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Author Topic: out of band amp operation  (Read 4273 times)
KE7KLY
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Posts: 65




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« on: January 02, 2012, 05:39:40 PM »

I thought about posing this question in the Elmers forum to get a broader audience, but know it really should be posted here under amplifers first, at least until I see the responce it gets.
I am a pilot and mariner as well as ham operator, and have handhelds for all of those frequencies. I own an MFJ/Mirage B-310-G 2meter linear amp that is rated at 5 in, 100 out between 144mhz-148mhz. The marine frequencies are about 10mhz above that in the mid 150's and many of the aviation comm frequencies are a little more than 10mhz below in the early 130's.
Question:  What would the affect be on the amplifier and on the quality of transmissions if I used the amp for either of those freq. ranges, ---- i.e., would I have high swr into the radio? would there be some sort of spurious noise in my transmission? would it overload the amps circuitry somehow? etc. etc.
I don't really have any intention of using the amp in this manner, but it's a question I have wondered about.   Any thoughts on it would be welcome.
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K6AER
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« Reply #1 on: January 02, 2012, 07:01:46 PM »

First off the amplifier won't go there.

Second the amplifier is not certificated for use in ether of those bands.

Aviation radios are very specific in AM operation and additional amplifiers are not allowed unless designed as part of the radio by the manufacture. Any thing added to an aircraft requires a 337 certificate and be inspected by the local GADO office and avionics shop.

Marine radios are also type accepted and any outboard amplifier will get your vessel being escorted by the Coast Guard for a long dry dock session.
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NA0AA
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« Reply #2 on: January 02, 2012, 09:03:10 PM »

Using the Mirage for AM would be...interesting, to say the least.

A fixed mount marine radio is 25 watts on FM, which usually goes to an antenna on top of a mast 40' tall or so - which is really good range for FM - adding more power won't help.  A service guy could probably tweak one for the band, but then it would not go back to amateur frequency service too - you have to re-tune the thing.

More or less same situation in your airplane - you are on top of how tall a tower?  5,000'? - I have flown all over the world in light airplanes, there are not many places you could even need an amp, if you do, you are much more likely to need an HF radio instead.

And, since both of these radios are designed for life safety, do you really want to put a bit of amateur gear in line just for a bit 'more power'?  I've considered amateur radio in an airplane but separate from the aviation communication stack.  IIRC, that's even in the statutes as well.
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KE7KLY
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Posts: 65




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« Reply #3 on: January 04, 2012, 06:50:58 AM »

As the person posting this inquiry I will stress the point that I have no intention, desire, reason, or anything else in actually performing the hypothetical proposal.  I am just curiouse about the latitude that amplifiers have.  For instance my Heathkit SB-200 is not able to cover all bands in the HF spectrum. 
I suppose my real question comes down to just what is it in the amplifier that allows for amplification at different frequencies, ---- ocillators? coils? traps? tuned circuits?  I am not an electronics expert, just curious. And as the one responder said, what about the differance between AM an FM (aviation being AM) Lot's of HAM operators still like to operate on AM as opposed to SSB are their amplifiers differant from ones that operate on FM ? 
Try not to flame me, I'm just curious.
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G3RZP
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« Reply #4 on: January 04, 2012, 07:21:31 AM »

Unlike the solid state HF amplifiers, which are inherently wide band through the use of broadband transformers, the VHF and UHF amateur amps usually use tuned matching networks, which is why they won't go far outside the band. Not all SS V/U HF amps are done like that, though - some of the ones used in mil aircraft can cover the full 225 to 400 MHz range: there are various techniques for doing that.

The filtering needed for marine work if you want to use duplex, rather than single or two frequency simplex working, is not so simple - a bit akin to the 2m amateur repeater, although the duplex spacing is much greater, which helps. The filter on the tx side needs to go after the final amplifer stage.

Hope this helps

73

Peter G3RZP
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W6RMK
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Posts: 680




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« Reply #5 on: January 04, 2012, 07:28:08 AM »

Yes, there's a difference between amps designed for AM, FM, and SSB..

