This FM
transmitter (FM Tx) is about the simplest and
most basic FM Tx it is possible to build and
have a useful
transmitting range. It is surprisingly powerful
despite its small component count and 3V
operating voltage. It will
easily penetrate over three floors of an
apartment building and go over 300 meters in the
open air. The circuit
we use is based on a proven Australian design.
It may be tuned anywhere in the FM band. Or it
may be tuned
outside the commercial M band for greater
privacy. (Of course this means you must modify
your FM radio to
be able to receive the transmission or have a
broad-band FM receiver.) The output power of
this FM Tx is below
the legal limits of many countries (eg, USA and
Australia). However, some countries may ban ALL
wireless transmissions without a license. It is
the responsibility of the builder to check the
legal requirements for the operation of this
circuit and to obey them.
CIRCUIT DESCRIPTION:
The circuit is basically a radio frequency (RF)
oscillator that operates around 100 MHz. Audio
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 (the 5 turn coil and the
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. The tank circuit is
connected in a Colpitts oscillator circuit.
CIRCUIT CALIBRATION:
Place the transmitter about 10 feet from a FM
radio. Set the radio to somewhere about 89 - 90
MHz. Walk back to
the Fm Tx and turn it on. Spread the winding of
the coil apart by approximately 1mm from each
other. No coil winding should be touching
another winding. Use a small screw driver to
tune the trim cap. Remove the screwdriver from
the trim screw after every adjustment so the LC
circuit is not affected by stray capacitance. Or
use a plastic screwdriver. If you have
difficulty finding the transmitting frequency
then have a second person tune up and down the
FM dial after every adjustment. One full turn of
the trim cap will cover its full range of
capacitance from 6pF to 45pF. The normal FM band
tunes in over about one tenth of the full range
of the tuning cap. So it is best to adjust it in
steps of 5 to 10 degrees at each turn. So tuning
takes a little patience but is not difficult.
The reason that there must be at least 10 ft.
separation between the radio and the Tx is that
the Tx emits harmonics; it does not only emit on
one frequency but on several different
frequencies close to each other.
NOTE:
You may experiment with using 6V or 9V with the
circuit to see how this increases the range of
the transmitter. The sensitivity may be
increased by lowering the 22K resistor to 10K.
Try it and see.