kit. The complete kit with BC-778 was designated as SCR-578.

photo left: Close-up of the Antenna Reel, the rubber insulated antenna lead wire with antenna clip and the Ground cable with threaded "sinker" plug. This is on the BC-778

photo above
: Accessories and case for the post-war AN/CRT-3. The box kite is partially assembled to show the shape. In front of the kite is the head signal lamp. To the right of the lamp are two balloons in cans. The tall canister is the H2 generator.

AN/CRT-3 - After WWII ended, the use for the "Gibson Girl" continued on. However many times the 500kc operating frequency wasn't efficient, especially if the downed aircraft happened to be on land rather than at sea. The effectiveness of the Gibson Girl on land was about 50 miles at best. See the movie "Island in the Sky" for some fairly accurate depictions of where, (the Canadian wilderness of Northwestern Quebec) and how the Gibson Girl would have been used on land, with antenna "strung up" in a tree (later the antenna is "strung up" off of the C-47 airplane.) Also, late in the movie, one crew member cranks the Gibson Girl while the radio op sends messages with the "key button." The Gibson Girl is referred to as "the coffee grinder" throughout the movie and is featured in many scenes as is a lot of other radio and navigation gear.

The need for a higher frequency of operations evolved into the AN/CRT-3 which provided both 500kc and 8280kc or 8364kc operations. No tuning was required. The transmitter would send for about 50 seconds on 500kc and then automatically change to 8280kc and send for about 50 seconds before switching back to 500kc. All "MANUAL" sending was on 500kc only. The transmitter was designated as T-74 but the entire kit was designated as AN/CRT-3.

The "Gibson Girl" was used post-WWII up into the late-sixties when smaller solid-state emergency transmitters and transponders became more common place. The USSR also produced a "Gibson Girl" look alike in their AVRA-45 which was being used at the end of WWII.

photo left: The T-74 (AN/CRT-3) version of the Gibson Girl. Note that this version can be set to 500kc only in MANUAL or in AUTOMATIC send on 500kc and then switch to 8280kc automatically repeated every 50 seconds. This example still has the leg straps and it's also functional.


photo above: The top label on the BC-778 showing basic operating instructions. Note that the downed pilot is shown in a life raft (FIGURE 1 in the artwork depiction.)


U.S. Army Air Forces

Aircraft Radio Communications Equipment, Radio Navigation & Radio Signal Intercept Gear

Signal Corps U.S. Army  -  RCA Manufacturing Company, Inc.

BC-224 Series and BC-348 Series

Various Other Contractor Companies


If ever there was a WWII military aircraft receiver that did "so much, so well, with so little," it was the BC-348. The double preselection superheterodyne receiver provided two TRF amplifiers, three IF amplifiers, Crystal Filter, front panel adjustable BFO, AVC-MVC, direct frequency readout on an illuminated, "band-in-use" masked tuning dial and an Audio Output stage that could be selected internally for 300Z or 4000Z loads. Unlike the proceeding aircraft receivers profiled above, the BC-348 was entirely "self-contained." All that was required was to connect the receiver to the +24vdc aircraft power buss, hook up the antenna, hook up the remote standby line and plug in the 'phones. The total weight of the receiver was < 30 pounds. Well-over one hundred and fifty thousand BC-348s were built during WWII making them one of the most common WWII aircraft receivers out there. It's unfortunate that over the past seven and a half decades hams (and surplus dealers) have managed to "cut and hack" almost every surviving example making it nowadays a real rarity to find an all-original, dynamotor-operated BC-348.

BC-224-H - 1942

RCA introduced the BC-224 Aircraft Receiver in 1935. It ran on the then popular 12 volt power systems used in most aircraft. The initial version of the BC-224 had the tuning dial on the left side of the front panel. This version is usually designated as the BC-224-A and the number produced was very small which was typical for pre-WWII military contracts. As aircraft power systems evolved, 24 volts became the standard voltage and that required changes to the radio equipment that was going to be installed in the newer airplanes. RCA redesigned the BC-224 to operate on 24 volts and this receiver was designated as the BC-348. With the redesign, both types of receivers had the tuning dial relocated more towards the center of the panel. The BC-224 continued to be built for installation into earlier aircraft while the BC-348 was produced for modern aircraft installations. Both receivers were built by RCA Manufacturing Co., Inc., a division of RCA-Victor that built all of the commercial and military radio equipment for RCA. When WWII began, several other radio companies became contractors for BC-348 construction,... Belmont Radio, Wells Gardner & Co., Stromberg-Carlson, to name a few. Only one contract for BC-224 receivers was built by another company other than RCA Manufacturing Co., Inc. and the last contract for the BC-224 appears in 1942. The BC-348 was produced through WWII and total quantity produced is certainly well over 150,000 receivers (over 50,000 receivers alone were produced by Wells Gardner & Company on just the "Q" model contract.)

The BC-348 operates on 24-28vdc with the high voltage (~+220vdc) provided by an internal dynamotor. It's quite likely that well-over 150,000 BC-348 (total of all versions) were built during WWII by many different contractors building many different versions within that time period. The circuit used eight tubes with the heaters originally wired in series/parallel for 24vdc operation (each of four 6 volt tube heaters in series would operate on 24vdc.) The early circuit provided two RF amplifiers, a Mixer, a Local Oscillator, an IF amplifier stage, a combination 2nd IF amp and BFO, a combination 3rd IF amp and Detector/AVC followed by a type 41 audio output stage (this was changed to a 6K6 in some versions.) These versions will have a 991 neon lamp acting as a regulator on the local oscillator and an antenna trim control. Construction makes use of four component boards resulting in extensive use of wiring harnesses. The Crystal Filter and BFO are in fully shielded cans. This early version is usually referred to as the "Grid Cap" version since these types of tubes are used. This early version was difficult to work on and was expensive to build. To reduce costs, the Q, N and J versions were introduced. These versions eliminated most of the component boards and used "point to point" wiring to reduce costs and ease rework. This version is called the "Single-ended Tubes" version since "non-grid cap" tubes are used. The later circuit used two RF amplifiers, a converter stage, three IF amplifiers, a duplex diode/triode provided Detector, AVC and BFO functions and a 6K6 provided the audio output. The later versions eliminated the shielded Crystal Filter, Antenna Trim control and the fully shielded BFO. The audio output impedance was internally selectable at "low Z" which was around 300 Z ohms or "high Z" which was around 4000 Z ohms (on later versions.) Some BC-348s will have a decal on the front panel indicating if the "low Z" was optioned. The dual dial lamps were adjustable for brightness and were wired in series through a potentiometer and fixed resistor. Frequency coverage was from 200-500kc (not on the B or some C versions) and 1.5-18mc. The military considered the two versions of the BC-348 to be interchangeable with virtually no difference in performance and operation.

photo left
: Top of the RCA Mfg. Co. BC-224 chassis which is virtually identical to all of the "Grid-Cap Tubes" versions of the BC-348 chassis. Note that this BC-224 does have the proper DM-24 12-14vdc dynamotor installed.

A selectable crystal filter was also included in the circuit. If working correctly, the crystal filter can narrow the IF bandwidth to about 1kc wide, which counters the popular contemporary opinion that the BC-348's bandwidth is "broad as a barn." A correctly operating crystal filter can be a valuable QRM-fighting tool,...even on AM Voice. However, another option is to try and "dodge the QRM" with off-frequency tuning. Most SSB signals are about 2.5kc wide consisting of one sideband of transmitted information and no carrier. An AM signal is usually 6.0kc wide and, being double-side band, information is in each sideband along with the carrier. It's quick and easy to tune off frequency (either above or below) a few kilocycles, drop the SSB QRM off the edge of one sideband or the other and still recover enough AM audio for good copy. This is the method I use most often to dodge QRM using any type of vintage receiver, not just the BC-348. The Crystal Filter can be used for more severe QRM or when the QRM is on both sides of the desired signal. Tune the AM signal "on the nose" and the bandwidth will be significantly narrower. Also, another thing to try for severe QRM is to go to MVC and only use enough VOL (RF Gain in MVC) to allow copy.

