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The image on the right shows a typical SCX bug that 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 subcarrier.
When set to FM, the transmitter behaves like a common radio bug
with Frequency Modulation (FM). In this mode, the bug is detectable
with all common intercept receivers. 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 SC mode, audio is Amplitude Modulated (AM) onto a carrier well outside
the audible range. To the average eavesdropper, the transmitter appears
to be sending a silent carrier, whereas in reality is carries the full
audio 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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The SCX was often supplied as a training device with Audiotel bug tracers
such as the Scanlock ECM
shown in the image on the right.
➤ More information
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© Crypto Museum. Created: Friday 15 May 2015. Last changed: Tuesday, 13 June 2017 - 06:56 CET.
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