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FM bugs
Simple FM radio bugs

In the mid 1970s large numbers of small FM transmitters, operating in the FM radio broadcast band (88-108 MHz) appeared on the market. It started with a self-build kit from the Danish manufacturer Jostykit [1] that allowed everyone to build a small FM transmitter for a few Euros.
 
Such transmitters generally consist of a single transistor oscillator with a simple resonance circuit, sometimes with an extra transistor that is used as audio pre-amplifier. The image on the right shows a few examples that were available in European electronics shops in the mid 1970s.

Many people bought such an FM transmitter for use as a Baby Monitor, but it was also used for industrial espionage and by people who wanted to monitor their unfaithful husband or wife. Due to a loophole in the law, these illegal FM radio transmitters were sold freely for several years.
  
Three simple FM radio bugs

Although using such transmitters had alway been illegal, in practice it was often difficult to prove that a bug (once switched off) had indeed been used by its owner. In the late 1970s the laws in some European countries were changed significantly, making the posession of any unlicenced transmitter illegal. As a result, the police no longer had to prove its use. After this, the miniature FM transmitters were confiscated on a large scale and were banned from the electronics shops.
 
Three simple FM radio bugs Collection of FM radio bugs Simple two-transistor bug with electret microphone Simple two-transistor bug with electret microphone  Single-transistor FM radio bug Single-transistor FM radio bug, powered by two 1.5V button-type batteries Home-made two-transistor bug with electret microphone, built after the Jostikit design. Close-up of the home-made Jostykit transmitter

 
Example
A well-known and easily reproducable design is the FM transmitter that was sold as a self-build kit by Jostikit [1] during the 1970s, of which the circuit diagram is shown below. It can be built with standard off-the-shelf components for just a few Euros in less than one hour. The first transistor (BC547) is a microphone pre-amplifier that accepts virtually any type of microphone, such a crystal earpiece, a dynamic microphone or even a speaker. The second transistor (2N2219) is the actual oscillator, the frequency of which is determined by a trimmer (2-22pF) and a coil (L).


The only special component in this design is the coil (L) which is made from 1mm thick copper wire. Make 4 windings on a 6 mm drill bit and then expand it somewhat, until it looks like the coil in the image below. In the original Jostikit design, the coil was part of the printed circuit board.
 
An antenna can be connected by making a tap after the first winding from the top (here visible as a black soldered wire). Use the trimmer to set the desired FM band frequency (88-108 MHz).

If the frequency range is insufficient, expand the coil some more and try again. Note that the design suffers from the so-called hand effect, which causes the frequency to change some­what when holding your hand too close to the circuit or when adjusting the trimmer with a metal screw­driver. Always use a plastic screw­driver or a wooden toothpick when setting the frequency.
  
Close-up of the home-made Jostykit transmitter

The transmitter shown here was built in the mid-1970s and measures just 1 x 2 cm. When properly built, it may have a range of several kilometres. Please note that the posession and/or the use of such a transmitter is subject to local laws and might be illegal in some countries.
 
Literature
If you want to know more about FM radio bugs and the various applications and improvements that were developed over the years, a good starting point might be the wonderful range of books under the title Minispione (micro spies) written by Günter Wahl in Germany. The books have been translated into various languages and were widely available in Europe during the 1970s and 80s.
 
References
  1. Wikipedia Denmark, Jostykit
    Retrieved April 2013.

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Crypto Museum. Created: Tuesday 30 April 2013. Last changed: Friday, 13 January 2017 - 18:14 CET.
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