Circuit : Andy Collinson
A modulated transmitter with variable tuning. The modulation is applied to the base and varies the frequency of the
tank circuit, thereby producing FM modulation. However there is enough cross distortion that the signal is also
received by AM radios. The antenna circuit is tuned and transmits using a long wire antenna.
Please Note. It is illegal to transmit on the broadcast bands in most countries, as such this circuit is shown for
educational purposes only.
Please read the disclaimer
on this site before making any transmitter circuit. It is
illegal to operate a radio transmitter without a license in most countries. This circuit is deliberately limited in output
power but although strictly FM provides enough cross modulation to be received on amplitude modulation (AM) . The ooutput
radio frequency is in the range 500kHz to 1600kHz with values shown.
You can apply differemt input values in the calculator below, remember to change drop down box to picofarads for
capacitance and microhenries for the coil. The coil is fixed at 200uH, the capacitor values can be varied and resonant
by using the calculator below.
Tuned Circuit Resonant Frequency Calculator
If winding your own coil then you may find Martin E Meserve page very helpful:
Single Layer Air Core Inductor Design
An alternative is to use a toroid core of appropriate material. Toroid's come in different sizes and colours, see the sample below.
A T130-2 core requires approximately 137 turns of 36 SWG wire.
Mike Yancey has a very useful Toroid calculator on his webpage, link below:
The circuit is in two parts, a microphone pre-amplifier built around Q1 and an RF oscillator circuit (Q2). The oscillator is a standard Hartley
oscillator which is tunable. Tank circuit L1 and C1 control frequency of oscillation,
the power in the tank circuit limited via emitter resistor R1. The transmitter output is taken from the collector, L2
and C2 form another tuned tank circuit and help match the antenna. L1,L2, C1 and C2 may be salvaged from an old AM radio if available. The antenna
should be a length a wire about 10 feet or more. In the schematic I have shown coaxial cable to be wired to the "longwire" antenna, the outer coax
shield returned to ground. Ground in this case is a cold water pipe, however even without a ground and coax cable a signal should still be possible.
L2 and C2 not only help match the antenna to the transmitter, but also help remove harmonics and spurious emissions in the transmitter circuit
caused by non linearity in the transistors.
Q2 forms the radio frequency oscillator. There are two requirements for an oscillator: a gain greater than unity and
regenerative feedback. The gain is provided by the transistor current gain and the impedance of the tank circuit. C4
decouples the emitter resistor at RF ensuring gain requirements are met. The regenerative feedback is via C3
connecting he tank circuit and ensuring is passed from collector, to emitter, via the internal base emitter resistance
of the transistor, back to the collector again.
Emitter resistor R1 has two important roles in this circuit. It ensures that the oscillation will not be shunted to ground via the very low internal
emitter resistance, re
of Q2, and secondly raises input impedance so that the modulation signal will not be shunted.
Q1 is wired as a common emitter amplifier, C7 decoupling the emitter resistor and realising full gain of this stage. Bias of this stage is
controlled by R4,R5 and R3. The microphone is an electret condenser type microphone, R7 setting operating current of the ECM and C6 providing DC
blocking. The amount of modulation is controlled by the 10k preset resistor PR1 which is also the collector load. The pre-amp stage is decoupled
by R6, C8 and C10. This ensures no high freq-ency feedback from the oscillator gets into the audio stage. Some electrolytic capacitors have a
high impedance at radio frequencies, hence the use of C10, a 10n ceramic to bypass any oscillator frequencies.