This FM transmitter
circuit is very simple and it has a acceptable transmission . The signal
transited from this FM transmitter circuit can be received at almost
300 meters in open air .The circuit require a 3volts operating voltage
and can be tuned anywhere in the FM band.The coil should be about 3mm in
diameter and 5 turns. The wire is tinned copper wire, 0.61 mm in
diameter.After the coil in soldered into place spread the coils apart
about 0.5 to 1mm so that they are not touching.
If you don’t have a trim cap you can use a fixed value capacitor and
you can vary the TX frequency by adjusting the spacing of the coils or
placing a small piece of ferrite inside the coil , but the better way to
change the transmission frequency is to use a variable capacitor
.Connect a half or quarter wavelength antenna (length of wire) to the
aerial point. At an FM frequency of 100 MHz these lengths are 150 cm and
75 cm respectively.