Proximity detection - tuned loops

I'm wanting to build a basic PIC based proximity detection device similar to those used in shops for anti shoplifting alarms, where a passive tuned loop influences a shift in another tuned loop - I'm wanting this device to detect tags within the loop but not outside it. I know it will influence it a bit if close to the rim but that's ok.

I'm think I may have the rough theory on it but no idea how to implement it in practice, I was thinking of having a slave tuned loop set to a slightly lower frequency than the master loop, and thinking the slave loop will pull the master loop down, from which I'd pick up the frequency shift with the PIC.

Does anyone know how to do this - how many turns of wire and what capacitance to use etc etc??

Thanks in advance

Maybe you should ask the same question here>>> http://www.thunting.com/geotech/forums/