Radio Transmitter

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Friday, March 27, 2009

Voice FM Transmitter Circuit

It is illegal in most countries to operate radio transmitters without a license so take care with transmitter circuits. This FM Low Power circuit may be tuned to operate over the range 87-108MHz band II, with a range of 20 or 30 metres.

Active components in this circuit are BC548 transistors. Although not strictly RF transistors, they still give good results. I have used an ECM Mic (Voice FM ) insert from Maplin Electronics, order code FS43W. It is a two terminal ECM, but ordinary dynamic mic inserts can also be used, simply omit the front 10k resistor. The coil L1 was again from Maplin, part no. UF68Y and consists of 7 turns on a quarter inch plastic former with a tuning slug. The tuning slug is adjusted to tune the transmitter. Actual range on my prototype tuned from 70MHz to around 120MHz.

The aerial is a few inches of wire. Lengths of wire greater than 2 feet may damp oscillations and not allow the circuit to work. Although RF circuits are best constructed on a PCB, you can get away with veroboard, keep all leads short, and break tracks at appropriate points.

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