An transmitter circuit which produces frequencies between 88 to 108 MHZ . should good in quality in respect of good transmitting capabilty With good area covering . This FM transmitter can cover upto 5 km area with stable frequency quality. The reason of use of voltage regulator 7809 is to regulated voltage power supply for stability of transmissing Frequency . If even some fluctuation of power supply remain, then oscillation may change and then output power affects. Adjustment is achieved by using the 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. This is Long range, very stable, harmonic free.The circuit of this project is little hard but can be built if we work carefully with it.
Two varactor diodes act like a variable capacitor when you adjust the pot to adjust the freq.Varactor Diode is a reverse biased p-n junction diode, whose capacitance can be varied electrically. As a result these diodes are also referred to as varicaps, tuning diodes, voltage variable capacitor diodes, parametric diodes and variable capacitor diodes.
By varying the diode capacitance the L1 + diodes circuit makes a resonance circuit for T1. 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 .T1 is used as an oscillator stage to deliver a low power stable frequency. 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.
56k 1 100uf 1
100k 1 10uf 1
18k 1 220nf 1
6.8k 1 15pf 1
330om 1 100nf 1
1k 4 22pf 1
22k 1 10pf 1
8.2k 1 1nf 5
10k 1 100pf 1
100Ω 1w 1
Variable(PRESAT LINEAR) 10k 1
T1 = T2 = T3 = T4 = BF199
T5 = 2N3866, 2N4427 or 2SC1970 for 1Watt / 2SC1971, BLX65, BLY81, KT920A or 2N3553 for 1.5 to 2W power.
L1 = 5 turns / 0.6mm / 4mm silvered copper
L2 = 6 turns / 0.8mm / 6mm enameled copper
L3 = 3 turns / 1mm / 7mm silvered copper
L4 = 6 turns / 1mm / 6mm enameled copper
L5 = 4 turns / 1mm / 7mm silvered copper
Also Read DC Motor/Fan Speed Controller circuit