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

[Dan Schaaf]

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.


Best Regards
Dan Schaaf
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-----Original Message----- 
From: Jim Lux
Sent: Saturday, December 14, 2013 9:09 AM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?

On 12/13/13 10:55 PM, Jim Brown wrote:

  4) The major issue with SWR at the
> transmitter is giving the transmitter a load that it is "happy" with.
> "Happy" means that it supplies full rated power, AND is CLEAN.
> Distortion rises when an amp is mismatched to its load and if it's not
> carefully tuned. Distortion = SPLATTER. That's why God made antenna 
> tuners.
>

I'm not sure this is true in general. What is the mechanism for the
increase in distortion?

Yes, if you adjust knobs to get 1 kW into 100 ohms from an amplifier
designed to get 1kW into 50 ohms, you're probably going to be clipping,
because you need more voltage for the same power.  But is that a
realistic scenario? Do people adjust their amplifiers this way (or are
they even adjustable.. most SS amps basically a "power brick")

Is that what you mean by "not carefully tuned"?  But is that not also
the case for 50 ohm resistive loads?  If I mistune the amplifier it's
going to perform non-optimally, regardless of the load Z

Devices operated in Class C will have different harmonic content
depending on the load impedance.  For instance, some L-band bipolar
transistors have significantly different 2nd and 3rd harmonic output
depending on the load Z, but that's more of a interaction between the
parasitics of the device and the external tuning components, so you
basically are "retuning a filter".


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