Here is the circuit diagram of an FM transmitter using the IC UPC1651. UPC1651 is a wide band UHF Silicon MMIC amplifier. The IC has a broad frequency response to 1200MHz and power gain up to 19dB.The IC can be operated from 5V DC.
The audio signals picked by the microphone are fed to the input pin (pin2) of the IC via capacitor C1. C1 acts as a noise filter. The modulated FM signal will be available at the output pin (pin4) of the IC. Inductor L1 and capacitor C3 forms the necessary LC circuit for creating the oscillations. Frequency of the transmitter can be varied by adjusting the capacitor C3.
Notes. * The circuit can be assembled on a Vero board. * Inductor L1 can be made by making 5 turns of 26SWG enameled copper wire on a 4mm diameter plastic former. * A ¾ meter insulated copper wire can be used as the antenna. * Do not give more than 6V to the IC. * Mic M1 can be a condenser microphone.
