I think your grandfather's estimation of what's happening is certainly close with the "too much power" thingy, but I don't think I'd narrow it down to the antenna being the problem without some more checking or thought.
It's a fact, changing power doesn't mean the SWR will change. It can, but if it does then there's a reason for that change. I'm not talking about the meter and calibrating it, I mean that something changes in the antenna system, OR, something changes with whatever is delivering that power to the antenna system, the radio or amplifier. I haven't ever used that antenna so can't say from first hand experience that it can handle 500 watts, 1000 watts, 3000 watts or whatever. I have to go by what other people have experienced with it, and the general consensus is that it whould handle 1000 watts give or take some.
The next thing is the coax used. Since the run isn't all that long at all with the average mobile, almost any coax should be able to handle 1000 watts if the antenna (load on the end of the coax) is tuned halfway decently. The coax you are using certainly ought to work fine. Checking coax with only an SWR meter is really "iffy" at best, but it can give you at least an indication of how it's working. That would mean putting a known load on the end of that coax for that test, and that's sort of "iffy" too, all things considered. Just for grins, try doing it like you've done before when the SWR goes up and wiggle the coax. Any change, good/bad with that SWR when you do that? How about the jumper between the amplifier and SWR meter, any changes when wiggled? You see where I'm going with that, right?
And the next thing in line is the amplifier. There are two paths for RF through that amplifier. One is the by-pass circuit which is active when the amplifier is off, just sort of goes around the amplifier circuit. From what you've said, that seems to be okay. The other path through that amplifier is the one that does go through the amplifying circuit, and where the problem seems to be, since the problem only happens when the thing is 'on', AND is producing more than 500 watts, give or take. that's where things get sort of more complex, sort of. The first thing the antenna system see on entering that amplifier is the output circuitry which matches the amplifier's output impedance to the feed line's impedance. That circuit is a 'no tune' thingy unless you have to tune the amplifier's output, which you don't have to do. Those 'no tune' circuits are NEVER exactly right, they are always a compromise. A "one size fits all" thing never fits everyone well, it always fits some better than others. If it ain't got 'elastic' in it to 'shrink' the size, it isn't going to fit very well, hats, pants, belts, shoes, you name it. Sometimes even with that 'elastic' in it, the elastic' is the wrong size or it's lost it's 'stretch'. That 'no tune' circuit is the 'elastic' in an amplifier. (Long 'stretch', but still holds true. And I love puns!)
No matter how careful the builder is, that 'no tune' circuit has to be very close to right, or it just isn't gong to work well. Unfortunately, it's one of the things that's very easy to get 'not right', and can certainly cause what you are seeing. You have two choices when checking that output circuit. There's a fairly simple way of checking that circuit, but most people don't have the dummy load to do it with. That requires a fairly large dummy load cuz of the power levels involved, 1000 watt range. Run that amplifier through an SWR meter into a dummy load of the right size and you should see a good SWR. If you vary the power, adjust the SWR meter correctly if it's needed, then that SWR shouldn't change except a very, very small amount. If it isn't better than something like 1.5:1, then I'd say that 'no tune' circuit isn't right. Wouldn't you?
Ain't got one'a them dummy loads of that size? Don't feel bad, most people don't (me too at the moment). Find someone who does, a technician, or
CB shop??
Those are the 'easier' and most common problem areas. If you don't find a problem in those areas, oh man are you gonna have fun, it gets complicated!
What leads me to think the problem might be in the amplifier is that there doesn't seem to be a problem when it isn't used.
The first thing to 'rule out' is the antenna system, make sure it's right. Then what are you left with?
- 'Doc
Sorry that was so long, but I don't know another way of saying all that, that's shorter.
(I'd make a joke about being paid by the word, but someone would probably believe it, I wish!)
I'm also not going back through all that to correct spelling or grammar! Forget it!
U 212
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