How to make a long range FM transmitter at low cost
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 thesurveillance transmitter and monitoring station receiver, or thedistance between them is too far so as to make a low powered devicefeasible. Whereas a typical micro transmitter 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 powersource, which may be anywhere between 6 volts and 30 volts d.c. Thebattery 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 issomewhat proportional to the current drain and so therefore bothmay be decreased by altering the value of R6 to a higher resistance, ora variable resistor with a value of around 1k may be introduced inseries with the existing R6, so as to give a variable power output. Thevariable resistormust 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 isformed by TR2 and associated components, is derived from apiezoelectric microphone which drives the simple audio frequencyamplifier TR1. The input of the audio amplifier is controlled by thegain pot R1, which selects the correct amount of voltage that isgenerated by the piezoelectric microphone, then connects this signalto the base of audio amplifier TR1 via C7.It may be found that there is insufficient housing space for a bulkypiezoelectric 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 relativelylarge, the problem of RF feedback may very well be encountered. Thismay 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
C1, 2, 3 = 330 pF C4 = 2-10 pF trimmer C5 = 4p7 C6 = 1 nF
C7,C8 = 40uF/25V Electrolytic