Hi, Can anyone help me with this. I have a rather limited knowledge of RF electronics.
The project is to feed a level into an analog input of an Arduino in proportion to any signal that can be detected in the FM band. The idea being that the Arduino monitors for any 'bugging' activity and notifies you of the signal strength and any thresholds that get exceeded.
I am fine with everything apart from the signal detecting bit, and when I found this:
Great, I thought, I can easily apply a bit of Ohm's law and some smoothing to get a suitable analog voltage out of that.
The problem. The coil is discontinued, so, thinking it was a broadband design and not too critical, I tried winding a selection or air core coils, using various thicknesses of wire and coil diameter and turns, based on air coil design applets guessing and the like.
I got it to work ok with Cellphone detecting, and then extrapolated how many more turns I would need to detect the much lower frequency FM car iPod type transmitter that I was trying to detect. But no luck - the needle doesn't even twitch when right next to the transmitter.
Any ideas for sources of similar inductors or why it is not detecting or pointers to alternative circuits would be much appreciated.
This is what the original design says about the inductor:
The inductor is a moulded RF coil, value of 0.389uH and is available from Maplin Electronics, order code UF68Y. (See my links page for component suppliers.) The coil has a very high Q factor of about 170 and is untuned or broadband.