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Re: [TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?

To: towertalk@contesting.com
Subject: Re: [TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?
From: Jim Lux <jimlux@earthlink.net>
Date: Sat, 14 Dec 2013 07:11:03 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
On 12/14/13 6:32 AM, Dan Schaaf wrote:

Most SS amps, at least my Tokyo HyPower amp, shut down when SWR get to a
point where 80 watts is reflected back into the amp. Does not matter
what the SWR is. It matters how much power comes back down the line.
Sometimes if you try to overdrive it, there will be a light flashing
that indicates overdrive or another light for overvoltage. In any case,
it wants the correct input and the correct SWR within 80 watt limitation.

Yes..
and at any point where the amplifier is not shutdown, does the distortion increase? I suspect that your amplifier (like many solid state PAs) has two states:
Operating into a match that it thinks is acceptable - and distortion is low
Shut down due to reflected power, overdrive etc - no output, so no distortion products to worry about.


What I'm getting at is Jim's assertion that a bad match leads to distortion and splatter. I think a load impedance that is beyond the amplifier's capability to be adjusted fits this case, but not when the amplifier is being operated within its limits.

With tube amps and manual tuning (characteristic of low cost amateur equipment), there's little or no protection against improper operation, and I can see someone having a terrible match, tuning up anyway, and causing a problem.

But with a SSPA using automatic shutdown, maybe not.

I guess there are also poorly implemented automatic controls (e.g. a clipper on the input to the amp)


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