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

[Jim Lux]

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".