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Old 22nd July 2005, 02:01 PM   #1
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Default RF amplifier

I am planing on starting a community radio station and have all the licence end of things completely sussed...

I am however still researching into the best way to get the signal production and transmission side of things all up and running...

I have worked out the kits required for the stage (in order);

Audio Compression/Limiter
Stereo Encoder
PLL FM modulator

but now need to find a solution for a reasonably priced RF amplifier to boost the signal strength significantly...

I found a website where someone was using the Motorola MRF 150 FETs to produce a 600 watt HF solid state amplifier that they were using for HAM radio (http://www.k0gkd.com/ssamp.html)... The schematics they used were derived from a book written by the actual chip designer who worked for motorola...

What I was wondering is if anyone knew if these mosfet could be applied to the situation i speak of (specifically the amplification of a FM modulated signal)???

I know this question might be of n00b status compared to some of the absolute electronic knowledge based discussions posed by individuals on these forums but I really dont know where to turn otherwise...

ANY help would be GREATLY appreciated even if it is jsut being steered towards a different place where this kind of thing is discussed if this in not the place for this question to be posed...

Thanks guyz & Gals
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Old 22nd July 2005, 03:43 PM   #2
Rahul is offline Rahul  India
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Hi!

First question that comes to my mind is do you realy need 600W ??

MF150s are expensive devises but most have been used for making broadband amps upto 50mhz. They will work on FM band with only slight decrease in performance.

I suggest have a look at RF Parts VHF device inventory that should give a good idea.

To begin with I suggest start with something like 20-25 watts. One more option is buy an old VHF band pager unit or high power commercial VHF transmitter they all work around 138-174Mhz and should be easy to bring down in frew with slight modification.

FM amplifiers are biased for Class C operation so are many VHF amps for NBFM communication. Even if you get a linear amp for reasonable price it is simple to disconnect the bias.

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Rahul
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Old 22nd July 2005, 08:08 PM   #3
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As far as I know PLL is FM de-modulator.
From the schematic of amplifier I conclude it is wide-band, so very good for FM.
Check if your modulator/encoder are suitable for FM audio broadcasting, I mean if they contain 'pre-emphasis' filter.
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Old 22nd July 2005, 09:20 PM   #4
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I don't know about Australia but in the United States, we are required to use Government approved equipment for broadcasting.

That being said, I have built a number of FM amplifiers (I'll never admit why I have done this). I never got to the 600 watt power level but I did modify a 160 MHz, 500 watt amplifier to operate in the 88-108MHz band.
The most important thing is to make sure the "Q" of the tuned circuits are low enough so as to avoid amplitude changes when you deviate the carrier.

I wouldn't start with solid state output devices. They can be very difficult to make stable. Also, because of their low output impedance, harmonic filters can be difficult to design.

Try to find a high powered VHF amplifier and modify it for use on the FM band.

Hint: If the amplifier operates at about 160-170MHz, double the turns on the output inductors. That will get you very close to the FM band.

Good Luck!
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Old 22nd July 2005, 10:35 PM   #5
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Quote:
I don't know about Australia but in the United States, we are required to use Government approved equipment for broadcasting.
Yes i have looked into how to obtain the licence and that is easy and relatively cheap too (Community Radio is Noncommercial and licences are about $1200AU). However, I have not looked into the equipment requirements as to whether they need to be standard approved equipment of whether they could be homebrew equipment that needs to just need to meet minimum requirements for singal deviations/ranges/etc... Will look into this now lol

Quote:
Check if your modulator/encoder are suitable for FM audio broadcasting, I mean if they contain 'pre-emphasis' filter.
Yes both the Audio Compressor/Limiter support pre-emphasis aswell as the Stereo Encoder and both are adjustable to either 50uS, 75uS or none...

Quote:
First question that comes to my mind is do you realy need 600W ??
Well yeah I kinda do need something with atleast this power as the area that I live in and intend to set it up in has terrain that will need something with a bit of "oomph", otherwise the signal will make it to about my front door (OK maybe a little further lol)

Quote:
One more option is buy an old VHF band pager unit or high power commercial VHF transmitter they all work around 138-174Mhz and should be easy to bring down in frew with slight modification.
Quote:
Try to find a high powered VHF amplifier and modify it for use on the FM band.

Hint: If the amplifier operates at about 160-170MHz, double the turns on the output inductors. That will get you very close to the FM band.
Thanks for the lead, I will look into this people and thank you for all your advice is has been really helpful
Quote:
Originally Posted by Frank Berry
Try to find a high powered VHF amplifier and modify it for use on the FM band.

