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50 W long range FM transmitter

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Feb 25, 2022
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I want to build a high power FM transmitter no matter the cost and time it takes. I am well versed in Audio amplifier design, as I have designed and build quite a couple of
PA amps. I am however completely new to RF. I have build 1 W spybugs and such so construction and shielding of RF circuits is not a problem. The problem is where to get the right scehatic. I searched on the web and i would like to ask you professionals, are these two schematics mergable? The first one as a transmitter circuit and the second as an RF power amp. I do not have a clue what all of those stabilization components are in the power section and furthermore, what kind of antenna should i use with this for the FM band? (88 to 108 Mhz).

RF Power amp.PNG

All those coils and capacitors need to be the right values (it helps if they're tunable) so the output is oscillations within a narrow range of frequencies. It's a tall order. The oscillations must be audio modulated. Another tall order. A 100 MHz oscilloscope seems essential.

I purchased a kit for $10 mail order. It has 2 transistors and the proper components. It let me hear my own voice come from a radio ten feet away.

Be careful how many Watts you build into your transmitter. Perhaps you wish to have your own micro-power FM broadcasting station? Then the FCC requires that you get a license and agree to a few obligations.

One watt is sufficient to send about a mile, say.

A very convenient reason to make a low-power FM transmitter is so you can send to your car's FM radio, playing your iPod, or phone, or tape player, or CD player sitting beside you. An antenna may even be unnecessary.

The first one is a RF Power Amplifier, the second one is a Transmitter Circuit.
They can work or not coupled together, nobody may say something without analyzing these circuits for the time being.I suggest you to realize the PA first and test it very well and ensure that the PA works well in your specifications, then realize the second one.
Because if you obtain a good response from the PA, second one will be easier.
But -as you said- if you're new to RF, I recommend you to read some related textbooks and application notes about RF, especially RF Power Amplifiers because RF is not like AF. RF Power Amplifiers particularly need very strong experiences and measurement techniques so it's really tough.
You have to find a appropriate Spectrum Analyzer, RF Power Meter, High Watt Attenuators,Signal Generator, proper cables etc. You have a long road to walk.
And don't forget to arrange a Transmitter Antenna.

As @BradtheRad mentions, the local regulating authority in your country (FCC in the US but there are equivalents in all other countries) take a dim view of unlicensed transmissions - especially if they interfere with other transmissions from licensed broadcasters.
Penalties can include fines, jail time and confiscation of equipment.
One thing that badly designed and/or constructed circuits can do is to 'splatter' signals in frequencies other than the main carrier frequency. You might think you have chosen a frequency that does not match any other in your area but you need a very carefully designed circuit to stop unintended interference in many other frequencies.

Passing over the situation about your license transmitting 50W in FM broadcast band, I would recommend to use one of the modern MOSFET transistors (as RD70HVF1) instead of using old BJT power transistors. They support higher VSWR at the output, have higher gain, and much better reliability. Check the net for schematics.

Take a look at Amatuer Radio Handbooks, full of info for Transmitters. You can borrow
them here -

Another resource is FM and Circuits handbooks

This site is loaded with various Radio Amateur magazines and books full of designs. Look at "Repeater" articles and the like as well.

Lastly the governing body, FCC, power limitations, un licensed and licensed.

Regards, Dana.

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