FM transmitters function by transmitting signals with a wave amplitude that remains constant as its frequency shifts because of variations in the amplitude of the audio signal. This technology allows the transmitter to instantly adapt its broadcast signal to the characteristics of the audio it is attempting to transmit.
Radios set to matching frequencies of the FM transmitter will receive signals broadcast by the FM transmitter. Depending on the range of the transmitter, signals received by the radio may be distorted if the radio is outside of the transmitter's optimal range.
FM transmitters rely on a small power supply and circuit board to operate. Schematics for these devices are widely available on the Internet.