Long Range FM Transmitter

Description

The use of transmitters which have a more powerful output than the ‘flea-power’ are sometimes required when there are many obstacles in the path of the surveillance transmitter and monitoring station receiver, or the distance between them is too far so as to make a low powered devicefeasible. Whereas a typical microtransmitter will produce an RFpower in the order of just a few milliwatts, i.e. a few thousandths of awatt, the VHF-FM transmitter described has a poweroutput of between around a half and 2 watts, depending on the power source, which may be anywhere between 6 volts and 30 volts d.c. The battery or batteries should be of the alkaline high power type, sincethe current drain will be found to be relatively higher when comparedto microtransmitter current drain. The power output of this device is somewhat proportional to the current drain and so therefore both may be decreased by altering the value of R6 to a higher resistance, or a variable resistor with a value of around 1k may be introduced in series with the existing R6, so as to give a variable power output. Thevariable resistor must not be a wirewound device because this wouldact as an inductor which will cause feedback problems.

The audio input to the power oscillator, which incidentally is formed by TR2 and associated components, is derived from a piezoelectric microphone which drives the simple audio frequency amplifier TR1. The input of the audio amplifier is controlled by the gain pot R1, which selects the correct amount of voltage that is generated by the piezoelectric microphone, then connects this signal to the base of audio amplifier TR1 via C7. It may be found that there is insufficient housing space for a bulky piezoelectric microphone, so with a slight modification to the circuit, it is possible to employ an electret microphone insert as shown Since the RF field that is generated by this transmitter is relatively large, the problem of RF feedback may very well be encountered. This may be overcome by placing the transmitter inside a metal enclosure, keeping all internal wiring as short as possible and the aerial wire

Component listing for 1 watt transmitter

Resistors Semiconductors
R1 = 27k TR1 = BC547
R2 = 330k TR2 = 2N2219 fitted with heat sink
R3 = 5k6 MIC = piezoelectric microphone
R4, 5 = 10k
R6 = 100R

L = 6 turns 22 gauge enameled wire wound on 3⁄16″ former

Capacitors

C1, 2, 3, 8 = 330 pF
C4 = 2–10 pF trimmer
C5 = 4p7
C6 = 1 nF