Topband: EZNEC 5.0 +

[Tom W8JI]

> >We can have 67% reflected power and still have nearly 100% of transmitter 
> >power getting into the antenna and being radiated.
>
> Then could someone please explain why the manufacturers of ham, broadcast 
> AM/FM/TV, and other transmitters specify the maximum SWR (e.g., minimum 
> return loss) for the loads they may drive at full, rated output power (no 
> foldback)?
>
> If "nearly 100%" of the r-f power output of such transmitters was radiated 
> by the antenna system regardless its VSWR/return loss, what would be the 
> need for such OEMs to specify a maximum load SWR?

Easy. It has nothing to do with reflected waves reaching the PA device. 
"Reflected power", or SWR, can actually make a PA run cooler.

The job of a tank circuit or output matching circuit is to establish a 
LOADLINE for the power amplifier device. This is found through a form of 
Fourier analysis of the time varying voltage and current at the output 
device, or through experimentation. This loadline is selected to provide 
either the best energy transfer from the device, the best linearity, or some 
other target goal or combination of goals.

When the load mismatches the output impedance transformation design 
termination impedance, the PA load impedance deviates from optimum.

As far as the PA stage goes, SWR really just indicates an impedance mismatch 
from ideal. The PA termination impedance change is transformed through the 
output matching to move the loadline.

If the tank system can be readjusted to a new value, you can have a 50:1 
feedline SWR and not hurt the PA system. The only limit is voltage and 
current rating of components up to the output device end of the matching 
system.

But again, a mismatch from the impedance the tank was adjusted for simply 
changes the loadline seen by the PA device.  Mismatch can make PA heat 
decrease, increase, or stay the same. It can make distortion increase, 
decrease, or stay the same. It is nothing other than a change in loadline to 
the PA device.

http://www.w8ji.com/Vacuum_tube_amps.htm

By the way, there is no source impedance to match in a typical PA.  The PA 
device impedance is varying constantly through the RF cycle. With class B or 
shorter angles, the PA device is switching completely off for a portion of 
the cycle. The only "impedance" is loss loading of the system.  This is why 
tank voltages on the anode of a tube can go ten or twenty times the DC 
supply voltages with certain termination impedances.

http://www.w8ji.com/demonstation.htm