Coil-Less FM Bug

Posted on Feb 4, 2014

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 h

Coil-Less FM Bug
Click here to download the full size of the above Circuit.

ave 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 it 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! No surprises there, it`s very similar to other schematics so therefore I won`t go into the working details. Actually I might argue this one is a bit on the low efficiency side, but at the moment I don`t have the equipment to further improve it`s design. But let`s not focus on that and instead let`s talk about the inductor / antenna, it`s based on a 12cm long 50-75Ohms coaxial cable with one of it`s ends soldered (mesh and core are joined together) this is a big plus, we don`t have to use a flimsy...

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