When the receiver was installed on its FT-154 shock mount and installed in the aircraft, an eight pin Jones plug (SO-143) mated with a receptacle (PL-103) and cable that exited from the rear of the mount containing the 28vdc input, the remote stand-by relay function and an audio output line. The BC-348 was generally interconnected with the transmitter to control boxes allowing the transmitter's control relay to provide antenna switching, receiver stand-by and providing side tone monitoring which allowed for full "break-in" keying. Since there are so many variations, military collectors have generally divided the BC-348 into two groups, early types (B, C, H, K, L, O, P & R) with grid cap type tubes (usually called the "Grid Capped Tubes" version, or GCT) and the later versions (J, N & Q) with single-ended tubes (called the "Single-Ended Tubes" version, or SET.) Most of the very early versions (B and C) were rebuilt into later configurations, especially to add the 200-500kc band. These were rebuilt by Belmont Radio and re-tagged as "R" versions. Many different contractors built BC-348s but Wells Gardner & Co. probably built the greatest quantity of receivers and is the most commonly seen manufacturer.

photo above: RCA Manufacturing Co. BC-348-C  - earlier "Grid Cap Tubes" version - ca: 1941. This BC-348-C does have the original style DM-28 dynamotor installed and the receiver is operational.

photo above: Wells Gardner & Co., BC-348-Q 1943 - this example is dynamotor operated. This one WG contract alone produced over 50,000 receivers. Note that these "Single Ended Tubes" versions eliminated the "ANT. ALIGN." (antenna trimmer) control that was located above the Antenna and Ground input terminals. Many other cost reductions will be found inside the Q, N and J versions. However, the great  performance is retained and these receivers are very easy to do any rework to.

A History of Hacking - The BC-348 became available as surplus almost immediately following WWII and was easily available for a couple of decades from many surplus dealers. Early purchases offered the receiver NOS in the original box. This allowed hams the ability to purchase a great performing receiver at a reasonable price - still in the crate for about $100. Many surplus dealer offered "AC power supply conversions" for as little as $15, so the buyer didn't have to do the work. The down side is that the dealers used the cheapest parts available and their installations show that little engineering thought (neither electrical nor mechanical) was applied to their AC conversions. Today it's almost impossible to find a BC-348 that hasn't been modified.

Typically, the dynamotor and filters were removed (and discarded) and an AC power supply was built onto the then vacant dynamotor chassis. Tube heaters were rewired for parallel connections. This mod was fairly simple and usually didn't compromise the receiver's performance too much (other than adding some hum to the audio output if the power supply wasn't designed correctly or wasn't well-filtered - which most weren't.) Unfortunately, most of the dealer power supply mods used a separate AC power toggle switch that resulted in a non-original hole on the front panel. Purists can't blame the just hams for shoddy mod design and workmanship because the dealers were actually charging the purchasers of the surplus BC-348s for the opportunity to destroy the originality of thousands of receivers. Additionally, all of the dealer-installed power supplies were just Pi-network filtered and as cheaply made as possible. Dual section filtering is necessary for proper hum reduction in the BC-348. Also, not realizing that there's a B- return for audio grid biasing, many "ham-ster modifiers" grounded negative leads on filter caps causing rampant hum in the receiver audio.

Certainly the "ham-ster" destruction of many of the BC-348 receivers was popularized by CQ magazine and their "Surplus Conversion Handbook" (there were multiple editions) in which page after page of "cutting and hacking" examples were presented. Every page seemed to promote the idea that the BC-348 needed "ham-level engineering" (after all, what did professional radio engineers that worked in the major radio corporations know about ham radio?) Adding an audio output section was a common mod. After all, what ham owned, or could afford, any sort of impedance matching transformer when they could build the mod using "junk box parts." The audio output mod called for the removal of the original audio output transformer which makes finding an original audio transformer today somewhat difficult. Removal of the 8-pin Jones plug power input connector or adding a useless S-meter were also common mods.

Additionally, many hams would sometimes drill the case with lots of .25" holes thinking the receiver needed extra cooling because of the added AC power supply rectifier tube. Somewhere along the line of destruction, the receiver base mount (the FT-154) always seemed to have been removed and discarded. Since the receiver's engagement pins presented a problem for "mountless" BC-348s, sometimes hams would drill out all of the pop-rivets and remove the entire receiver base and then add rubber feet to the bottom of the cabinet. Of course, this ruined the cabinet for future use if the FT-154 ever turned up. Decades of "ham-ster mods" have pretty much made an "all original" BC-348 a true rarity. Of the well-over one hundred and fifty thousand built during WWII, few examples have made it to the present time without some type of modification. However, with diligent searching it's still possible to find unmolested examples. However, not so rare and usually not so pricy are the receivers that just had the AC power supply thoughtfully and carefully added. These examples usually aren't too difficult to restore back to proper operating condition. The Grid Cap Tube versions have many Micamold-brand capacitors that will require replacement but the Single-ended Tube versions eliminated these troublesome capacitors and many of the SET versions can operate quite well on original components. Both types of BC-348s are great performing receivers if restored to original or when conservatively modified (a properly designed AC PS only.) An thorough and accurate IF-RF alignment and repair of the crystal filter are absolute necessities for top performance (and must be done before any critical comments regarding IF bandwidth are even considered.)

If you're interested in restoring your BC-348 or BC-224 receiver back to its original configuration with the installation of the correct DM-28 or DM-24 dynamotor, go to our web article "Rebuilding the BC-348 Family of Receivers" This article tells you about all of the "pitfalls" that might await you in this seemingly easy project. Also includes information on other aspects of the restoration of these fine performing receivers, such as building (or re-building) a minimally invasive AC power supply that operates the receiver correctly. Although the proceeding write-up seems to imply that original dynamotor operation for the BC-348 results in best performance, the receiver will also operate extremely well on a correctly designed and properly installed AC power supply.  

photo above: Inside the BC-348 "Single Ended Tubes" version. Note that the shielded cans in the IF section has been reduced from six to only three cans. Many other cost-reduction differences can be found when comparing the construction of the SET version to the GCT versions of the BC-348 receivers. Dynamotor is the DM-28 24-28vdc operation.


Signal Corps - U.S. Army Air Forces - General Electric

BC-375-E - Aircraft Transmitter

Though the BC-375 was initially designed in the early thirties (AA-191 transmitter) and utilized parts and technology from a decade earlier, it found a long-term usage with the Signal Corps due to its ease of operation and reliability. Its earlier kin, the BC-191, was the first version built from the mid-thirties on up to mid-WWII. Around the beginning of WWII, an airborne +24vdc version was necessary and the BC-375 became the designation for a slightly different transmitter for use in larger aircraft. GE got a manufacturing contract for close to 100,000 BC-375 that were built through the first half of WWII. Commonly used on B-17s, the SCR-287 was found on thousands and thousands of those bombers. Towards the middle of WWII, the ARC series of transmitters-receivers were introduced, along with the Collins ART-13A, to replace the BC-375. At the end of the war, thousands of BC-375-E transmitters remained unopened in their original crates (ready to flood the post-war surplus market.)

The BC-375 uses four VT-4-C triode tubes (type 211E) and a single VT-25 (10Y) triode with one VT-4 used as the Master Oscillator, another VT-4 as the Power Amplifier and two VT-4 tubes for the P/P Modulator. The VT-25 serves as the speech amplifier in the Voice mode (AM,) as a 1000hz oscillator in the Tone mode (MCW) and as a sidetone oscillator in the CW mode. Power is provided by the aircraft battery/charger system (+24 to +28vdc) and by a high voltage (+1000vdc) dynamotor (PE-73.) The BC-375 provides full break-in keying by allowing the elaborate internal antenna relay to control the receiver antenna and the receiver standby circuit. Additionally, external inputs via the PL-64 cable allow remote microphone and key operation along with remote power control. The aircraft was usually set-up to allow the pilot to also access the transmitter/receiver for various reasons. 

BC-375-E Transmitter with the BC-306-A LF Antenna Tuner

SCR-287 display at WHRM in 2009

After WWII, the BC-375 was available surplus, initially for about $125 (in the crate price,) but soon prices plummeted down to as little as $15. The various TUs were also available at "give-away" prices. This lead to many hams buying  the BC-375 for an economical way to get "on the air." Unfortunately, most hams tried to run the transmitter at full power on automotive batteries or tried to rebuild the transmitter into something that it was NEVER intended to be - a ham transmitter. The end result was a bad reputation that the BC-375 was unstable, sounded awful, created horrible TVI and was only useful as a resource for parts to build other ham projects. Though TVI was a major issue in the fifties, today's HD digital, uW signals routed though strong TV or satellite dish systems are not affected by the BC-375 operation. The transmitter can produce excellent "military" audio if it is carefully operated after a thorough checkout that includes a dynamic adjustment of the neutralization, using a good quality carbon microphone and, probably the most important,...not running the PE-73 dynamotor on batteries but running it from a high current, +28.5vdc power supply, e.g., the PP-1104.

