FM bug detector with Arduino

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:

http://www.zen22142.zen.co.uk/Circuits/rf/bugdetector.htm

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.

In effect what this is is a simple crystal set. FM car iPod type transmitters are very low power so you will have trouble detecting this with this sort of equipment anyway. How are you terminating the transmitter?

I suspect another reason is that your coil does not have a high enough Q factor. However for playing with RF you need to have some test equipment, a signal generator and an oscilloscope are the absolute minimum.

Thanks Mike,
The transmitter does not have anything that I could immediately identify as an aerial (is that what you mean by terminating?). It does have a range of perhaps 10m, receiving on a normal FM radio.

I have a scope and only an AF sig gen. I understand the concept of Q factor, but have no way to test an inductor I make. I can see that with a sig gen working at 100MHz frequencies, I could inject a signal and take amplitude readings at various frequencies and make sure the signal I want is being passed. Much as I’l like to buy more test kit, I’m moving house, so my other half might not see it as an essential purchase just at the moment!

So, what is it about a coil that gives it a high Q factor?
Do you think that circuit is just too insensitive to be practical anyway. Would a multi-stage RF amp design be better?
Or better still any one recommend a nice IC with all the FM amp on it?

I have ordered a $7.50 'bug detector' from Deal Extreme. The reviews indicate that it does work quite well, I should be able to adapt that to extract a signal.

Thinking about it, the FM iPod transmitter is only marginally stronger than the broadcast signals. So, this would only really work if the signal were orders of magnitude stronger than the commercial FM broadcasts. Which maybe a real FM bug is.

While Q applies to a tuned circuit it also applies to a simple inductor and is an indication of the dielectric loss in the core of the coil. So for example coils wound on certain types of ferrite can have a low Q or high Q depending on the type of ferrite.

The coil here is acting as little more than a pickup coil coupling RF into the rectifier circuit. You also need to be using a diode that has a low capacitance and a high frequency response as well.