Category: RF

A simple Coil-Less FM Bug

Posted by on September 22, 2010

A simple FM Bug for beginners

For months I’ve been looking for a simple FM BUG project, the ones online require inductors which you either have to acquire or build, if you don’t have a LCR meter it becomes rather hard to get the circuit working, specially if you’re a beginner without an oscilloscope! – Sometimes they don’t even tell you which inductance is required and you have to calculate an estimate, which is the main reason why many high frequency RF projects fail in the first place.

From all the projects out there I’ve only seen one which didn’t require an external inductor since ┬áit simply used a pcb / trace inductor, however the board was big and the circuit itself had lots of stability issues, etc. I wasn’t going to waste my time with it.

My first FM BUG was based on one of the many schematics out there, it seldom worked. It was microphonic (due to the air core inductor) but the electret capsule itself did not modulate the output at all, needless to say it was very unstable and it never worked properly.

This circuit on the other hand performs pretty well, even if you’re manipulating the board or touching the coax it will stay within the tuned frequency (unless you touch the transistor or timing capacitor!). The power is very low, so don’t expect great distances, specially in populated areas! More…