The SCR-287 comprised a complete liaison radio station installed onboard various bombers and transports during WWII. The transmitter used was the BC-375. The other components shown in the photo to the left are the BC-348-Q receiver which does run on its original dynamotor from the battery supply, the Lionel J-47 telegraph key and the Shure Bros. T-17 carbon microphone. The speaker is an LS-3, although these were never used in the SCR-287 or onboard the aircraft. Four BC-375 Tuning Units are mounted in their CS-48 containers on the wall. The olive-drab console is not a WWII vintage item - it's homebrew. I designed and built the desk console as an easy way to display the BC-375/BC-348 and to have all interconnections neat and to have the equipment easily accessible. The console wasn't equipped with casters in Virginia City but I did install them with the move to Dayton. The panel to the left of the BC-348 has all of the remote connections for receiver audio output, receiver stand-by, xmtr CW sidetone select, xmtr microphone input and xmtr key input. The console features a fold-down desk, a sound-proof (almost) compartment for the PE-73 dynamotor and a bottom shelf for the four storage batteries.

IMPORTANT OPERATIONAL NOTE ABOUT USING STORAGE BATTERIES: The storage batteries NEVER worked correctly and were the major source of problems when operating the BC-375. The PP-1104-C military power supply/battery charger solves almost all of the operational issues with the transmitter. A high-current power supply and dynamic neutralization will cure 95% of the BC-375 operational problems. Batteries alone will never supply the proper input voltage of +28.5vdc to allow the dynamotor to spin fast enough to produce the +1000vdc B+ and to have the dynamotor armature rotating with enough inertia to stabilize that voltage under load. Less than +26.5vdc input will have the dynamotor output at about +900vdc and the slower rotation of the armature results in that voltage varying under load which results in FMing and non-symmetrical modulation on VOICE and "blooping" on CW. Trying to run the BC-375 only using batteries is a waste of time. If you must use batteries you can compensate for the low voltage instability by reducing transmitter power to about 40 watts, reducing the length of your transmissions and re-charging the batteries when in the "receive mode." This can produce an acceptable, albeit low power, signal. The PP-1104-C allows +28.5vdc at 50 amps input with no variation under load. The BC-375 will easily put out 75 watts 100% modulated and sound good doing it. Use a good quality, new carbon mike element for best audio results. 

For the ultimate information source on how to successfully operate the BC-375 "on the air" without enduring endless criticism of your signal's lack of audio quality, go to our web-article "Successfully Operating the BC-375 on the Ham Bands Today" - Use Home/Index below for navigation.

The End of Operational BC-375 Transmitters?

Shown to the right is the interior of the BC-375 showing the four VT-4/211E tubes along with the 10Y/VT-25 tube to the far left. The demand by audiophiles (actually by "vacuum tube investors") for the 211E has had a catastrophic effect on many future restorations or rebuild attempts of the BC-375 transmitters. It's not at all uncommon for a single 211E in good usable condition to sell for $250 (that's Fair Radio's 2021 price) and, if the tube is NOS in the original box, it would sell for double that on eBay. Power triodes that interest tube investors always sell for more in a set, so a good condition "quad" of 211E (necessary for a "tubeless" BC-375) could easily sell for $1200 to $2000. Luckily, these insane prices are only found on eBay and only when dealing with tube investors from Asia (or USA sellers that ONLY want to sell to tube dealer/investors in Asia.) Between military radio enthusiasts a more common price is about $125 a piece for a good usable 211E but that's still $500 for a quad if you have a "tubeless" BC-375 (check Aug 2021 update below.) The 10Y is still not expensive (except the Western Electric VT-25 version.)

There is a fairly common VT-4 substitute tube, the 805. However, although the 805 is an identical tube to the 211E, the external structure is different in that the 805 employs a plate cap where the 211E uses a base pin connection. There's ample room for plate leads without drilling holes so the incorporation of 805s into a BC-375 can be accomplished fairly easily. The only problem is that the tube dealers have now discovered the 805 also and the price of that tube has started to climb. 805 tubes are still cheaper than the 211E,...but for how long?

The high prices of power triodes like the 211E or the 805 have certainly halted or, at least, slowed down most BC-375 rebuilds. Today, the "standard condition" is to find most BC-375 are for sale "without tubes." Unfortunately, the high cost to "retube" a BC-375 has relegated "tubeless" transmitters to a "parts set" status. And, if a complete "tubed" BC-375 is for sale, the "tube investor" price will certainly be factored in.

The military radio enthusiast should take a look at the easily available ART-13 transmitter. These transmitters use tubes that are of no interest to vacuum tube investors. ART-13s are easy to rebuild, easy to get going and they produce a 100 watt carrier, 100% modulated with excellent audio.

UPDATE: AUG 30,2021 - Chi-Comm VT-4C/211E "new tubes" - Fellow BC-375/BC-191 enthusiast Charlie W4MEC mentioned in an e-mail that he had rebuilt a BC-375 for installation into a B-17 restoration that he equipped with Chi-Comm VT-4C/211E tubes and that these tubes were a fraction of the price of USA-made vintage VT-4C/211E tubes. The price was about $70 each for new tubes. Charlie indicated that these replica tubes functioned fine in the transmitters. In trying to find out more information as to the current availability of these new tubes, he checked the website  but it seemed the VT-4C/211E replicas are on "back order." Charlie related further that he had a friend that had a business that relied on his products being shipped from China. The friend was having problems getting his product because there is a shipping container shortage in China. The shipping container shortage has resulted in "who ever has the most money gets the container" sort of cut-throat hierarchy in the use of the available containers. The result is, if there is someone willing to pay more to rent the container, the items inside that container are unloaded and stored in a warehouse while the new renter loads and ships his product. It seems there are many items that were scheduled to be shipped but remain in warehouses in China. The order status of a set of replica VT-4C/211E tubes right now is "on hold" until they are back in stock and that could be a long wait. Check the link provided above regularly if you're interested in bargain VT-4 tubes.  


The "SAAMA Fugitive" ART-13A with a Belmont BC-348-R

ARC-8 Airborne Radio Gear
ART-13A Aircraft Transmitter, BC-348-R Aircraft Receiver

When the ART-13A transmitter and the BC-348 receiver were installed in an aircraft, the pair was generally designated as the ARC-8 station. However, to be absolutely correct for the ARC-8 designation, the ART-13 and the BC-348 should be operating in an aircraft from the +28vdc battery-generator buss and each should be powered by their respective dynamotors. That's not the case with the station shown to the left since it operates from a house and is uses AC power supplies that operate from the 120vac line. The ART-13A is a "SAAMA fugitive" that ended up at that facility, the San Antonio Air Materiel Area, in the 1950s. SAAMA was part of Kelly AFB and was given the SAAMA designation in 1946. SAAMA could and did rework almost any type of Air Force equipment. Unfortunately, this particular ART-13A seemed to be a problem for their technicians. Some how it made to the surplus market in "non-working" condition where it was sold. Some time after the sale it was disassembled. Then some parts were "cut out" and other parts lost. I obtained this ART-13A as a "basket-case" about fifteen years ago - actually three large boxes of parts along with several pieces of sheet metal. I thought it was just a parts set for quite a while. Eventually, I inventoried the parts and was amazed that I had almost everything to "build" an ART-13A. Lost forever were the three wire wound resistors located in the PA/MOD compartment but Fair Radio Sales supplied original replacements. The metal pocket for the cal book needed to be replicated. The audio module had been "gutted" so a replacement was purchased on eBay. The Plate/Grid/Batt meter needed to be replaced (from Fair Radio.) That pretty much was enough to get the SAAMA fugitive working up to a point. Apparently, at SAAMA, the meter function switch was replaced. Somehow, the SAAMA techs had mis-wired the switch causing the grid drive not to work (two wire positions swapped.) A few other problems were repaired and this ART-13A was pretty much resurrected from the junk pile. Over the years I've replaced the non-matching meters with a "matched set" of meters, added the O-17 LF Oscillator, added the calibration book and many other minor items that have added to the overall completeness of the transmitter.

The Belmont BC-348-R was obtained from a local ham. It was in excellent cosmetic condition and was fairly original only having had an AC supply added to replace the DM-28 dynamotor at sometime in the past. That PS was poorly designed and had excessive hum due to having the pi-filter input capacitor negative terminal connected to chassis. I built a completely new dual section filtered power supply that fit exactly into the dynamotor position and utilized the still present "dynamotor harness." The new PS used two chokes, three filter caps, solid state rectification and utilized the AVC-OFF-MVC switch to turn the receiver on and off. I also used the PL-103 rear connector so that the FT-154 shock mount could be used correctly. A full IF/RF alignment was performed. Also, the Micamold brand capacitors were replaced (notorious for shorting,...and two were.) It's a nice example of a minimally modified (AC PS only) "grid cap-type" BC-348-R.

