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FM radio sub-carrier bug
The FM/SC is an eavesdropping device (bug)
that can pickup an conversation
in a room and transmit it via radio, using Frequency Modulation (FM) or
modulated onto a Sub-Carrier (SC). The transmitter was developed by
Audiotel
in Corby (UK) and was intended for training sweep teams on their
Scanlock receivers,
but was also suitable for actual covert bugging operations.
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The image on the right shows a typical FM/SC bugthat operates
at approx. 170 MHz. The device measures 7 x 6.5 x 2 cm and is powered
by an standard internal 9V battery.
At the front are two switches: one to turn the power ON and OFF,
and one to switch between FM and SC.
When set to FM, the transmitter behaves like a common radio bug
with Frequency Modulation (FM). In this mode, the bug is detectable
with most types of bugfinder. When set to SC however, the sound that
is picked up by the microphone is modulated onto a subcarrier (SC).
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In this mode (SC) audio is modulated in AM onto a non-audible
carrier. To the average interceptor, the transmitters appears to be sending
a silent carrier, whereas in reality is carries the full sound picked up
in the room. The sound can only be demodulated with a suitable SC receiver
or with a bug tracer, such as the Audiotel
Scanlock Mark VB or
later receivers.
Checking the signal of the transmitter on a
spectrum analyzer reveals
what is happening. If we monitor the
signal from the bug in FM mode,
we see a carrier of which the frequency is altered by the audio signal.
Watching the
same signal in SC mode,
shows a fixed carrier with two sidebands.
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© Copyright 2009-2013, Paul Reuvers & Marc Simons. Last changed: Thursday, 16 May 2013 - 07:28 CET
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