Below is the circuit diagram and PCB layout of FM wireless microphone:
The range of frequencies for the FM broadcast band is 90MHz (MHz = Megahertz or 90 million cycles per second). Because the FM microphone has a variable tuned circuit, it can be tuned to a quiet spot on your local FM broadcast band for the best reception. When the small microphone element is struck by sound, it converts the audio to a change in current through resistor R1 (see schematic diagram).
This electrical change is amplified and eventually frequency modulates the transmitter. The transmission range of the FM microphone is approximately 100 feet, depending on the efficiency of the antenna (properly tuned or not) and the quality of the FM radio receiver.
Get an FM radio and tune it away from any FM stations. You should hear only noise and no programs. Place the unit about 2 feet from the radio with the microphone facing the radio speaker. Remove the foam cover and tune the RF coil with the tuning stick and listen for a howl in the radio. This indicates that you have tuned the transmitter to the FM radio frequency. Place the transmitter away from the radio until the howl disappears. Talk into the microphone and you should hear your voice on the radio. If your voice comes through the radio distorted, speak softer (you are over-modulating). Push the foam cover onto the case when tuned.
Have a friend listen to the radio and move the transmitter about 100 feet away. Your voice should still be heard over the radio. To obtain further distance, add a longer antenna.
Download the FM Wireless microphone kit manual: