> If the SWR causes excessive voltage in the PA, it will BLOW
> the output transistors. If the SWR causses current in the PA,
> it will cause excessive heat. Hopefully, the SWR protection
> circuitry is designed to prevent both, especially the high
> voltage scenario since that would be fatal.
I think that sums it up pretty well, except to say voltage is
generally limited by the operating Q of any resonant circuits (they
can ring) AND the design of the PA. With class AB or A PA's in
push pull, it's tough to have voltage more than double the supply
voltage because one transistor clamps while the other is off. Single
ended PA's (even linear devices) are different, and can easily go
into a "switching mode" where they ring the tank system and
generate voltages many many times the supply voltage.
I occasionally gave a demonstration in technical classes with a
single MRF-150 FET driving a pi-network, with 12 volts on the drain
of the FET. By mismatching the load the right direction, over 100
volts appears on the drain! Of course in push-pull, that would not
occur because the opposite FET would clamp the voltage at twice
the supply voltage if conduction angle was 180 degrees or longer.
But that demonstration illustrates why tube PA's sometimes blow
bandswitches or arc capacitors when the antenna fails, the feedline
or a trap arcs, or if the gas tube used for lightning protection
flashes over while transmitting.
Depending on load impedance and the design of the PA, almost
anything can happen. Things can run cooler, hotter, better, worse,
or not change at all with SWR.
The results are largely unpredictable, and that is why it is best to
follow what the manufacturer says. They probably have experience
with how their product reacts to loads that are outside the normal.
The only thing I wanted to point out, and drive home, is that tuners
almost always operate better with loads OTHER than 50 ohms,
and SWR doesn't matter a bit with them. While standing waves are
useful in analyzing long transmission lines, they are nearly useless
for understanding what happens inside the box on our desks.
73, Tom W8JI
W8JI@contesting.com
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