Medium-Power FM Transmitter

Medium-Power FM Transmitter

Circuit Description

The range of this FM transmitter is around 100 meters at 9V DC
supply. The circuit comprises three stages. The first stage is a
microphone preamplifier built around BC548 transistor. The next stage is
a VHF oscillator wired around another BC548. (BC series transistors are
generally used in low-frequency stages. But these also work fine in RF
stages as oscillator.) The third stage is a class-A tuned amplifier that
boosts signals from the oscillator. Use of the additional RF amplifier
increases the range of the transmitter.

Circuit diagram:

Circuit

Coil L1 comprises four turns of 20SWG enameled copper wire wound to
1.5cm length of a 4mm dia. air core. Coil L2 comprises six turns of
20SWG enameled copper wire wound on a 4mm dia. air core. Use a 75cm long
wire as the antenna. For the maximum range, use a sensitive receiver.
VC1 is a frequency-adjusting trimpot. VC2 should be adjusted for the
maximum range. The transmitter unit is powered by a 9V PP3 battery. It
can be combined with a readily available FM receiver kit to make a
walkie-talkie set.

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