76-110MHz band FM radio transmitter

This is a simple 76-110MHz FM transmitter that can transmit your voice or audio via a regular FM radio within the FM broadcast band. It can transmit voice using a microphone or music from any music player. The frequency is changed by adjusting a 5.5-turn inductor coil. The transmitter is powered by a 9V battery or a 3V-9V power adapter. The transmission range is 100 meters, but this can be increased with a better antenna or RF amplifier.

In open areas, the transmission range is 100 meters. The transmission distance depends on the length of the antenna used. I used only a simple 10-centimeter-long wire. Using a longer antenna of about 50 centimeters and a 9V power supply, the signal can be transmitted much further.

An electret microphone (MIC) converts natural sound signals into electrical signals. Capacitor C2 is coupled to the base of transistor 2SC9018. When a signal comes from the microphone, the junction capacitance of the transistor changes, which helps to change the oscillation frequency.

Resistor R1 (2.2K) is the microphone bias resistor; a preferred value is typically between 2-5.6K. R2 provides bias to the transistor base. C4 and inductor L (5.5 turns) form the oscillator tuning circuit; you can change the values ​​of C4 and L to change the transmission frequency.

Testing the transmitter: Using an FM radio, turn on the power and volume, and tune the frequency to approximately 88 MHz. Connect the transmitter to a power source, point it at the radio, and use a screwdriver to adjust the coil until the radio receives a signal. Then slowly increase the distance between the microphone and the radio, while adjusting the radio (or handset) volume and tuning knob as needed, until you hear the clearest sound.