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.
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