Can you build a very small fm/or am receiver that only receives a single frequency?

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Almost all FM/AM receiversuse a variable capacitor to adjust frequencies. If you use a Capacitor of a fixed value, then you will be able to receive only a fixed frequency (depending on the value of the capacitor used.)
Smokedasphalt is correct in this. Though you may have a difficult time finding capacitors with arbitrary values, unless you're familiar with series-parallel capacitor calculations. A more practical solution might be to include the variable capacitor anyways, and tune it before you put it in a project box (or whatever have you). Epoxy or hot glue the top of the dial so it doesn't shift on its own, and just leave it inside the case. That way nobody can change it from whatever you've set it to.
Yeah, as ACProdigy says, you can hot glue it. Or rather, tune it and keep it inside the case so that nobody changes it. This will eliminate the work of searching for a capacitor of a particular value and also, you can always change the frequency if the need arises!
Well said mate.
rickharris8 years ago
Yes there are lots of kits for building spy/surveillance radios It is easier to buy one of them.
True, but if I was using one of those, I would be using it for a hidden bug in an ink pen I left where I wanted it, or conceled as a button, like our FBI uses. Short ranged to about 700 ft. Needs no batteries, as it uses the power of a local radio station for it's power source for the bug. Learn how to do in second phase electronics engineering school.
Thanks a lot for the Best Answer!!
@ACProdigy : Thanks bro! :)