Description.
Here is the circuit
diagram of the simplest FM transmitter using a transistor. Great
performance or range is not guaranteed here, because this is an
elementary design. General purpose radio frequency transistor BF 494
(Q1) is used here for obtaining FM modulation. A condenser mic is used
here to pickup the sound. The condenser mic converts the sound to
electrical variations and this variations are fed to the base of Q1,
which performs the amplification as well as modulation. The capacitor
C2 and L1 determines the frequency of transmission. The circuit can be
powered from a 9V transistor radio battery.
Circuit diagram and Parts list. 
Notes.
Notes. - The coil L1 can be made by winding 8 turns of 1mm thick enamel coated copper wire on a ball pen refill. The coil should be tapped at the center for connecting the antenna.
- A 30 cm wire can be used as an antenna.
- Remember! This circuit is an elementary circuit. No good performance or range is not guaranteed. Ideal for demo applications only. I got only 8 meter range with some decent sound quality.
- Battery is strictly recommended because mains powered supply may induce additional noise.