FM (and PSK) is "constant envelope": only the frequency and phase of the signal change.  So you can run an amplifier hard into saturation (the output looks like a square wave instead of a big sine wave), and then filter out the harmonics.  This gives you good efficiency (the transistor or tube is acting as a switch).

AM and SSB are varying envelope. To amplify them, you need an amplifier that is linear (the definition of which means that if you put twice the input in, you get twice the output out).  Now, the active device has to be more like a variable resistor, rather than a switch, so the amplifier isn't as efficient at turning DC power into RF.

There ARE clever ways to improve the efficiency of the latter:  for instance, you could have a FM type saturated amplifier where you vary the saturation level.. that's basically what an AM transmitter operating Class C is doing... Or you can have a conventional linear amplifier where the max output is always tracking the input signal level (look up Kahn or EER amplifers).


As for frequency bands..

There's two factors going on:
1) In amplifiers which use tuned circuits for harmonic control (e.g. the saturated FM amp) the tuned circuit needs to be different for different frequencies.
2) People who are putting an amplifier in a system like to see that the input and output impedance of the amplifier is reasonably constant.  Unfortunately, semiconductor devices and tubes don't have constant input and output Z over frequency, so you add a matching circuit to make it so.  Depending on how weird the active device's characteristics are, that could either be a simple or complex circuit.  Narrow band circuits are almost always easier to make (you only have to match over a small range) (hence the bandswitch on most power amplifiers)
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WB2WIK
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« Reply #6 on: January 04, 2012, 09:15:08 AM »

I thought about posing this question in the Elmers forum to get a broader audience, but know it really should be posted here under amplifers first, at least until I see the responce it gets.
I am a pilot and mariner as well as ham operator, and have handhelds for all of those frequencies. I own an MFJ/Mirage B-310-G 2meter linear amp that is rated at 5 in, 100 out between 144mhz-148mhz. The marine frequencies are about 10mhz above that in the mid 150's and many of the aviation comm frequencies are a little more than 10mhz below in the early 130's.
Question:  What would the affect be on the amplifier and on the quality of transmissions if I used the amp for either of those freq. ranges, ---- i.e., would I have high swr into the radio?

Yes, you will.  But also the amplifier drops off in gain and output power a LOT if you change frequency much.  In fact, the B-310-G specifically really doesn't even cover the entire two meter band with flat gain and output power (I've had several of these amps); you can optimize it for the "low end" of the band (CW-SSB) at 144 MHz, or for the "high end" of the band (FM) at 147 MHz, and those are two different tuning points.  It does have internal tuning trimmer capacitors on both the input and output to optimize it for the end of the band you intend to use.  The factory sets them at 146 MHz, in an attempt to cover the whole band, but it really can't quite do that.  At 130 or 160 MHz, the gain drops off to nearly "zero," and you'd be just as well off without the amp.

BTW the B-310-G can only handle "AM" operation at greatly reduced power without overheating very quickly.  Using a linear amplifier on AM is very inefficient; the B-310-G specifically "tests out" okay at about 20W carrier power on AM, which results in 80W PEP AM.  It won't melt down at that level.  If you tried to use it at 100W output on AM, you'd have no modulation (the amp will be so saturated it won't be able to reproduce the modulation) and the amplifier would overheat within a minute or two.

Of course, the legal implications have already been pointed out.
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KE7KLY
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Posts: 65




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« Reply #7 on: January 04, 2012, 02:15:03 PM »

Once again, I was the original poster of this thread,
Thank you sooo much to those of you who quite clearly are very knowledgeable about these things for adding your comments.  Although I am not very clever about this stuff, I am able to follow your remarks, even if not fully understanding all of it.  Wish I understood it all better, but at 60 years of age I'm not really wanting to get that far into it.  For all you others, I am (except for having a current medical), an Airline Transport Pilot, and also past multi-ton Marine Captain. But those glory days are over. Now I just enjoy Amateur Radio.
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