There's no doubt that today the ART-13 is by far the most popular transmitter used by military radio enthusiast/hams in their vintage military radio stations. The ART-13 is easy to rebuild, easy to power up, it provides over 100 watts of carrier power in the AM mode and does this while maintaining a truly excellent stock audio response. The ART-13 started out as the Navy ATC and soon became the T-47/ART-13. It wasn't long before the USAAF wanted a version specifically built for their requirements. This slightly different transmitter was the T-47/ART-13A. The following write-up profiles both types of transmitters,...

Navy Dept. - Collins Radio Company  -  ATC, T-47/ART-13 (Zenith Radio Corp. was also a contractor)

Signal Corps USAAF - Various Contractors - T-47/ART-13A,
T-412/ART-13B, AN/ART-13 & AN/ART-13A

The Collins T-47/ART-13 is a 100 watt carrier output, AM-CW-MCW transmitter that was initially used in USN aircraft and later used in USAAF/USAF aircraft. The ART-13 could also be found onboard USN ships as the TCZ installation. There were even some vehicular uses for the T-47/ART-13. The T-47/ART-13 was developed from the earlier Collins ATC Navy Aircraft Transmitter that appeared around 1941. By 1943, Collins was building the T-47/ART-13 for installation in Navy aircraft and other applications. The USAAF also wanted to use the T-47/ART-13 and a very slightly different transmitter was produced for their use, designated as the T-47A/ART-13. Many of the T-47A/ART-13 transmitters were built by contractor Stewart-Warner and also some were built by Zenith Radio Corporation (Zenith also built some of the ATC transmitters.) Documentation was changed near the end of WWII to designate the transmitters as AN/ART-13 with the A suffix included. When combined with other equipment the set-up designation is for the entire installation. Hence, the combination ARC-8 describes the ART-13 combined with a BC-348 receiver along with the other necessary auxiliary equipment for a complete aircraft radio installation. The ART-13B was a modified ATC or ART-13 that installed a plug-in COMCO crystal oscillator that provided 20 HF and 4 LF channels. The mod also incorporated a frequency extension that allowed using the transmitter down to 1.5mc.

The T-47/ART-13 power requirements were supplied by a dynamotor that ran on the aircraft +28vdc battery/charger system. The aircraft battery buss supplied the +28vdc@10Amps necessary for the transmitter's tube filaments and relay operation while the dynamotor provided a dual output of  +400vdc and +750vdc. The dynamotor would have the two B+ levels connected in series for the HV Plate ( +1150vdc) below 20,000 to 25,000 feet altitude but a barometric pressure switch (located inside the dynamotor housing) would separate the outputs at higher altitudes and only allow +750vdc maximum to prevent arc-over. There were at least three types of dynamotors used, the DY-17, the DY-11 and the DY-12 (after WWII an improved DY-17A was produced.) The shipboard TCZ featured two types of power supplies, a 115vac operated power supply (of enormous proportions) that supplied the required +28vdc, +400vdc and +1150vdc directly to the transmitter. Additionally, the 115vac unit had a motor-generator that provided +14vdc and +28vdc (the +14vdc was required for relay operation inside the ac or dc operated TCZ power supply.) The 115vdc operated TCZ power supply used two dynamotors that ran on 115vdc input and provided +14vdc and  +28vdc output on one dynamotor and +400vdc and +1150vdc on the second dynamotor. The USMC had a vehicular set-up that installed an ART-13 transmitter with a BC-348 receiver that operated from the back of a Jeep and ran on the +28vdc battery system with HV provided by a DY-12 dynamotor. The antenna used was a whip.

photo above:  ART-13A "SAAMA Fugitive" with the CU-32 LF Loading Coil on top. I used this set up on 630M CW working into a 163 ft end-fed wire. This ART-13A is powered by the "DynaSim" AC power supply, a homebrew SS AC supply that uses a +440vdc and a +700vdc supplies in series to create the +LV (+440vdc) and the +HV (+1140vdc.) I made 2X CW QSOs on 630M as far away as Pennsylvania NO3M and South Dakota WØSD.

photo above: Not an Airborne Station - Navy-Collins T-47/ART-13 on top of a vacuum tube-based homebrew AC power supply in an amateur vintage military radio station. Note that this transmitter has the earlier style shock mount rails. Also the LFO is the Navy type O-16. The receiver is a Navy-Wells Gardner & Co. RAO-3. The Mill is WWII Underwood S I I. Although most of this equipment is from the same time period in WWII, it would not have been used together at that time. The RAO-3 was a surveillance receiver used with the S I I Mill at USN shore stations. This Navy ART-13 is the first T-47 transmitter that I rebuilt and got operating. The vacuum-tube AC PS is the first one that I built. This photo is from late-2020 before I had finished the ATC transmitter restoration. The last photo at the bottom of this ART-13 write-up shows the finished ATC set-up in this position now.

The T-47/ART-13 featured an advanced Autotune system that would automatically tune up to 10 preset channels selectable by a front panel switch. The Autotune system would tune the transmitter frequency and output network to mechanical presets that then would match a properly selected antenna. The Autotune cycle took about 25 seconds to complete. Switch position MANUAL would allow manual adjustment of the tuning without disturbing the Autotune presets.

The T-47/ART-13 uses an 837 as the variable frequency oscillator, two 1625 tubes are used as multipliers, an 813 as the power amplifier and two 811 tubes as the P-P modulators. There are also two small modules. One provides the audio amplifier and sidetone amplifier using two 6V6 tubes and a 12SJ7 tube and the other module, the MCW/Frequency Calibration Indicator, uses two 12SL7 tubes and a 12SA7 tube. FCI allows the operator to calibrate the frequency of the transmitter by providing a 50kc calibration signal derived from a 200kc crystal oscillator.

The transmitter frequency range is from 2.0mc to 18.0mc, however many Navy T-47/ART-13 transmitters were equipped with a plug-in Low Frequency Oscillator (LFO) module that allows the transmitter to operate from 200kc to 600kc or 200kc to 1500kc. Early LFOs (O-16) have a frequency range of 200kc to 1500kc in six ranges while the later LFOs (O-17) cover 200kc to 600kc in three ranges. The LFO module uses a single 1625 tube. There are some indications that the Navy preferred the 200kc to 1500kc LFO while the USAAF used the 200kc to 600kc LFO. Many versions of the T-47/ART-13 will have a blank plate installed where the LFO module was installed (along with a resistive load substitute for the LFO's 1625 filament.) After WWII, the USAAF/USAF didn't use the LFO module but the USN still did. This statement is according to the USAF Extension Course 3012 book on "Radio Mechanics" although this book is from the 1950s and may reflect the uses of the LFO at that time rather than during WWII.

LF operation does require an external tuner called a "loading coil." The Navy used the CU-25 or CU-26 while the USAAF used the CU-32. Also most installations on aircraft included a small Remote Control Panel that allowed the pilot to operate the transmitter from the cockpit. There are a couple of different remotes that could be used with the ART-13. Many transmitter installations also used three selectable condensers to allow easier loading into various antenna impedances at lower frequencies.

To the right is a photo showing the chassis of the Collins-built T-47/ART-13. This transmitter has the Navy version LFO installed. Also, this is a fairly early version of the transmitter so there are some differences when compared to the T-47A/ART-13 versions. Of note is the lack of an interlock switch which on the early versions allows you to easily operate the transmitter with the lid off. The module to the lower right is the Audio Amplifier unit and directly behind it is the 837 VFO tube. Behind the VFO tube is the FCI/MCW module (later three tube version) and to the left of it are the two 1625 multiplier tubes.The module in the center of the transmitter is the LFO. In the section at the rear of the transmitter, to the left side is the modulation transformer from which its plate leads connect to the two 811 modulator tubes. To the right of the 811s is the 813 PA tube. The left-center section of the transmitter contains the matching network and the LF relay (next to the LFO module.) On the far left is the vacuum TR switch and behind it is the keying relay. The round ceramic unit in front of the vacuum TR switch is the inductive pickup for the Antenna Current meter.

The somewhat later USAAF T-47A/ART-13 (ART-13A) version added some minor improvements to the transmitter with a vernier scale on the VFO Fine Tuning, a top lid interlock switch, a different bottom plate with built-in guides for the shock mount and a white ceramic insulator bell on the antenna connection being among the most apparent changes. There was also a T-412/ART-13B that added a COMCO selectable crystal oscillator in place of the LF module. The COMCO crystal oscillator normally has 4 LF/MF channels and 20 HF channels. All ART-13Bs are retrofitted earlier models and it is possible to find even early ATC transmitters that were converted to the ART-13B version.

