Here is a simple FM broadcast transmitter, power output to the quarter wave antenna is less than 0.5mW so it has a range of a few tens of meters using a standard FM broadcast RX, it makes a very cheap baby monitor / bug. The output frequency is adjustable any where in the broadcast band ( 88 - 108 MHz ) by tuning the tank variable capacitor - chose a frequency well away from any broadcast stations
Tr1 forms a grounded base VHF oscillator. FM modulation is acheived by D1, a 1N4001 power diode, due to its large junction capacitance it acts as a crude varicap diode. Maximum capacitance occurs with zero reverse bias across the diode, minimum capacitance is acheived with maximum reverse bias voltage across the diode. Essentially the capacitance exhibited by D1 can be varied by varing the size of the depletion region within the PN junction of the diode by altering the amount of reverse bias. A electret mic insert followed by a single stage AF preamp feeds modulation voltage to D1.
The circuit is built on some copper clad board with islands cut into the board using a pad cutter ( G - QRP club ). The board should be mounted in a metal box for screening. A small mains power supply could be mounted in the same box if desired but screening would be needed between the transformer and the rest of the circuit to avoid hum pick up.
Barry Zarucki M0DGQ
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