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Phone mike
Telephone microphone radio bug

The phone mike bug was a small radio transmitter that was disguised as a normal microphone of the handset of a standard telephone set. It was sold during the 1980s by security shops like the Spy Shop in Amsterdam (Netherlands). As it was an illegal radio transmitter, it was 'export only'.
 
Back in the 1980s, in the days when the national telephone companies (PTT) had not yet been privitized, most (if not all) handsets of domestic telehone sets contained similar microphones and speakers. When the carbon microphone of an old telephone was broken (which happened rather often), it could easily be swapped for a new one by unscrewing the mouth-piece of the handset.

The radio bug shown in the image on the right is built inside the casing of a standard microphone element, making it virtually indistinguishable from an original microphone used in those days.
  
Placing the radio bug inside the handset

As all telephone sets of the 1950s though to the mid-1980s used a carbon-based microphone element, the phone had to provide a DC bias voltage of 8 to 10 Volt for the microphone. When small electret microphones (with built-in amplifiers) and transmitters became available during the 1980s, it became relatively easy to exploit this voltage to power a small radio transmitter.
 
The device shown here works in the 100 MHz FM band and has a small screw, hidden behind one of the circular holes, to adjust the frequency. It has an operational range of 25 to 50 metres.

One of the great things of the early telephone systems, is that nearly all telephone sets, world-wide, used a microphone with the same form factor. As a result, the microphones of all these sets are interchangeable, allowing the phone mike bug to be used virtually anywhere in the world. The image on the right shows a typical German W48 phone that was used in the 1950s.
  
The microphone of a vintage W48 handset

Bugs of this type were easily installed in an existing telephone without any technical knowledge or skills. They were therefore frequently used for industrial and private espionage during the 1980s and 1990s. A standard domestic FM receiver in a nearby room was enough to intercept any conversion. The transmitter was switched on automatically when the user picked up the handset. The user would not notice the presence of the transmitter as the device would act like a normal microphone. Due to leakage, the transmitter would pick-up both sides of the conversation.

The Phone Mike Bug can bee seen as a professional radio bug. Due to its appearance, its high build quality and the fact that it was only on the air during the phone conversation, the device was difficult to trace. It was often sold as a test transmitter with professional bug tracing equipment, such as the Audiotel Scanlock ECM, and was used to train bug-sweeping teams.
 
Phone mike bug in the original storage box Rear view of the phone mike bug Close-up of the phone mike bug (note the frequency adjustment at 10 o'clock) Phone mike bug aside a normal microphone Standard Dutch T65-TDK telephone of 1979 Placing the radio bug inside the handset German W48 phone from the 1950s The microphone of a vintage W48 handset

 
References
  1. Crypto Museum, Phone mike bug
    Date unknown. Inventory number CM300497.

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