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Pager bug
Remote controlled transmitter

Pager Bug 1 was a remote controled covert listening device (bug), consisting of a mains-powered crystal-controlled transmitter with electret microphone, and a commercial Motorola pager [2] that acted as a switch receiver, made during the 1980s by a Dutch group of skilled hobbyists, known as Counter Observation Team (COT) [1]. It was used by criminals in the Amsterdam underworld.

The disadvantage of most radio bugs that use continuous wave (CW) transmission, is that they are easy to trace when active. Battery operated devices have the added disadvantage of limited battery life, reducing the bug's effectiveness.

The bug shown here, overcomes both issues. It has a built-in mains power supply unit (PSU) that gives it a virtually unlimited life, and is remote-controlled by means of a pager, so that it can be switched OFF when it is no longer needed. This is the case when no conversation is taking place (e.g. at night) or when a sweep-team is in action.
  
Remote controlled FM transmitter with Motorola pager

The pager – normally powered by an internal 1.5V AA-size battery – is also powered by the bug's
PSU
, so that its standby time is no longer limited to a few days. The bug is activated by sending a special code to the pager via the public telephone network, and can be deactivated with another code. When active, the radio transmitter has an operational range between 200 metres and 2 km, depending on obstacles and circumstances. When inactive, the device is virtually undetectable. 2

  1. The real name of this device is unknown, but as it is remote controlled by means of a Motorola pager, we have nicknamed it Pager Bug.
  2. Except when a Non-Linear Junction Detector (NLJD) is used by a skilled operator, but this was little known at the time, and certainly not common practice.

Remote controlled FM transmitter with Motorola pager Motorola Bravo Express pager glued to an FM radio bug Pager bug interior Crystal-driven FM radio bug with mains power supply unit Inside the transmitter Transmitter, switch circuit and power supply unit (PSU) Switched RF transmitter with electret microphone
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Remote controlled FM transmitter with Motorola pager
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Motorola Bravo Express pager glued to an FM radio bug
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Pager bug interior
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Crystal-driven FM radio bug with mains power supply unit
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Inside the transmitter
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Transmitter, switch circuit and power supply unit (PSU)
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Switched RF transmitter with electret microphone

Operation
The diagram below shows how remote-controlling of the bug worked. At the centre is the target area with the actual bug. At the right is the nearby listening post, where a regular telephone set (or a phreaked mobile phone) is used to dial out the pager's subscriber number and the numeric message (1). The call is passed by the public network (
POTS
) to the Semafoon paging service (2).


At the Semafoon service, the message is queued for transmission (3). As soon as the network is free, the message is broadcast nationwide via a network of cell towers (4). When the message reaches the pager, it is decoded (5), and the transmitter is enabled. When the bug is active, its microphone picks up the sound in the room, and transmits it to the nearby listening post (6). When bugging is no longer required, another message is sent to deactive the transmitter again.


Interior
The actual bug is housed in a standard black plastic enclosure, the interior of which is shown in the diagram below. At the front is the actual transmitter. It was probably an off-the-shelf part, that was modified to make it fit. One of the corners has been removed, and the quarz crystal is mounted at an angle. The electret microphone – with built-in pre-amplifier – protrudes the case.


Behind the transmitter is a home-made decoder/switcher. It takes the signal from the (modified) Motorola pager, and converts it into an ON/OFF signal. At the far side is the mains power supply unit, which consists of a miniature transformer, a stabilising circuit and two voltage regulators.

The Motorola pager is used here as a remote control unit, or switch receiver, for the transmitter (bug). It was a standard device that could be purchased in a telecom shop, and could be activated from the national paging network (Dutch: Semafoon netwerk). A message was sent by calling the pager's subscriber number (in this case: 06-59565008) 1 followed by the numeric message that had to be displayed on the pager's LCD screen; in most cases, the number that had to be called.

  1. The Semafoon paging network is no longer operational, and the 06-59... numbers have since been reused for the GSM telephone network. Similar paging services still exist today however (2020).

References
  1. Anonymous donor, Switched FM radio transmitter with Motorola pager - THANKS !
    Received January 2020.

  2. Wikipedia, Pager
    Retrieved February 2020.

  3. Wikipedia, Semafoon
    Retrieved February 2020 (Dutch).
Further information
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Crypto Museum. Created: Tuesday 04 February 2020. Last changed: Thursday, 06 February 2020 - 21:05 CET.
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