The T-47/ART-13 and its variations had a very long life. Introduced around 1943-44, actively used during and after WWII and well into the fifties (sometimes found still being used well into the sixties and early seventies.) The USSR also produced a copy of the ART-13 that they used up well into the 1980s (the R-807.) Because of its long useful life, most T-47/ART-13 transmitters found today will have had many scratches and a few dents and paint scrapes. Non-matching modules are common and will be encountered with some parts having MFP applied and others that are bare. A book containing brief instructions and the calibration settings for specific frequencies was usually stored in the metal pocket underneath the transmitter. This book is also usually missing on most transmitters although the same information is in the standard manuals. Luckily, many tens of thousands of T-47/ART-13 were built and spare parts are very easy to find which allows for the fairly easy restoration and maintenance of these durable and potent transmitters.

photo above: Chassis of the Navy-Collins T-47/ART-13. The MCW/FCI module is the later, three tube version. The Collins chassis are usually painted gray and have a "winged emblem" embossed near the 837 VFO tube. When I first got this Navy T-47 it was partially disassembled and there were several boxes that contained many of the small parts. Everything went back together since it was just a reassembly job. Once completed, I built the vacuum-tube AC power supply which then allowed operating the transmitter. It has always operated reliably and is an example of what to expect from ART-13s that are in reasonably good condition.

Nowadays, the T-47/ART-13 has become one of the most popular aircraft transmitters being used in amateur vintage military stations. It can provide plenty of power with excellent audio. It's relatively easy to restore since there are many, many parts sets out there. Though the ART-13 can run on +28vdc and its original dynamotor, most users opt for a homebrew AC power supply. The +HV can be safely increased to around +1400vdc to provide even more output power and some brave users will run the HV up as high as +2000vdc (not for the timid and distortion might be encountered at this level of HV.)

After years of operating, repairing and restoring ART-13 transmitters, it's my opinion that the transmitter operates very well and quite reliably with an AC operated power supply that provides +27.5 to +28.0vdc at 15 amps minimum available current, +420vdc to +440vdc for the +LV and +1100vdc to +1200vdc for the +HV. The transmitter will easily and reliably produce 110 to 120 watts carrier output power on 75 meters - 75 meters is where most of the vintage military radio nets are located.

The Audio Module has a "fixed-level" gain setting that was designed to work with specific WWII vintage military microphones. The carbon mike bias resistor R203 was a 15K value that provided sufficient bias voltage for most "then new" carbon mikes. After WWII, many ART-13 audio modules were modified by changing the carbon mike bias resistor R203 from 15K down to 4.7K. In fact, the ART-13B schematic shows the R203 value as 4.7K. Audio modules with 4.7K bias resistors will have no problem providing plenty of carbon mike response. Check the value of R203 if you're having carbon mike problems. An easy way to achieve proper modulation levels is to use an Astatic TUG-8 stand with a D-104 or 10-D microphone "head" (with "DYNAMIC" selected on the Audio Module.) These mike stands have a built-in, adjustable gain amplifier that provides ample audio output to drive the fairly low input Z of the T-47/ART-13 (~ 250Z ohms.) An oscilloscope should be used to monitor the transmitter output when trying out different mikes as it will be very apparent on the 'scope whether proper modulation is being achieved. Due to the unbalanced, low reactance typical ham antennas used on 80 meters, the T-47/ART-13 will require an auxiliary capacitor connected to the COND terminal to ground for proper loading of these kinds of antennae. These external capacitors should be high voltage rated ceramic types. A fixed value 75pf to 150pf capacitor will work fine on 80M. 40M depends on the antenna used - a 40M dipole won't require an auxiliary capacitor if the C control is at 7 or higher for proper loading and antenna tuning (7 and above becomes a Pi-network.)

photo left
: Close to an Airborne set-up, sort of - this is the "Wasp's Nest ART-13A" - a Stewart Warner contract ART-13A SN: 2054 shown with a Collins R-648/ARR-41 Aircraft Receiver (1957 vintage.) Both the transmitter and the receiver run on their respective dynamotors and are powered by a PP-1104C +28vdc 50A power supply (actually a depot battery charger but it works great for high current voltage requirements such as running transmitter dynamotors.) Station location is out in the shop. This ART-13A was infested with "mud dabber" wasp's nests when found. Restoration (in 2016) of this ART-13A is detailed in Part 3 of the "Rebuilding the ART-13" article. Use Home-Index link below for navigation.

photo above:  USAAF T-47A/ART-13, aka AN/ART-13A - this is the "SAAMA Fugitive" back when I had just finished its rebuild, probably around 2011. Note the blank panel installed to replace the LFO. Also note that the two meters don't match. It's very common to find mismatched meters ART-13 transmitters and was probably a result of depot repair work, in this case from SAAMA (San Antonio Air Materiel Area, those guys worked on a lot of ART-13s.)

photo above: Chassis of the "SAAMA Fugitive." Note the "gold" appearance from the MFP coating (SAAMA laid it on thick.)

In case you haven't figured it out already, I have four working ART-13 transmitters that I use:
1. Navy-Collins - T-47/ART-13
2. SAAMA Fugitive - ART-13A
3. Wasp's Nest - ART-13A
4. Pinky -ATC/ART-13

"Pinky" - The photo to the right shows the latest (Feb. 2021) ART-13 restoration, actually a Navy-Collins ATC version that was MWO'd into the ART-13 configuration by SAAMA post-WWII. Six MWOs were installed but enough of the original ATC parts were still present to identify the transmitter's origins.

This Collins ATC/ART-13 was found in a rented storage unit in Carson City, NV. The ATC was at the very bottom of a six foot tall "stacking" of electronics gear. It also had several stripes of "hot pink" Latex paint applied on the top and front of the transmitter. The left side was "crunched" and the left grab handle broken with the entire central section missing. The right side also had a smaller "crunch" that actually was much more difficult to correct. Inside, the EMISSION switch was broken and needed to be replaced. The vacuum antenna switch was broken (shattered, actually) but, luckily, I had a NOS Sperti replacement to install. The 813 was missing and one 1625 tested bad. One glass tube in the CFI module was broken so it was replaced with a metal version. RF Plate meter flange was broken and a good "parts set" replacement meter was installed.

Lots of sheet metal work to straighten all of the panels. The lid was a disaster (after all, six feet of gear was piled on top of this poor transmitter.) Someone had cut a notch cut of the lip just to the left of the Antenna Current meter. The pink paint was everywhere but could be easily removed with Isopropyl Alcohol (worked because the paint was Latex.) Lots of "touch-up" with Mars Black acrylic paint.

Restoration is detailed in Part 4 of "Rebuilding the ART-13" write-up - use Home/Index below for navigation.


For copious amounts of detail and information on the Restoration and Operation of ART-13 Transmitters, including four schematics of different approaches to building an AC operated power supply, go to the web-article "ART-13 Transmitter - Restoration to Complete and Operational Condition"  - there are four parts to this write-up - go to Home/Index for navigation.


Setchell Carlson, Inc.  -  Model 524 Beacon Receiver

U.S. Army Signal Corps Designation:  BC-1206-CM

The Model 524 Beacon Receiver is a small size, light weight aircraft receiver that covers 195kc up to 420kc. The circuit is a five tube superheterodyne utilizing loctal type tubes. The receiver is entirely powered by the +28vdc aircraft battery-charger buss. No higher voltages are required to operate the 524. The "PHONES" output is 300 ohms Z although internally the output Z can be switched to 4000 ohms Z, if desired. The IF is 135kc. The tubes used are RF Amp 14H7, Mixer 14J7, IF Amp 14H7, Det-1st AF 14R7 and AF Output 28D7.

The 524 had a rather interesting use during WWII. These small receivers were installed into the instrument panels of airplanes that were going to be flown to specific destinations by WASPs (Women's Airforce Service Pilots.) This would generally be smaller fighter types of aircraft but did include larger aircraft as well. Some aircraft manufacturers had access to adjacent runways or the airplanes could be ferried from the manufacturer to either an airport or an export facility. Since the destination was known, the 524 provided a way to fly to a specific airport via the system of Airways and using Radio Range Beacons that provided continuous navigation signals. The pilot would use a homing loop to find the direction of the beam and then keep the airplane "on the beam" by course corrections during the flight. At that time, Airway Radio Range beacon stations also had other transmitters operating on an adjacent frequency that provided weather reports and other information necessary for piloting aircraft. Once the airplane was delivered, the 524 was usually removed from the instrument panel and returned to the aircraft factory where it was eventually recycled into another aircraft destined for delivery.

