This FM transmitter circuit project is a very simple and powerful transmitter circuit with a range up to 1 kilometer in the open. This FM transmitter circuit uses an RF transistor in its output stage and two BC547’s for the first two stages. Distance of transmission is critically dependent on the operating conditions (in a building or out on the open), type of aerial used (single wire or dipole), operating voltage (12V is better than 6V) and if the circuit is peaked for maximum performance.
The FM transmitter circuit is basically a radio frequency oscillator that operates around 100 MHz. Audio signal picked up and amplified by the electret microphone is fed into the audio amplifier stage built around the first transistor.
Output from the collector is fed into the base of the second transistor where it modulates the resonant frequency of the tank circuit (L1 coil and the red trimcap) by varying the junction capacitance of the transistor. Junction capacitance is a function of the potential difference applied to the base of the transistor T2. The tank circuit is connected in a Hartley oscillator circuit. The final stage built around T3 amplifies the output RF signal.
The 10pF ceramic capacitor in parallel with the red trim cap will enable you to tune the transmission in the 98 MHz to 105 MHz range of the commercial FM band.
Greater range from the transmitter can be obtained by replacing the half-wave antenna (the length of wire about 160cm long) with a dipole antenna .
L1 and L3 must coils must have 6 turns from Cuem 0.5 mm wire on a 5mm support and L2 coil must have 8 turns on a 5mm diameter support.