1W Long Range FM Transmitter

Posted on Feb 4, 2014

Long range, very stable, harmonic free, FM transmitter circuit which can be used for FM frequencies between 88 and 108 MHz. With good antenna transmitter can cover 5km range. It has a very stable oscillator because it uses LM7809 voltage regulator which is a 9V stabilized power supply for T1 transistor. Frequency adjustment is achieved by using th

1W Long Range FM Transmitter
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e 10K linear potentiometer. The output power of this long range RF transmitter is around 1W but can be higher if you use transistors like KT920A, BLX65, BLY81, 2N3553, 2SC1970 or 2SC1971. T1 is used as an oscillator stage to deliver a low power stable frequency. To adjust the freq. use the 10k linear potentiometer like this: if you trim down, towards ground, the freq. will drop and if you adjust it toward + it will rise. Basically the potentiometer is used as a variable power supply for the two BB139 varicap diodes. Those two diodes act like a variable capacitor when you adjust the pot. By varying the diode capacitance the L1 + diodes circuit makes a resonance circuit for T1. You can use transistors like BF199, BF214 but do not use BCs. At this moment you don`t have yet the long range fm transmitter because the power is quite low, no more than 0. 5 mW. T2 and T3 works as a buffer stage, T2 as a voltage amplifier and T3 as a current amp. This buffer stage is very important for freq stabilization because is a tampon circuit between the oscillator and the preamp and final amplifier. It is well known that poor transmitter designs tend to modify freq. as you adjust the final stage. With this T2, T3 stage this won`t happen anymore! T4 is a preamplifier for the FM transmitter and is used as a voltage power RF amplifier and will deliver enough power to the final T5 transistor. As you can see T4 has a capacitor trimmer in its...

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