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

[Jim Lux]

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)