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(c) 1999,2018 Peter McCollum


 Beacon transmitters were used by agents to signal the location of a desired air drop.

The RT/A-3 Transmitter, a part of the BN-2

The RT/A-3 is based on a 'standard' RT-3 transmitter, but with the following modifications:

·         The tuning chart plate has been replaced by a plate that reads "RT/A-3 PART OF BN-2 TUNING RANGE 1500-1800 KC". The original RT-3 ID label is still there.

·         The band switch has been removed, and replaced with a screw that plugs the hole (to keep the unit watertight). Inside, all of the bandswitch components are gone.

·         A fixture has been added to the FT-243 crystal socket: it allows a crystal to be installed in the horizontal position, laying across the BN-2 label plate. The original crystal socket is still usable, and the second socket (for crystals with wider-spaced pins) is still there, also.

·         The antenna tuning cap has been replaced by a dual-section 365 pf unit, with both sections wired in parallel for a total of 730 pf. The cap looks like a commercial broadcast radio type, with a compression trimmer on each section. To mount the cap, three holes were drilled and countersunk in the front panel. The original watertight shaft bushing has been mounted upside-down on the top of the panel, and there is a rubber washer under it. This allows everything to remain watertight, but allows the new capacitor to mount close to the panel. However, since the shaft bushing is mounted on the "wrong" side of the panel, the antenna tuning knob is now higher above the panel than the original.

·         There is a schematic (marked "RT/A-3 TRANSMITTER") glued to the inside of the case. On one RT/A-3 unit, many points in the schematic have a small hand-drawn check-mark next to them - it's as if the technician was checking off the modifications as he did them, then he glued the schematic in the box when he was finished.

·         The ant. current indicator is a #43 lamp, instead of a #47 (this is indicated on the schematic, also). The lamp's parallel resistors are a much smaller value than the original. Most of the rest of the circuit is the same - although the plate-tank is a toroid transformer (two separate windings), and the oscillator tank is also a toroid (single winding). These new toroids are about 1" diameter, and are mounted on plastic standoffs with nylon screws.

·         Because of the change in the antenna current lamp circuit, it only glows when the antenna impedance is low (around 20 ohms or less). This probably indicates something about the intended type of antenna for the RT/A-3 (perhaps a bottom-loaded vertical whip?).

·         The outside of the case and lid has a 2"-wide yellow stripe painted on it.

·         The original code-key is there, and works normally, although the units were adjusted so that the contacts were closed all the time. So, the unit would transmit a carrier as soon as it was powered up.


RT/A-3 transmitter. Note the missing bandswitch control, the modified crystal socket, and the yellow stripe on the case. Author's collection.


Close-up view of the RT/A-3's modified crystal socket, with a crystal installed.

Another style of RT/A-3 modification (probably later). The markings are printed directly on the panel, and the crystal holder is different.

The BN-2 is a designation for a beacon transmitter which includes the RT/A-3, and was used to signal for air drops, or to mark an airstrip. To support this theory, consider the following:

Many types of aircraft in the 1950's were equipped with navigation equipment that tuned in the area of the broadcast band; such as the ARN-59 which tunes 190-1750 KC. So, the RT/A-3 could have been used by clandestine teams in remote areas as a way of providing a navigation beacon for supply air-drops, or to mark a temporary airstrip. The RT/A-3 would have been very familiar to the agents who were already trained with the RS-1. Because of the crystal-socket modification, it could have been delivered to the field with a crystal already installed (a wide variety of frequencies may not have been needed for a beacon that was used only occasionally, and the signal could be easily 'hidden' in the AM BC band). With the key 'locked down', it would be easy for a single person to set up and operate the transmitter with a GN-58 generator; and transmitting an unmodulated carrier allows it to be more easily overlooked by the enemy. The January 1998 issue of "Air & Space" magazine has an article about the CIA's air operation in Tibet in the late '50's. The author mentions a case where a C-130 was returning, low on fuel, and the pilot needed to find a CIA-run emergency airstrip in Thailand that had "only a non-directional radio beacon". The RT/A-3 could fit a situation like this.

An R&D Lab report in March 1959 mentions a “DZ Location System, BN-1”, based on a “short pulse radar unit”. This project was not pursued because of the very limited range.

Radio Set AN/URC-4

The URC-4, often referred to as a “pilot's rescue transceiver”, was sometimes part of an agent's equipment, to use as a beacon for air-dropped supplies [ref 105, 108, 109, 111, 114, 115, 125, 129, 133, 144]. The standard URC-4 was modified to mount on a camera tripod, and the dipole antenna was replaced with a ground plane, and tuned for a frequency other than the standard 121.5 MC [ref 111]. Since the antenna was a ground plane, there was a strong “null” in the signal as the aircraft flew over, to indicate when to make the drop. Agency testing of this system was performed with C-47 aircraft with AN/ARA-8 homing equipment. Two criticisms of the system: the specialized battery could be difficult to obtain in the field; and users were nervous about the long and continuous transmit time that was required, plus the super-regenerative receiver that radiated a signal as well.

The URC-11 was also used in the beacon role when it became available in early 1957. It was less than half the size, because of employing transistorized audio stages. Agency testing was performed in January 1957, along with the SARAH beacon and another system.

The picture below is from a portion of a picture of a Soviet press briefing in 1957, showing captured spy hardware. The item in the center is a URC-4, with a non-standard groundplane antenna, and is mounted on a small tripod. In the foreground appears to be an RS-6 set.


RT-37/PPN-2 Beacon “Rebecca-Euraka

The PPN-2 is a transponder beacon, used primarily for marking a ground location for aircraft to use as a homing signal. The PPN-2 Beacon, and the aircraft-mounted APN-2 interrogator, were known to the British as the "Rebecca-Eureka" system. See "Wireless For The Warrior, Volume 4" for details on the system.

The CIA used the PPN-2 on some missions until at least late 1952 [ref 106, 111, 112a].

A “Eureka” unit. Image courtesy of the late Bill Howard.


Image courtesy of the late Bill Howard.

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