The proposed long range transmitter circuit really is very steady, harmonic free design which you can use with standard fm frequencies between 88 and 108 MHz.

Technical Specifications of the Transmitter


This will likely encompass 5km spectrum (long range). It includes an extremely consistent oscillator for the reason that you employ LM7809 stabilizer that is a 9V stabilized power source for T1 transistor and for frequency realignment that may be reached by means of the 10K linear potentiometer.

The output strength of this long range rf transmitter is approximately 1W however may be more significant should you use transistors like KT920A, BLY8, 2SC1970, 2SC1971…

Transistor T1 is employed as an oscillator stage to present a small power steady frequency. To fine-tune the freq. apply the 10k linear potentiometer this way: should you moderate, in the direction of ground, the freq. would probably decrease but when you fine-tune it in direction of + it would climb.

Essentially the potentiometer is needed just as a flexible power source for the a pair of BB139 varicap diodes.

Both of these diodes function as a changeable capacitor whilst you regulate the pot. By tweaking the diode capacitance the L1 + diodes circuit renders a resonance circuit for T1.

Feel free to employ transistors similar to BF199, BF214 however be careful not to use BCs. At this point you don’t receive yet the long range fm wireless transmitter due to the fact that the electric power is fairly reduced, a maximum of 0.5 mW.

How it Works


The proposed transmitter circuit works in the following manner:

Always encase the oscillator stage in a metal guard to avoid parasite frequencies destabilizing the oscillating stage.

Transistors T2 and T3 functions as a buffer stage, T2 as a voltage amplifier and T3 as a current amp.

This buffer stage is vital for freq stabilization simply because is a tampon circuit between the oscillator and the preamp and final amplifier. It happens to be renowned that bad transmitter layouts normally change freq. whenever you alter the finalized stage.

Using this T2, T3 stage this won’t occur again!

T4 is a preamplifier stage and is employed as a voltage power rf amplifier which enables it to produce adequate power to the ending T5 transistor stage.

As is demonstrated T4 carries a capacitor trimmer in its collector, this is definitely accustomed to render a resonance circuit designed to drive T4 to promote more advantageous situations and do away with those undesirable harmonics.

L2 and L3 coils has to be at 90 degrees perspective one to another, this is to prevent frequency and parasite coupling.

The concluding stage of the long range rf transmitter is equipped with any rf power transistor containing no less than one watt production power.

Utilize transistors like 2N3866, 2N3553, KT920A, 2N3375, 2SC1970 or 2SC1971 should you wish to produce a professional fm transmitter with ample power to take care of an extended spectrum zone. Should you use 2N2219 you will definitely get a maximum of 400mW.

Make use of an effective heatsink for the T5 transistor because it becomes slightly warm. Make use of a reliable 12V/1Amp balanced supply of power.

How to Set-up the Transmitter


Begin by building the oscillator stage, solder a tiny wire to T1 10pF capacitor out and hearing a fm radio, tweak the 10k pot until it is possible to “hear” a blank disturbances or maybe if you connect an music base you could listen to the melodies.

With a 70cm cord it is possible to take care of a 2 – 3 meter region simply with the oscillator stage.

Next carry on and construct the remaining of the rf transmitter, utilize correct shielding as suggested in the above explanation.

As soon as you have completed the transmitter design, hook up the antenna or more effectively a 50 or 75 Ω resistive load and make use of this as a rf probe, feel free to use 1N4148 diode in place of the probe diode.

Fine-tune yet again the 10k pot to favored freq. thereafter go to T4 stage and scale down the initial collector trimmer for highest voltage signal on the multimeter.

After that carry on with the subsequent trimmer and so forth. After that get back on the very first trimmer and readjust yet again until you receive the maximum voltage on the multimeter.

For one watt rf power you could possibly ascertain a twelve to sixteen Voltage. The method is P (in watt) is equivalent to U2 / Z, wherein Z is 150 for 75Ω resistor or 100 for 50Ω resistor, nevertheless one should keep in mind that the proper rf power is lesser.

After those modification, in case things are heading nicely hook up the antenna, keep on employing the rf probe, readjust once more all of the the trimmers right from T3.

Guarantee you don’t have harmonics, verify the TV and radio set to determine if there exists fluctuations on the band. Verify this in an alternative area, a long way away from the fm transmitter or antenna.

The unit is all set up to be used for exchanging music, talks, chats across the suggested range and bands.

Circuit Diagram


 



All Inductors are air cored

L1 = 5 wounds / 23 SWG / 4mm silvered copper
L2 = 6 wounds / 21 SWG / 6mm enamelled copper
L3 = 3 wounds / 19 SWG / 7mm silvered copper
L4 = 6 wounds / 19 SWG / 6mm enamelled copper
L5 = 4 wounds / 19 SWG / 7mm silvered copper

T1 = T2 = T3 = T4 = BF199
T5 = 2N3866 for 1Watt / 2SC1971, BLY81,or 2N3553 for 1.5 to 2W power.

Feedback from Mr. Himzo (a dedicated follower of this website)

Hello Swagatam,

I have few questions about your long range fm transmitter.

Firstly about the shielding, what is the most simplest solution to avoid those "parasite frequencies"?

Secondly, what means those 1nF capacitors at the top? Can they be simple in parallel connection or they need to be separated to every transistor like in scheme?

Thirdly, I sent you a photo of transmitter, I didn't turn on amplifier part because my heatsink is coming. Where can I put antenna for testing without amplifier (T5 stage)?

And lastly, how can I modulate those trimmers if I dont have plastic screwdrivers?

Thank you very very much, this is great project.

Your fan, Himzo.

 



Solving the Circuit Problem

Hello Himzo,

the simplest and the only way to shield the various sensitive stages is by using metal walls between the stages...

the 1nF capacitors should be positioned exactly where these are indicated in the diagram.... the picture which you have shown will never work... transmitter circuits require extreme care as far as their construction and positioning of the components are concerned.

You can never build a long range transmitter successfully on a breadboard, you will have to do it on a well designed PCB which should have a grounded track base layout encompassing all the thinner tracks, only then you can expect the transmitter to work...that too after careful optimization of the trimmers and by employing a compatible antenna.

Regards

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