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AU-020
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.

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
  
Transmitter

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].

  1. Based on the construction and the choice of components, it seems likely that this device was made in Japan.
  2. The transmission frequency changes when you hold your hand close to the device.

Transmitter
Transmitter closeup
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Transmitter
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Transmitter closeup

Setup
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).


Interior
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.

Perspective view (1)
Perspective view (2)
Top view
Side view
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Perspective view (1)
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Perspective view (2)
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Top view
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Side view

Specifications
  • 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
References
  1. Wikipedia, FM broadcast band
    Visited 6 April 2022.

  2. Wikipedia, Airband
    Visited 6 April 2022.

  3. Advanced Intelligence, Telephone monitoring voice transmitter UA-020
    Retrieved December 2021.
Further information
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Crypto Museum. Created: Wednesday 06 April 2022. Last changed: Thursday, 07 April 2022 - 07:18 CET.
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