Long range FM transmitter

This circuit is a highly stable, harmonic-free, long-range FM transmitter designed to operate within FM frequencies between 88 and 108 MHz. It is capable of covering a range of up to 5 kilometers. The circuit features a stable oscillator powered by an LM7809 voltage regulator, providing a 9V stabilized power supply for the T1 transistor. Frequency adjustment is facilitated by a 10K linear potentiometer. The output power of this long-range RF transmitter is approximately 1W, which can be increased by using transistors such as KT920A, BLX65, BLY81, 2N3553, or 2SC1970/2SC1971. The T1 transistor serves as the oscillator stage, generating a low-power stable frequency, adjustable via the potentiometer. Lowering the potentiometer reduces the frequency, while increasing it raises the frequency. The potentiometer acts as a variable power supply for two BB139 varicap diodes, which function as variable capacitors. By adjusting the capacitance of these diodes, the L1 + diodes circuit creates a resonant circuit for T1. Alternative transistors like BF199 and BF214 can be used, but BC series transistors should be avoided. At this stage, the transmitter outputs a power of no more than 0.5 mW. The T2 and T3 transistors act as buffer stages, with T2 functioning as a voltage amplifier and T3 as a current amplifier. This buffer stage is crucial for frequency stabilization, serving as a buffer between the oscillator and the preamplifier/final amplifier, preventing frequency modulation during adjustments. T4 is a preamplifier for the FM transmitter, acting as a voltage power RF amplifier that delivers sufficient power to the final T5 transistor. A capacitor trimmer in T4's collector forms a resonant circuit, enhancing T4's amplification and suppressing unwanted harmonics. Coils L2 and L3 should be positioned at a 90-degree angle to each other to avoid frequency and parasitic coupling. The final stage of the transmitter is equipped with an RF power transistor capable of at least 1 watt output power, with recommended transistors including 2N3866, 2N4427, 2N3553, BLX65, KT920A, 2N3375, BLY81, 2SC1970, or 2SC1971 for optimal performance. Using a 2N2219 will yield a maximum output of 400 mW. A robust heatsink is necessary for the T5 transistor due to heat generation. A stable 12V/1A power supply is required for operation. The construction begins with the oscillator stage; a small wire should be soldered to T1's 10pF capacitor output, and an FM receiver should be used to tune the 10K potentiometer until a blank noise or music is heard. The oscillator stage alone can cover a distance of 2-3 meters with a 70 cm wire. Proper shielding must be implemented as per the circuit schematic during the assembly of the RF transmitter. Once construction is complete, connecting the antenna or a 50 or 75-ohm resistive load is essential, and an RF probe can be utilized (a 1N4148 diode may substitute the probe diode). The 10K potentiometer should be adjusted to the desired frequency, followed by trimming the first collector trimmer in the T4 stage for maximum voltage reading on a multimeter. Subsequent trimmers should be adjusted in sequence, returning to the first trimmer for final adjustments to achieve the highest voltage reading. For 1 watt RF output, a voltage measurement of 12 to 16 volts is expected. The power formula is expressed as P (in watts) = U^2 / Z, where Z is 150 for a 75-ohm resistor or 100 for a 50-ohm resistor, noting that actual RF power may be lower. After completing these adjustments, if all is functioning correctly, the antenna can be connected, and all trimmers should be readjusted starting from T3. It is critical to check for harmonics and ensure there is no interference on the band by verifying reception on a TV and radio set in a different room.This is very stable, harmonic free, long range fm transmitter circuit which can be used for fm frequencies between 88 and 108 MHz. This can cover 5km range (long distance). It has a very stable oscillator because you use LM7809 stabilizer which is a 9V stabilized power supply for T1 transistor and for frequency adjustment that can be achieved by u

sing 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, 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 collector, this is used to make a resonance circuit that will force T4 to amplify better and get rid of those unwanted harmonics. L2 and L3 coils must be at 90 degrees angle one to another, this is to avoid frequency and parasite coupling.

The final stage of the long range rf transmitter is equiped with any rf power transistor that has at least 1 watt output power. Use transistors like 2N3866, 2N4427, 2N3553, BLX65, KT920A, 2N3375, BLY81, 2SC1970 or 2SC1971 if you want to have a pro fm transmitter with enough power to cover a long range area.

If you use 2N2219 you will get no more than 400mW. Use a good heatsink for the T5 transistor as it gets a little hot. Use a good 12V/1Amp minimum stabilized power supply. Start by construction the oscillator stage, solder a small wire to T1 10pF capacitor out and listening to a fm receiver, trim the 10k pot untill you can hear  a blank noise or you you plug in an audio source you can hear the music. With a 70cm wire you can cover a 2 3 meter area just with the oscillator stage. Then continue to build the rest of the rf transmitter, use proper shielding as indicated in the circuit schematic.

When you finished the transmitter construction connect the antena or better a 50 or 75 © resistive load and use this rf probe, you can use 1N4148 diode instead of the probe diode. Adjust again the 10k pot to desired freq. and then go to T4 stage and trim the first collector trimmer for maximum voltage indication on the multimeter.

Then continue with the next trimmer and so on. Then go back to the first trimmer and readjust again untill you obtain the highest voltage on the multimeter. For 1 watt rf power you can measure a 12 to 16 Voltage. The formula is P (in watt) is equal to U2 / Z, where Z is 150 for 75 © resistor or 100 for 50 © resistor, but you must remember that the real rf power is lower.

After those adjustment, if everything is going well connect the antenna, continue using the rf probe, readjust again all the trimmers starting from T3. Make sure you don`t have harmonics, check your TV and radio set to see if there is disturbance on the band.

Check this in another room 🔗 External reference