RF Detector

http://www.imagenisp.ca/jsm/Crystal.html

A cat whisker (sorry, Del and Hen), a bad filling, holding a scope probe in your hand, a radio receiver, a radio telescope. What frequency and strength are you looing for?

Nice collection, PW!

As you can see from the posts already given, any radio is a detector of radio frequencies. Of course you have to tune the radio to the frequency you want. If you do not want to tune, then a diode in parallel with a VOM will do. I used to use that with a short piece of wire to tune my CB radio. It was a crude form of a 'Field Strength Meter', and not very sensitive, so it had to be close to the transmitter.

Generally, the rf detector is specially designed to block radio-based communication devices from operating. In other words, rf detector interfere with communications by broadcasting radio frequency and radio transmissions on the particular frequencies used by targeted devices.