(c) 2001,2008 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.