Hint: If the amplifier operates at about 160-170MHz, double the turns on the output inductors. That will get you very close to the FM band.
Thanks for the lead, I will looks into this people and would like to thank you for all your advice, is has been really helpful/insightful
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Old 22nd July 2005, 11:28 PM   #6
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oh btw

Quote:
As far as I know PLL is FM de-modulator.
When I said "PLL FM modulator" i more so probably meant "PLL/Modulator/Freq. Generator" which was my understanding to basically be all in one circuit that generally flies under the flag "FM Exciter"....

Please correct me if i'm wrong here...
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Old 23rd July 2005, 12:02 AM   #7
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OK, I have looked into the technical specifications for broadcast services in Australia and short of saying OMG at the technical aspect of it , I was unable to find anything that said that the equipment had to be of certain make or model or the like so at the moment there is still forward momentum...

They are very specific though on what they expect from your signal (p61-69 Apndx II - Tech Info) and here are some examples;

Quote:
  • The width of the radio frequency channel shall be 200 kHz.
  • The channel spacing shall be 200 kHz with the lowest channel centre frequency being 87.7 MHz and the highest channel centre frequency being 107.9 MHz. A carrier frequency offset of 100 kHz may be specified.
  • Unless otherwise specified, the frequency of the unmodulated carrier shall be maintained within 1000 Hz for a transmitter fed with a baseband signal or 2000 Hz for a transmitter fed with a signal derived from another transmitter by frequency conversion
    (i.e. not demodulation).
  • The main carrier shall be centrally located in the channel and shall be frequency modulated by the baseband signal.
  • The maximum frequency deviation of the main carrier (or the sub carrier for that matter) shall not exceed 75 kHz.
  • A positive value of the baseband signal (i.e. when the signal crosses the time axis with a positive slope) shall correspond to a positive frequency deviation of the main carrier.
  • The frequency of the (suppressed) sub-carrier shall be 38,000 4 Hz.
  • Pre-emphasis of the signal S shall be identical with that of the compatible signal M and shall be the same as that for monophonic transmission. The pre-emphasis characteristic of the audio signal shall be in accordance with the admittance/frequency characteristic of a parallel resistance-capacitance circuit having a time constant of 50 microseconds.
This basically summarises alot of what they expect but alot of these specifications come from the FM Exciter region rather than the actual RF amplifier... I can still see that the SNR is going to have to be pretty damn good. Even ensuring that things like a 4Hz (yes thats right 4Hz!) deviation of the 38kHz signal dont happen!!!

All i can see is even if one could get it all together it would be a nightmare trying to find someone with the technical equipment required to ensure that your signal met these specifications rather than the brodcasting authority being the first one to pull you up and give you some fine or somehing... Hmmm...

If anyone wants to offer any help in breaking any of this down a bit further and looking at apects of any part of the process than I would jump at some help lol... Any help with regards to RF amplification, especially if ideas could be coupled with some ideas on harmonic and band pass filtering that would help meet these outlined specifications would be a god send!!!
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Old 23rd July 2005, 04:42 AM   #8
Rahul is offline Rahul  India
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Hi!

Now since you have made it clear that 600W is must a broadband amplifier would not be suitable in my opinion. It is better to go in for tuned one. This would ensure a cleaner signal and better efficiency. Low pass filtering would be must to keep spurii down below the carrier. The filter would be air cored but would require a spectrum analyzer and some 10-20db tap for their proper alignment.

Modding a commercial equipment is still the easy way out.

If you wish to make your own I suggest first try out with cheaper Class C amp using APT MOSFET for industrial use , some app notes are there will look around.

One more aspect do give a thought to antenna system as you can get lot of gain , minimising on power. A 4-6 element colinear should give abt 10dbs ie with only 60W an ERP of 600W.

Regards

Rahul
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Old 23rd July 2005, 07:06 AM   #9
PRR is offline PRR  United States
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I'm sure Australia's broadcast laws are roughly similar to the US, Canada, and the UK.

Which means: while you CAN build-your-own, you have to "proof" it against ALL the regulations (you barely scratched the surface), and that really costs more than the transmitter. You don't build one transmitter; you build and proof one and then make dozens to spread the cost around. i.e. commercial gear.

Fer eggample: Transmitters have harmonic distortion just like audio amplifiers. Simple example: you transmit 600W on 90MHz, a Class-C amp with simple tuned circuit may transmit 5 or 10 watts on the second harmonic 180mHz. If aircraft, police, military, or other services are working on 180MHz, that leakage could wipe out their communications and make them very unhappy. You may be granted the right to be the dominant signal in your area at 90MHz, but no right to cause any interference on 180, 270, 360MHz, or the cross-modulation products that a sloppy transmitter can spew all over the RF spectrum. If you can prove rather strict out-of-band requirements, you have a leg to stand on if someone complains of OOB interference (you may still be asked to cooperate to mitigate the trouble).

