200M FM Transmitter

Posted on Feb 5, 2014

A very stable and simple FM transmitter circuit is given here. With a matching antenna, this transmitter can attain a range of around 200 meters. I assembled this transmitter few years and got very good results. Let us see how the circuit works. A condenser microphone (K1) is used to pick the sound to be transmitted. Capacitor C1 is a DC decoupler

200M FM Transmitter
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and the sound signal is coupled to the base of Q1 which wired as a preamplifier. R2 and R3 are the biasing resistors of Q1. Amplified sound signal will be available at the collector of Q1 and it coupled to the emitter of transistor Q2 through the capacitor C4 and 1mH inductor L1. Capacitor C4 decouples the DC component from the preamplifier output. Q2 performs the job of oscillator and modulator. Inductor L2 and variable capacitor C5 forms the tank circuit necessary for creating oscillations. Capacitor C6 is the feedback capacitor. The modulated FM wave will be available at the collector of Q1 and it is transmitted using the antenna A1. This simplified formula will help you in making inductors, L = (d ²n ²) / (18d+40l). Where L is the inductance of the coil in uH, d is the coil diameter in inches, l is the coil length in inches and n is the number of turns. Just try to make the inductor your self using this formaula. If you can`t, then comment here. I will give you the winding details.

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