Circuit Description

The circuit diagram of Long Range FM transmitter having a power output in the range of 0.5 -2W has been explained here. High power transmitters are required if the transmission path contains many obstacles or the transmission distance is very high. A micro transmitter can produce a power of a few thousandths of a watt. The battery using here should be of the alkaline high power type, because the current drain will be high when compared to micro transmitter current drain. The power output of the device can be altered by changing the value of the resistor R6. This can be done by putting a variable resistor of 1K in series with R6 so that it can be adjusted to get a variable output power.

The audio input to the power oscillator is formed by TR2 and associated components, is derived from a piezoelectric microphone. This is used to drive the simple audio frequency amplifier TR1. Potentiometer R1 is used to control the input of the audio amplifier which is done by selecting the correct amount of voltage that is generated by the piezoelectric microphone. Then this signal is connected to the base of TR1 by means of capacitor C7. RF feed back problem due to large RF field is solved by employing a metal enclosure for transmitter and keeping internal wirings short.

Component listing for 1 watt transmitter

Resistors: R1 (27K), R2 (330K), R3 (56K), R4, R5 (10K), R6 (100 ohm).

Semiconductors and other Devices: TR1 (BC547), TR2 (2N2219) fitted, MIC (piezoelectric microphone), L (6 turns 22 gauge enameled wire wound on 3⁄16″ former)

Capacitors: C1, C2, C3, C8 (330 pF), C4 (2–10 pF trimmer), C5 (47 pF), C6 (1 nF), C7, C8 (40uF).

Circuit diagram

Long Range FM Transmitter Circuit
Long Range FM Transmitter Circuit

Notes

  • DC power supply ranging from 6V to 30V can be used in this circuit
  • The variable resistor must not be a wire wound device since this would act as an inductor which will cause feedback problems.
  • It is possible to use an electret microphone instead of piezoelectric microphone.