Simple FM Radio Bug

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 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 shows a few examples that were available in European electronics shops in the mid 1970s. 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 kilometers.

A well-known and easily replicated 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 (BC548) 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 winding 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.”


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