FM Beacon Broadcast Transmitter (88-108 MHz) Circuit

Frequency modulated broadcast band of 88 Mhz to 108 Mhz shall be used to transmit an audio tone. A distance of 100 yards is a working distance for such a broadcast band to be used by this circuit.


A popular timer 555 is being used to produce the tone that is nearly 600 Hz. The frequency modulates a heartly oscillator. The oscillator frequency is governed by an inductance and a capacitance.

The inductor is primarily an air core inductor which is built around a G.I. or M.S. bolt having 3*16 inches diameter. The bolt is basically a plain Hanger bolt termed as #8x32. Five turns are wound on the bolt and then the bolt is carefully removed by unscrewing.

After the coil is made it is stretched to 3* 8 inches and tapped at the centre. The frequency of the oscillation should be kept for best results at the centre of the band, i.e. 88 to 100 MHz. This can be shifted high or low by expanding or compressing the inductance coil. The 555 timer which produces the tone of around 600 Hz modulates the heartly oscillator. The output from the J-FET (2N4392) has the same phase as a signal at its gate and has the same voltage as input, where as the current is being amplified and thus acting as a current buffer.

A small signal diode (IN914/ IN4148) is being used here as a varactor Diode. This varactor diode is also termed as variable capacitor diode or variable reactance or variable cup diode whose capacitance varies as a function of the voltage across its anode and cathode and thus also being termed as a tuning diode. The total capacity in parallel with the inductor varies at the audio rate causing the oscillator frequency to change accordingly.

The ramping wave at Pin 2 and 6 of the timer circuit is being applied to the reverse bias diode through the IM resistance. This enables the capacitance of the diode to change as the ramping voltage changes. This alters the frequency of the tank circuit. An alternative methodology could have been employed so that an audio signal is fed to the IM resistance to modulate the oscillator but it would have required an additional pull up resistance to reverse bias the diodes.

The principle of the varactor diodes is very interesting as they operate in reverse bias condition. The thickness of the depletion Layer varies with the applied voltage and despite that there is no current through it. Its capacitance varies with the applied voltage. Actually the thickness of the depletion region varies to the square root of the applied voltage, as capacitance is inversely proportional to the depletion region thickness.

All components used in the circuit are readily available from radio shacks. The J-FET transistors must be of high frequency response.