Back to Contents...
(c) 2001,2015 Peter McCollum
A Home-made "Agent Radio"
The author used a 'junker' T-784 transmitter to build an experimental agent radio. Its purpose is to allow low-power CW or AM transmissions in a portion of the standard AM broadcast band. A case officer could use such a transmitter to communicate with nearby agents, without the need to equip the agents with a shortwave radio. A BCB radio can be kept in an agent's apartment without arousing any suspicions, and without the agent needing to hide a clandestine radio. AM capability is added to the transmitter, since a typical BCB radio does not have a BFO for CW reception. It can be operated from the standard RP-1 or RP-2 power supplies.
Since the signal is within the standard broadcast band, it would be easy for an adversary to overlook the signal, especially at night when many DX signals may be present. Or, the frequency can be chosen to be very close to a strong local station (this is called "snuggling").
The transmitter operates with crystal control from about 1400-1700 KC, with a CW power input of about 8 watts. The AM power input is about 3 watts, and the modulation is about 20-30%.
The construction of the transmitter uses many components of the T-784. The important changes are as follows:
The RF portion of the transmitter is a single tube. A 6AG7 is used instead of the 6AC7, to allow greater power input. The oscillator configuration is the same as the original, except that there is no longer a cathode resistor, and the plate load resistor is reduced.
The 2E26 is now used as a plate modulator for the 6AG7. The modulator tube is configured as a 'series voltage regulator', like a DC power supply regulator, except that it's output voltage is controlled by an audio signal. This approach is simple and effective at relatively low power levels, and avoids the weight and expense of a modulation transformer.
The oscillator tank was modified to operate in the upper portion of the AM band. A new tank coil was wound by hand, and an extra 47pf added to the tuning cap.
The antenna current indicator was modified to be more sensitive at lower power levels.
A microphone and speech amplifier was added to drive the modulator. The 2-tube speech amp uses subminiature tubes and components from an SB-329/ARC-44 audio control box. The microphone is a hearing-aid type earphone element.
Click here to view the schematic of the transmitter.
The front panel of the BCB transmitter. The toggle switch on the right selects AM or CW. For AM, the key is used as a PTT switch. The mic is permanently attached.
A view of the chassis. In the foreground is the tank coil and the pi-net tuning cap. The 6AG7 is under the tank coil. At the top is the speech amp sub-chassis, with 3 unused tube sockets.
Another view of the chassis. The 2E26 is visible in the foreground.