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Need schematic for FM transmitter ~100 Watt to start Pirate Radio Stat

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#1 alhadi


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Posted 15 June 2011 - 09:25 AM

Hello i need a schematic for a ~100+ watt FM transmitter. I want to be able to broadcast for 10-20 KM. 100 Watt should do the trick, right?
Anyways, can anyone provide a circuit?
I will be broadcasting on the 100MHZ Frequency band in the FM. My Antenna is mounted 50 feet above ground. Is this high enough?

I looked up at a transmitter, but I don't want to shell out $500 for this, i'd rather buy the parts and build it myself.

Please, I really want to do this.
Also, What length does my antenna have to be?
Yes, I will also stream it online so others can listen.

:wink: :wink:

#2 counter440


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Posted 17 June 2011 - 05:10 PM

Good to see that there are other people out there taking interest in the good old FM transmitting ;P
I have been working with illegal FM transmitting for a year now, and I gotta say that I have learned a lot about it.

As for the 100W FM transmitter you are trying to build. That isnt as simple as it sounds like , and it requires a lot of adjustment to make the spools fit the frequency so that you dont end up transmitting to the closest Flight tower or Police station ;) And 100W in itself is wayyyy to much for a pirateradio in my opinion. I can transmit about 20km with my 30W placed at a good altitude, so try imagining how far a 100W could go.

Would you mind giving some more information about where you are going to broadcast from , like in a city ?

#3 phaedrus


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Posted 12 August 2011 - 08:32 PM

I'm with the other poster. You are better off starting with a much lower power output, and learning about antenna's and how to make them more efficient, than sending out a dirty noisy overdriven signal that will get everyone in the neighborhood's back up and hunting for you.
Radio ham's manage to get 1/4w to get great distances even on fm line of sight by clever antenna design, careful impedance matching, high quality coax and low loss plugs and careful antenna placement. Admittedly you can't use a highly directional antenna like a multi element yagi or a quad of some description to gain a lot of db because you want to radiate in all directions to your audience, but you can still build say a vertical half wave dipole antenna that is omnidirectional (the radiated signal is donut shaped, but the strongest bit is straight out where you want it) and apply the same principles to get everything matched correctly to everything else, low swr so that your transmitter isn't overworked for nothing and your not splattering harmonics everywhere, and keep it out the way of obstructions that distort the radiated field direction, place it as high as possible etc.

I also think his advice to drop your expectations down to the 10-20w output range is solid, and if you follow that good advice a lot more options for tx'ers become viable. If you really need more later on, its then not difficult to build a class A linear amp setup and just amplify what you already have once it is all working correctly. Or just buy one in.

To actually answer one of your questions, if you build a halfwave dipole, it has to be (nearly) half wave. At 100Mhz, wavelength is 3m. So half wave is 1.5m in total making each leg 0.75m long. Which will give you near to 75 ohms impedance at the centre feed point, mating up nicely to 75 ohm coax. You can fine tune the impedance by altering slightly the length of the arms. You can measure the mismatch between the two with a swr meter as usual.

If you make it out of larger diameter tubing instead of wire, it will have more bandwidth but be nearer 0.47 wavelength instead before it resonates. So then each leg of the dipole will be 0.70m long.
Also worthy of a mention, folded J match dipoles (slim jim's), someone's already done all the work for you :-


Finally, mount it as high as possible. Because of the curvature of earth, and objects getting in the way of the line of sight waves, height is your very best tool. I used to run a half wave vertical dipole bolted the back of a house, and a tiny royale discone on the chimney pot some 5-6m higher, and on 11m the discone with a really high (>4) swr reading outperformed the dedicated 11m antenna with its 0.3 swr by a huge margin...

Enough waffle from me... Good luck.

#4 bitTwiddler


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Posted 30 August 2012 - 07:13 PM

Any updates? I am interested to hear how your project is proceeding.

Edited by bitTwiddler, 30 August 2012 - 07:13 PM.

#5 ticom


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Posted 03 September 2012 - 09:16 PM

Homebrewing 100 watts on any frequency, while definitely within the hobbyist realm of possibility, is not something a beginner can slap together and hope works, especially without the right tools and test equipment. Since you're asking the question on this forum, it's safe to assume that you have pretty close to nothing when it comes to test equipment and knowledge. The former is going to cost you some money. The latter not so much, and with a little bit of knowledge you can build a lot of test equipment and not have to shell out as much money.

Your unattainable $500 price point for equipment is kind of interesting. I saw a commercial FM BCB exciter for $10 at a MIT Flea this past season. The thing stayed on the table until the end of the hamfest, when my fellow former IIRG member all but threw it at me because he didn't want to load it back in his van. Similarly, a local broadcast engineer was giving away 300 watt Harris PA decks from his FM station after a recent equipment upgrade. Turns out with a little matching network modifications they work just fine on both the 2 and 6 meter ham bands. The stuff is out there, and it's pretty cheap. You just have to know what to look for and where to look.

In matters concerning pirate radio it comes down to whether the individual is a techie looking to play with radio, or an activist type who thinks they can magically throw together a station (or get a techie to do it for them) so they can go push whatever sociopolitical agenda they are into. Can't do much with the latter unless they want to take the time and effort to also become the former (rare, but it happens.) Now as far as the former are concerned, I'd start by reading through the NEETS series to get a good education in electronics, followed by a recent copy of an ARRL Handbook For Radio Communications, and maybe even a copy of Experimental Methods In RF Design. Concurrently, establish contact with a local group of "cool" radio hobbyists who can show you some pointers, like cheap radio gear and test equipment that's probably sitting right under your nose.

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