The 524 is very sensitive with a specification of 3uv for 10mw output. The receiver shown in the photo to the left does function quite well and receives many NDBs and other signals in the 195kc to 420kc range. However, the lack of a BFO does limit the reception to only fairly strong NDB signals. Since the original application was to receive Radio Range Beacons that were MCW signals, a BFO wasn't necessary. It's very small weighing only about 4 lbs and measuring 4" x 4" x 6.625". The four holes in the front panel surrounding the dial plate are tapped and are provided to allow mounting the 524 into a standard instrument panel opening (3.125".) Other manufacturers manufactured similar Nav-receivers with the same dimensions for the same purpose during WWII. 



R-45/ARR-7  SN: 732   ORDER NO: 631-DAY-44  Contractor: The Hallicrafters Co.

MFP Coated: 1944     Reworked: 20 OCT 1950 at SAAMA

San Antonio Air Materiel Area - Kelly AFB, Texas

Signal Corps USAAF - The Hallicrafters Co. & Belmont Radio Corp.
Radio Receiver R-45/ARR-7   SN: 732

The R-45/ARR-7 was an airborne countermeasures search and surveillance MF and HF (.55 to 43mc) receiver that was primarily used for visual analysis of enemy radar signals and the visual analysis of other types of enemy signals. The Panadaptor and Video Outputs were designed to feed into specific airborne versions of typical panoramic adapters and oscilloscopes. The oscilloscopes normally utilized external oscillators to create lissajous patterns for audio analysis of incoming signals (Video output is from the 6V6 audio stage of the receiver.) The panoramic adapters monitored the output of the Mixer stage of the receiver and provided a visual representation of the spectrum surrounding the receiver's tuned RF frequency (but down-converted to the IF.) This allowed the operator to "see" signals that were outside the receiver's IF passband and couldn't be heard - but they could be seen on the panadapter, allowing the operator to tune to the signal for investigation and to analyze its RF characteristics. The motor-drive tuning could be set up to scan just small segments of a particular band or span wide portions of selected tuning ranges. The motor-drive tuning would automatically reverse at each adjustable end stop so when "programmed" the receiver would automatically keep scanning the same selected frequency segment until the motor was switched off. The motor was powered by the aircraft battery-charger buss but the tube heaters and B+ are supplied by the PP-32 power pack that ran off of the aircraft's 115vac 400 cycle power. Besides Hallicrafters, Belmont Radio also built R-45 receivers on a 1945 contract. Interestingly, the Belmont built R-45 receiver S-meters are marked "the hallicrafters, inc."

Airborne SX-28A? - The R-45's circuit has a few vague similarities to the SX-28A, although considerably "stripped down" to the essentials and lightened for aircraft use. 12 tubes are used (not including the rectifier that is located in the PP-32 power pack.) Some of the similarities to the SX-28A are the use of the same Micro-set coils in the front end, double pre-selection above 3.0mc and the six selectivity steps with three utilizing the crystal filter. The differences from the SX-28A are a Noise Limiter that is just a clipper circuit (not a Lamb Noise Silencer,) use of a Re-radiation tube circuit, use of a VR tube, no bandspread, no antenna trimmer and the "militarily basic" audio output system which is just a capacitive coupling from the 6V6 plate to drive the headphones. The audio output wasn't intended for "pleasure listening" but was provided to drive a set of 'phones that were only necessary to aurally analyze signals in complement to the visual analysis of received enemy signals. Audio output was spec'd at 600Z ohms impedance.

The Re-radiation Tube - Double preselection receivers with additional shielding usually provided enough isolation of the LO to prevent excessive leakage to the antenna (the Navy spec was <400pW of leakage on the antenna terminal.) Apparently, even more isolation was needed for the particular function of the R-45 as a countermeasures receiver and the necessity of the receivers to mutually not interfere with the other measuring equipment. The R-45 has a "Re-radiation tube" added into the antenna input to block the LO radiation from leaking back into the antenna. The Re-radiation tube is a 6AB7 tube that has the antenna input capacitively-coupled to the grid of the 6AB7. The plate of 6AB7 has B+ supplied through the primary windings of the antenna coils. Essentially, the Re-radiation tube is a buffer stage with unity gain that's inserted between the antenna and the RF amplifier to reduce LO leakage onto the antenna input.

Signal Losses? - Like other Hallicrafters receivers, the SX-28A for instance, the two lowest frequency bands only use one RF amplifier and the Re-radiation tube is coupled to that single stage RF on those bands. The four higher frequency bands use both RF amplifier stages with the Re-radiation tube. Some users feel that the Re-radiation tube actually causes signal loss and results in reduced sensitivity (it should be noted that Band 1 will be somewhat "desensitized" by the Q-spoiler resistor R59.) It's normal for the stock R-45's S-meter to never exceed S-9, even with a RF signal generator connected directly to the antenna input "pumping in" a 1.0 vrms sine wave. However, when the receiver is in good operating condition with a full IF/RF alignment having been performed and connected to a resonant antenna, signal reception seems very good, easily able to receive ample 20M DX or pick-up Asian coastal marker beacons in the 25M region of the spectrum. Surprisingly, the manual vaguely spec'd the sensitivity at something less than 10uv.

Review of Modifications That ARE NOT Recommended - If the Re-radiation (buffer) tube is thought to be limiting signals, a simple experiment might be to unplug the Re-radiation tube and install a simple capacitive coupling between the grid and plate tube socket pins (making a plug-in adapter would be easy) to see if there's any real difference. Unfortunately, there are some circuit conflicts with this simple approach. Better results would require disconnecting one end of R2 to isolate the B+ from the primary coil windings, disconnect one end of R53 to isolate the AVC from the grid of the Re-radiation tube socket (which is the antenna input circuit) and install a jumper across C25 to connect the primary coil return to chassis. Then install a wire jumper from grid to plate on the Re-radiation tube socket. Coupling from the antenna input will have a 100K shunt to chassis and will be coupled to the primary coil windings through an 85pf capacitor. That would result in a fairly standard antenna input circuit to the antenna coils and then to the RF Amplifier stage. The downside is that without the Re-radiation tube or an antenna trimmer, the antenna impedance will have to be closely matched and will vary with the tuned received frequency.

It's unlikely that the reconfiguration of the antenna input with the elimination of the Re-radiation tube will do too much for the S-meter operation. The S-meter is in the IF amplifier plate line and is heavily shunted with a 100 ohm resistor (and a series 500 ohm adjustment pot.) It doesn't respond to strong signals with any enthusiasm by design. In the original airborne countermeasures installations, the S-meter really had very little useful function since a panoramic adapter was connected and could provide much more useful information than the S-meter. The heavy shunt was probably to reduce the possibility of "pegging" the S-meter during the testing and measuring functions during in-flight operations. If the ultra-conservative S-meter is a concern, it's possible to slightly increase the value of the 100 ohm meter shunt resistor to allow a little more current to flow through the meter coil. But, one should remember why the S-meter is set-up the way it is (besides, an S-meter is not an accurate measurement instrument but was only provided for tuning AM signals and a way to compare relative signal strengths - and then that's only in the AM-AVC mode - the S-meter is actually "turned off" when the BFO is turned on.)

photo above: R-45/ARR-7 SN: 732 (non-operating pose)

photo above: W7MS' R-45 is setup with the proper APA-10 Panadaptor.    photo by W7MS

Any of these mods, of course, aren't necessary and aren't recommended. They're only presented here for information as to why the modifications really aren't needed and aren't desirable. As always, better performance improvement can be gained by thoroughly checking out the receiver, by using tubes that test "as new" in a quality tube tester, by going through the entire circuitry checking for defective components (be sure to check all ERIE Corp. resistors for drift - they are notorious for drifting "non-functionally" out-of-spec after 75+ years of existence, luckily there are only a few in the R-45,) by troubleshooting to repair any non-functional circuits and then the real necessity is performing a complete IF/RF alignment. Most important to good sensitivity and low noise reception is to use a resonant or matched antenna (remember, there's no antenna trimmer.) A correctly operating stock receiver with a full IF/RF alignment will perform quite well, receive ample DX and allow Q-5 copy on the ham bands when used with a resonant antenna. It's very likely that the damped S-meter gives the operator the impression that the receiver isn't performing as it should. But, ignore the S-meter and you'll find that the receiver can provide solid copy even on very weak signals. Remember, the R-45 was intended to drive 600Z ohm 'phones, not a loudspeaker. You'll hear everything when using 'phones.

R-45/ARR-7 SN: 732 was thoroughly serviced and a full alignment performed but still S-9 was about the maximum that the S-meter would show and that was with a signal generator input. Typical "over-the-air" signals rarely exceed S-6 (Radio Havana on the 49M band at night - with the dominate signal on the band - will consistently "almost" reach S-6.) The receiver is very sensitive with the ability to perform very well using a "tuned-dipole wire antenna" up to about 18mc (about the same as the SX-28A.) Higher frequency operation would certainly be possible using a "gain antenna" such as a yagi or a quad. I haven't installed any modifications to my stock R-45 since I believe that it's operating per the "design intent" of the receiver.

