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Old June 2nd 11, 10:49 PM posted to rec.radio.amateur.homebrew
Joel Koltner[_2_] Joel Koltner[_2_] is offline
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First recorded activity by RadioBanter: Dec 2007
Posts: 133
Default Killing an RF amp with high VSWR: ZlZ0 vs. ZlZ0?

Something I've wondered for awhile now is this: Since you get the same VSWR
with a load both higher than or lower than your "expected" load (we'll call
this Z0, and the actual load Zl), either Zl/Z0 or Z0/Zl respectively, is one
siutation more likely to kill an amp than the other?

When ZlZ0, you tend to (ignoring phase effects here) end up with a higher
voltage sitting on the finals than when Zl=Z0... but less current than the
matched case. With ZlZ0, it's typically lower voltage but higher current.
Based on what I know from, e.g., power supply design, there's usually more of
a margain from overvoltage than there is from overcurrent, in that you can
often find parts that inherently withstand a 2:1 or better overvoltage margain
with just a bit of extra cost, whereas overcurrent will tend to overheat the
power devices and getting rid of, e.g., 2-4x as much heat (vs. matched case)
can require significantly greater heatsinking which often isn't cheap.

In the extreme case, I'd expect that far more amps will blow when shorted than
when open-circuited. But I don't really have much experience in this area...
so... could anyone comment on what you tend to see in real designs? Perhaps
also compare and contrast tube-based amp with solid-state ones?

Thanks you,
---Joel