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POTS telephone FM bug
AU-020 is a miniature covert listening device
for analogue telephone lines (POTS),
also known as a telephone bug,
developed in the 1980s by an unknown manufacturer. 1
It is connected in series with the telephone line and consists of a
free-running oscillator that transmits
in the VHF/FM band between 100 and 140 MHz over a distance of 400 m.
It is also known under other names.
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The transmitter is fully potted and is housed in a black metal
enclosure that measures 35 x 14 x 9 mm and weights just 8 grams.
It has two fixed wires with miniature crodile clips at the end,
for quick connection to an analogue telephone line.
The device takes it power from the telephone line, and uses the same
telephone line as its antenna. At one of its sides is a recessed
screw for adjustment of the transmission frequency. As it covers a
wide frequency range (100-140 MHz), it will be difficult to adjust
it accurately.
Furthermore, it suffers from the so-called hand effect. 2
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The metal enclosure reduces this effect somewhat, but does not completely
eliminate it.
The device was intended for operation in the FM broadcast band [1], so that
it could be monitored with any regular broadcast
receiver. In particular the 'quiet' area between 104 and 108 MHz was a
popular range for this type of bugs [3].
The use of this device is not only illegal – it may interfere with
legitimate radio stations – but also potentially dangerous,
as it can also transmit in the airband [2] where it might interfere
with navigational aids or airplane and ground communications.
The diagram above shows the frequency range in which the AU-020 operates.
The device was available around 1991 for a price of US$ 73 [3].
For more advanced eavesdroppers, a crystal-controlled variant that
operates in the UHF band — the AU-026 — was available for US$ 385 [3].
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Based on the construction and the choice of components, it seems likely
that this device was made in Japan.
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The transmission frequency changes when you hold your hand close
to the device.
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The diagram below shows how the bug is installed.
One wire of the subscriber line
(between the exchange and the telephone set) is cut and the crocodile clips
of the bug are connected to the wire ends (the red wire in this case).
This means that the bug is connected in series with the line.
The subscriber line is used for three purposes: (1) it provides power to the
bug, (2) is delivers audio to the bug and (3) is acts as an antenna. The bug
becomes active as soon as the telephone's handset is lifted from the
cradle (off-hook). Due to the fact that the telephone line is used as an
antenna, the operating range will be approx. 400 m. The frequency of the bug
is not very stable and may drift over time when it warms up.
For this reason it is necessary to use a broadcast receiver with Automatic
Frequency Correction (AFC) at the Listening Post (LP).
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As the AU-020 is potted in a hard resin, we are unable to show the interior
of the device. We have therefore made a couple of x-ray images that reveal
its contents. The device compares favourably to the somewhat simpler
POTS bug. It has more components and has a longer operational
range.
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Model AU-020 Manufacturer Unknown Purpose Telephone radio bug Type Free running oscillator Range 400 m Modulation FM Frequency 100 - 140 MHz (adjustable) Harmonics f2 -10dB, f3 -30dB Dimensions 35 x 14 x 9 mm Weight 8 g
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© Crypto Museum. Created: Wednesday 06 April 2022. Last changed: Thursday, 07 April 2022 - 07:18 CET.
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