2 Transistor FM Voice Transmitter
Warning:
Take care with transmitter circuits. It is illegal in most countries to operate radio
transmitters without a license. Although only low power this circuit may be tuned to
operate over the range 87-108MHz with a range of 20 or 30 metres.
Notes:
I have used a pair of BC548 transistors in this circuit. Although not strictly RF
transistors, they still give good results. I have used an ECM Mic 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.
One final point, don't hold the circuit in your hand and try to speak. Body capacitance is
equivalent to a 200pF capacitor shunted to earth, damping all oscillations. I have had some
first hand experience of this problem. The frequency of oscillation can be found from the
theory section,and an example now appears in the Circuit Analysis section.