FM Crystal Radios

Figure 1 

The possibility of producing FM crystal radio is shown in this project which can produce a sound with high fidelity that is as good as or better than more expensive AM radios when connected as a receiver.

The device uses not only the radio transmission but also a crystal detector without requiring any batteries or AC power. In order to be heard in earphones without any such extra power, an FM crystal radio must be able to detect and receive FM signals well enough. The design of the circuit looks similar to a classic AM crystal circuit but is even simpler to build. To resonate at higher frequencies, the components were reduced in dimension by using smaller capacitors and coils as well as the antenna.

Two trimmers are used as the air variable capacitor which can be adjusted for best reception. This can be done by using a commonly available vernier dial and knob which will nicely fit the capacitor. Due to an effect called slope detection that modulates amplitude, a detected FM signal is converted to AM. Near the transmitter, this FM crystal set will work best with not very loud sound.

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