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[TowerTalk] ++ High SWR and Modern Transceivers

To: <towertalk@contesting.com>
Subject: [TowerTalk] ++ High SWR and Modern Transceivers
From: W8JI@contesting.com (Tom Rauch)
Date: Wed, 12 Dec 2001 11:15:27 -0500
> 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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