This wireless FM microphone is simple to build and it has a useful transmitting range (over 300 meters in the open air). Despite its small component count and a 3V operating voltage it will easily penetrate over three floors of an apartment building. It may be tuned anywhere in the FM band (87-108MHz) and its transmissions can be picked up on any standard FM receiver.
The coil (L1) should be about 3mm in diameter with 5 turns 0.61 mm copper wire. You can vary the Tx frequency by simply adjusting the spacing of the coils. The antenna should be a half or quarter wavelength long (for 100 MHz 150 cm or 75 cm).
Circuit description: The audio amplification stage (T1) is a standard common emitter amplifier. The 47nF capacitor isolates the microphone from the base voltage of the transistor and only allows AC signals to pass.
The LC tank circuit is constructed with T2, the feedback capacitor C5 and the parallel LC circuit L1, C4. The coupling capacitor(C6) directs the signal to the RF amplifier (T3).
Circuit calibration: Place the transmitter about 10 feet from a FM radio. Set the radio to somewhere about 89 - 90 MHz. Spread the winding of the L1 coil apart to tune to the desired frequency.