NOTE: It seems obvious that the R-45/ARR-7 must have been the subject of a Surplus Conversion article (or an ER article) somewhere along its post-war history. It's very difficult to find any examples of the R-45 that haven't been "ham-stered" and literally ruined for any useful purpose,...well ,... other than for parts. Certainly the re-radiation tube "ham-ster myth" hack jobs seem to be the most common but removal of the motor-drive and original power connector are close seconds followed by audio output modifications. It's unfortunate that the R-45 has shared a similar fate of the BC-348 in that nearly all examples found will be compromised in some manner. It's likely that the "Airborne SX-28A" myth is responsible for many of the modifications that tried to rebuild the R-45 into something that it was never intended to be, entertainment, shortwave listener, ham receiver, pleasure band cruiser.

UPDATE: Aug 15, 2021 - I've been using R-45 SN: 732 as a vintage military radio receiver paired with another SAAMA fugitive, the "basket-case" ART-13A. Actual "on-the-air" performance on the 75M band is very good with all stations copied Q5 even though the S-meter's normal reading is about S-6 for most of the stronger AM signals. Despite what the S-meter indicates, the reception using the R-45 is excellent and even the very weak check-ins can be copied quite well. Selectivity is acceptable but due to the usual adjacent frequency activity I normally have to keep the SELECTIVITY on IF SHARP. Stability seems consistent with the age of the design and isn't a problem for ham band use on AM. For CW or SSB monitoring, frequency drift (LO and BFO) is normal for a 1940s design and doesn't cause any problem with copy. I use a matched 600Z ohm loudspeaker for audio reproduction and the audio output can be very loud. Complaints would be the lack of a remote standby function. However, the AC power supply's 6.3vac powers up the R-45 tube heaters as soon as the power supply is switched ON. Then the R-45's front panel POWER switch turns on the B+ and this switch can be used for a "standby" switch. Also, the depth of the receiver is typical of later aircraft receivers but it can be a problem on some benches. Depth is 20.5" plus a slight bit more for the shock mount depth.  NOTE: Several times, on the Vintage Military Radio Net on 75M, we'll have up to three different stations successfully using all-original R-45 receivers for net operations,....W7MS, KØDWC and myself, WA7YBS. It's a capable receiver, capable hands.

photo left: Top of the chassis. Front end is in the center, IF section top-center, Xtal Filter in front of IF section, detector, bfo and audio at the rear of the chassis. The rear pot adjusts the motor-drive speed. The pot lower-front is the S-meter zero pot. The metal tube just below the glass VR tube is the Re-radiation tube. The blue dots on the tubes are just my indication that I've tested the tube and it was in good operating condition.

photo right: Bottom of the chassis. Band switch is at the top. Next long shaft is the Audio Gain, then the BFO shaft. The BFO air variable capcitor is located in the shielded box. Note how the RF coils have been "locked in place" using brown sealing wax. Probably done at SAAMA (luckily, during alignment, only a couple of coils needed L adjustment.) Also, all that red paint on the wiring harness was done at SAAMA. 


Signal Corps USAAF - The Hallicrafters Co. - Radio Receiver R-44/ARR-5

This VHF receiver was generally paired with the R-45 HF receiver and provided similar airborne search capabilities. The design intent was to allow airborne search and analysis of enemy radar or other signals. The R-44 is a 14 tube superhet tuning from 27.8 to 143 Mc in three bands and receiving AM, CW or FM signals. Motor-drive tuning provided a "search" scanning mode. Like the R-45, outputs for visual indicators were provided. A special "stub" antenna was used (AT-38) and a separate power pack (PP-32) provided the power for the receiver (and added three more tubes, although these were to provide B+ for three individual receivers.) Sometimes this receiver is called the "Airborne S-36" based on its vague similarity to Hallicrafters' VHF receiver, the S-36.

Up to three combinations of the R-44 or the R-45 receivers could be powered by the PP-32 power pack that provided the heater voltage (6.3vac) and the B+ (+275vdc) but the scanning motor drive was powered by the aircraft battery system (+24vdc.) Additionally, the PP-32 operated off of 115vac 400 cycle provided by the aircraft's AC system.

Apparently, when the R-45 and the R-44 were sold surplus after WWII some of the dealers also offered a very well-designed power supply kit with all of the proper military connectors and an interconnecting cable. I was given one of these types of power supplies with this R-44 receiver I obtained from Bill Mitchell of Yerington, NV about 20 years ago. Mitchell's father had originally purchased the R-44 surplus (never issued) along with the power supply kit in the late-forties. The sheet metal work makes it obvious that the power supply chassis and cabinet were professionally built pieces. W7MS also has one of these types of power supply kits that came with his R-45 receiver, so they were a fairly popular addition to the surplus R-45/R-44 purchase. The power supply provides 6.3vac for tube heaters and +230vdc for B+. The power supply doesn't provide +24vdc to operate the motor drive. The power supply is housed in a professionally-made black wrinkle finish aluminum cabinet. I use this power supply for either the R-45 or the R-44.

r-44.jpg (22294 bytes)
R-44/ARR-5 VHF Receiver


1952 Collins R-105A/ARR-15

USN, USAAF and USAF - Collins Radio Company

R-105A/ARR-15 Aircraft Radio Receiver

The R-105/ARR-15 was developed late in WWII specifically for the Navy but it wasn't long before the USAAF and later the USAF also became major users. There were some versions built and installed in aircraft in 1945 but most of the receiver's career was post-WWII. Collins originally designed the R-105 for the Navy to be paired with the ART-13 transmitter. Since the ART-13 could be set-up to "autotune" to ten selected channels, it made sense that the new aircraft receiver should also have the same capability. The autotune function would allow either a radioman to change frequency at his location or, via remote controls (C-733/ARR-15,) the pilot or co-pilot (or anyone else setup with a remote) would also be able to select control over the receiver (and transmitter via its remote.) With the possibility of several remotes being in possible control of operation, priority was determined by having the last remote switched "ON" having "control" until another remote would be switched "ON" and that action would pass the control of operation to the remote just switched "ON" by switching "OFF" all other remotes. Each time a remote was activated, the proceeding remote was deactivated which left control only on the "last activated" remote.

The R-105 uses 14 tubes in a single conversion superheterodyne circuit that tunes from 1.5mc up to 18.5mc in six tuning ranges. The receiver is powered by the aircraft battery-generator buss at +24vdc to +28vdc with the rated input voltage at +26.5vdc drawing 8.5 amps when auto tuning and about 3.1 amps in normal operation (dynamotor instantaneous surge current is about 16 amps.) The B+ is supplied by an onboard dynamotor that provides +220vdc at 100mA. Only one RF amplifier is used along with two IF amplifiers. To maintain better stability, a VFO (PTO type 70E-2) that tunes 2.0mc to 3.0mc operates with a Frequency Multiplier to provide the correct combining and excitation to the Mixer stage. The IF can be variable from 450kc up to 550kc when using the BFO-CALIBRATION control but when set to the detent at "0" the IF operated at 500kc. In CW, the BFO provides a heterodyne for demodulation. However, the BFO control also operates the CAL 100kc CFI oscillator when in the MCW-CAL position so when the BFO is tuned off of "0," it turns on the CFI calibrator. When setting up an autotune channel, if the frequency 7225kc was to be set, then the TUNING and BAND were set to 7225kc and the BFO was set to +25kc. This would have the receiver responding to the 100kc CAL harmonic at 7200kc plus the +25kc offset so that a heterodyne was heard and was fine tuned to zero beat using the TUNING control (not the BFO.) The autotune controls were then locked and another channel selected and the procedure repeated for that channel's desired frequency setup. The differences between the R-105 and the R-105A are slight and involve an improved autotune and a few minor circuit and component changes in the "A" version.

IF Bandwidth in MCW/AM Voice - The IF bandwidth isn't adjustable and is usually considered by most AM operator's to be very broad (specs are 15kc at 6db down.) This really only affects AM Voice reception and was undoubtedly a design decision that must have been impart due to the mechanical nature of the "autotuning" and the repeatable accuracy of the system (although the autotune accuracy is excellent and generally < 1kc off the set point) along with the accuracy of the transmitted signal's channel frequency. If retuning a signal would be necessary that involved "unlocking" the autotune for the manual tuning function which then changed the autotune settings for that channel. These possible autotuning issues made it mandatory that the receiver reliably "come up" on the selected channel frequency with the completed autotune "stop" ultimately having the received signal within the IF passband. For Voice reception, a wide bandwidth could provide enough recovered audio for good copy even if the receiver or the transmitted signal (or both) were several kilocycles "off frequency." This allowed the radio op to leave the R-105 "locked." It's important to adjust the SENS pot when in MCW. If the SENS is adjusted to maximum the increased noise will make weak-signal detection difficult and will also tend to broaden the apparent bandwidth of stronger signals (strong AM signals are fairly broad anyway.) The SENS pot is not used in CW. 

The CW mode seems to have a much narrower bandwidth probably due to the BFO action (being a Navy receiver, CW was the primary mode of reception at the time.) Also, that the AVC is eliminated, the VOLUME control adjusts the RF gain and the audio bandwidth is narrowed rolling off the high-end at about 1200hz helps narrow the apparent bandwidth. When receiving CW, the BFO can be adjusted to compensate for off-frequency reception to optimize the CW signal.

The 24 pin Cannon Connector - The installation onboard the aircraft required an aircraft wiring junction box and audio phone jack patch boxes (as needed) along with the necessary remotes and cabling. The original shock mount also included the mating connector for the R-105's rear mounted input-output connector which was a large rectangular Cannon 24 pin male receptacle. Besides power input, audio output and a remote standby there are ten pins dedicated to the remote channel selection and a few other pins for remote priority sensing for the receiver. Since the mating connector was part of the shock mount which seems to have always been left mounted inside the airplane, most R-105s today don't have the original mating connector or the original type of shock mount. There are other types of shock mounts available that fit correctly but don't have the rear connector mounting bracket (the R-105A shown in the photo is installed on a newer "generic" Barry Mount aviation shock mount.) The 24 pin Cannon connector was only used in one other application that might be encountered, the ARC-2, so finding an original plug can be somewhat difficult. There are several methods to interface power and other required inputs and outputs, so the lack of an original Cannon plug won't present too much of a problem. In the case of the R-105A shown, the former owner had a delrin block machined and bored to accept the proper size Molex female receptacle pins. The holes were counter-bored to provide "non-movement" of the Molex pins (once inserted) when either pushing in the plug or removing it. Though all the parts for the plug were included in the purchase, I've never completed assembly of the plug since most of the R-105A pin outs are for using the C-733A/ARR-15A remote box (examples of which seem to be "unobtainium.")

Actually, about the only rear connector-plug wiring absolutely necessary for local operation of the receiver will be the +26.5vdc and chassis ground/-26.5vdc connections along with disabling the remote standby control line and maybe using the audio output line (instead of the front panel PHONES jack.) All other pins are for remote channel select and remote priority operations. Powering up for check-out, only pin 9 (Chassis, -26.5vdc,) pin 17 (+26.5vdc) and a jump from pin 3 to pin 22 are required for receiver-only operation. For a station setup, Pin 3 and pin 22 would be used for remote standby function with NC for receive and NO for transmit. Antenna isolation has to be provided (the ART-13 has an internal vacuum T-R switch for receiver connection to the antenna.) I had lots of spare Molex pins that were the correct size, so I just made up a test cable with two 14 gauge wires with Molex pins on one end for power and a 22 gauge jumper about 3" long with Molex pin ends to jump pin 3 and pin 22. For testing, this works fine (actually, unless a remote turns up, this setup works fine for operation in a vintage mil-ham station too except that the jumper from pin 3 to pin 22 has to be removed and two wires substituted and then routed to the ART-13 sending relay.) The proper size Molex pin is "HCS 125 Socket" with a Mfg PN: 18-12-1602 (or a Mouser PN of 538-18-12-1602.)

Dynamotor Servicing - After test operating the R-105A a few times, I noticed that the dynamotor seemed pretty noisy (the receiver was out of the case) and the dynamotor was running very hot. Normally, you always have to service any dynamotor since most of them have never been serviced in their entire existence. The old style grease used in the ball bearings at that time will harden with age and inactivity, resulting in the bearings running with a minimal amount of lubrication. Also, internally, most dynamotors will have a lot of brush carbon residue all over the inside and that carbon can be somewhat conductive if there's enough residue. Almost always the commutators will need thorough cleaning along with de-glazing and leveling their surfaces. The brushes will also need inspection for length and proper seating. This dynamotor's condition was as expected,...probably never properly serviced and with lots of carbon residue all over everything internally. Servicing requires some disassembly and consists of pulling both end bells and removing the brushes (mark for correct reassembly.) At this point, I tested how freely the armature turned. It didn't move freely at all and was actually fairly resistant to any rotation. With the brushes removed the armature should easily rotate with virtually no resistance to movement. This "resistance to rotation" was probably what was causing the over-heating of the dynamotor (more current required for rotation resulted in more watts dissipation in the form of heat.)

I proceeded with further disassembly by disconnecting the wires going to the brush barrel terminals, removing the two long screws that hold the bearing housings to the dynamotor body and dismounting each bearing housing piece. Now the armature could be taken out. The armature will have commutators on each end. I used 600 grit AlOx paper to clean, smooth and level the surface of the commutators and then washed with Isopropyl Alcohol. I then used a wooden tooth pick that was sharpened to clean out any residue between each segment of the commutators and then cleaned them again with alcohol. The ball bearings are a press-fit on the armature shaft but the bearings can easily be cleaned and flushed with WD-40 and then repacked with modern wheel bearing grease. Pushing the new grease into the bearing will force any remaining "old" grease out the back of the bearing and the old grease will have to be carefully cleaned off making sure to keep any grease residue off of the commutators. The dynamotor body was thoroughly filled with carbon deposits that all needed to be cleaned out. I used alcohol and a combination of small acid brushes and Q-tips to remove all the carbon residue. I cleaned the brush barrels, the brushes and brush connector terminals with alcohol.

I carefully reassembled the dynamotor except for the end bells. This was so I could see the brush to commutator contact and make sure there wasn't going to be any "sparking" which is an indication of a rough commutator surface or poorly fitted brushes. I didn't see any sparking during this test operation, so the end bells were installed and the dynamotor installed into its receptacle with the slide-clips used to "lock" the dynamotor in place inside the R-105A.

Rubber Mount Problems - The newly serviced dynamotor ran much quieter and a lot cooler. However, with a little bit longer operation time, the vibration noise progressively worsened and the vibration could be felt when touching the case. Out of the receiver, the dynamotor ran quietly although some minor vibration could be felt. I tried rotating the rubber mounts to see if that would help since they were obviously old and leaning to one side but this didn't do anything to reduce the vibration. Next, I inserted extra rubber cushioning between the dynamotor and the mount. This seemed to reduce the noise from the imparted vibration significantly. It seemed that the original rubber cushions were just worn out. I ordered new small vibration isolation mounts (the size is 0.375" square and 0.50" tall with 8-32 studs 0.190" long for mounting - easily available on eBay.) Once installed, the new mounts had the dynamotor setting squarely and noticeable higher off the mount. When operating, virtually no vibration can be felt on the receiver chassis. When installed back in the case and on the shock mount the operation is now quiet and no vibration can be sensed when feeling the case.

NOTE: Rubber Isolation Mount Distortion: I think this might be a common problem on the R-105A due to how the rubber isolation mounts were designed to work. In a normal set-up, the receiver would have been horizontal and rightside-up with the weight of the dynamotor directing a force downward vertically onto the rubber mounts, that is, compressing the mounts. Years later, with most receivers minus their shock mount and in a "storage condition," it became a common sight to see R-105A receivers "stored" vertically on their backs with the front panel facing up or, if horizontal, they might be upside-down or on one side or the other. This places the weight of the dynamotor being supported at a right angle to the rubber mounts. With enough vertical storage time (decades?,) the rubber mounts become distorted and stretch with the result being that they are no longer able to support the dynamotor correctly when the receiver is placed back into the horizontal rightside-up operating position. Replacement of the motor isolation mounts will be required and afterwards always try to keep the receiver rightside-up and horizontal (although the new rubber mounts should last decades.)


photo above: R-105A chassis (top) showing the Autotune motor at the left (front of receiver) and the dynamotor at the right (rear of the receiver.) This particular receiver has been given the MFP treatment hence the gold color of most of the metal pieces. Also, note the main PTO 70E-2 (lower PTO) and the PTO for the BFO (upper PTO.) Between the two PTOs is the crystal controlled CFI module. There are two tubes inside the CFI module, also one tube under the shield in each PTO. The lower three tubes are the RF/Mixer section and the upper six tubes are the IF/AVC/Audio section. The R-105A chassis is 20.5" deep which became the standard aircraft radio receiver size for the time.


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