If you are going to run even a Watt in the suburbs or 50 watts out in the desert, you NEED a Licensed Engineer. US FCC regulations have changed a LOT since I was a wee lad: we used to have to have a specific "Ticket" manning the transmitter any time it was on. It seems that advancing technology (more stable equipment that self-logs and will auto-shut-down at the slightest flaw) have reduced this to an annual check-up. Still, for initial set-up, you NEED a Broadcast Engineer to guide you through the maze, both regulations and practical details. I used to get the US broadcast magazines, and the classified ads were full of companies who did broadcast technical consultation.

Your best plan may be to buy a used transmitter. The transmitter companies are often the best source: they sell new stuff because it takes less oversight than the old stuff, and then they have to take the old stuff as trade-in. If you can work under a lesser level of technical oversight and logging than an urban broadcaster (if nobody is going to pitch a complaint when you broadcast 630 watts on a 600 watt license, as they do in the overcrowded city) an older transmitter may be just your thing. Or maybe not: if it is prone to fail once in a while, and you have to order expensive parts from far away, you may quickly pay more for repairs than a new transmitter. Another reason transmitters get retired is the new switching power supplies are more efficient than the old big-iron power supplies: the 1KW power bill will add-up over the years.

There must be broadcast engineers' forums around the Web. There may be one for Australian BEs. Or maybe not: there are only 272 commercial radio stations in the country (we must have almost that many in-range of the NYC area). You know there is funding (and surely support and advice, though probably massive bureaucracy) for Australian Community Broadcasting; see http://www.cbf.com.au/ and "Funds are available to help long term licensed community radio stations purchase necessary transmission equipment." More grants if you can claim Ethnic or Indigenous service.

I didn't say this, but: if you are really so "far out" that you need big power to reach a radio-starved audience, you might get away with a bootleg (unlicensed) operation. Get one of the Ramsey Kit "export" transmitters; they have more power than several of the radio stations in my area. You are still liable if you knock-out an aircraft radio at a critical moment, etc, so you didn't get this idea from me. Unless your police-state is better than ours, if you don't do any harm, the first legal step (assuming anybody notices) would be a "DESIST!" order. When you go off the air, you will find out how much your audience loves you, and can leverage their outrage into a more legal setup.
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Old 23rd July 2005, 07:58 AM   #10
PRR is offline PRR  United States
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> if these mosfet could be applied to the situation i speak of (specifically the amplification of a FM modulated signal)???

Yes, but not like that.

The plan shown is a single-sideband "linear" amplifier. In audio terms, it is a class AB amplifier biased for an idle current of about 3% of the maximum peak current (pretty darn "B" by tube-audio standards, though transistor folks would call it "AB"). Its output is proportional to its input. That works for AM or for SSB, though AM is so inefficient that you almost never bother with "linear" amplifiers.

It would work for FM but it isn't right. In FM you do NOT want your output power to vary at all. So all this tricky bias stuff is pointless. And this amp is not made to run full-power 100% of the time, it taps full power only on speech peaks. For FM work you would want to bias and drive into class C: hard-off when drive fails, and always driven hard-saturated for maximum efficiency. Total fuzz-box. That leads to more re-optimizations than I can think of.

And his filtering scheme may be legal for amateur radio, which has considerable leeway for experimental gear, but may be unacceptable for the broadcast bands.

If I am reading it right, he only built for up to the 10 meter band, which is around 30mHz, far below your ~100MHz. Yes, the citation implies up to 150mHz, but working the last octave of a device's "potential" is usually a job for Experts.

> the area that I live in and intend to set it up in has terrain that will need something with a bit of "oomph", otherwise the signal will make it to about my front door

For short-range over hilly country, consider 1MHz (AM band) instead of 100MHZ (FM band). A typical hill is "small" compared to a 600KHz wave, the wave just slides over the top and into the valley like treacle. But a hill is "large" compared to a 100MHz wave, a perfect obstacle, you are just warming outer space and not getting any power to the far side of the hill.

Do not confuse the short-range case with the more familiar situation of driving 100 miles of of the city into the hills, and AM going in and out. AM "skips" off the ionosphere and at 100 miles you will get both the ground-wave and a sky-wave, generally out of phase but similar amplitude. Small variations in either one can reach a near-perfect null, or worse the imperfect nulling of one sideband more than the other. AM over a distance in hills is bad; short-range AM tends to flood hills well.

OTOH, an efficient AM-band antenna is a quarter-mile tall and a Major Investment. You can comprimise the size but at the cost of power required. OTOOH, if most of your audience is in one direction, and you can erect telephone poles over a large field, a Rhombic antenna 1,000 feet long and 50 feet off the ground can throw a lot of 1mHz horizontally. (That's basically how the US Navy throws so much 20KHz power that it can crisscross the world and penetrate several feet underwater, to reach hidden submarines. Their exact size is IIRC secret, but a couple miles of wire on towers and a few megawatts does the trick. Scaling-down, 500 watts should throw hundreds of miles.)
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