Joel Koltner wrote:
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? ...
A fairly tricky question
You are right that voltage margin is usually less expensive to obtain
than current margin, but in RF devices this is not always true; e.g.,
the classic MRF316 can withstand 9 amps and 35 volts, for a nominal
output (VHF) power of 80 W, and is guaranteed to survive a 30:1 mismatch
(with any phase angle), but other transistors are less tolerant - e.g.
the widely known MRF455 is rated for 18V (only) and 15A for a 60W power
(HF).
But the main question is that the output matching network transforms
load impedances in a way not easy for the final user to predict, so a
short on output may appear to the device as something very different,
whilst a reactive load could well appear like a short or open circuit or
something like that. Adding to this the transformation of load impedance
caused by the transmission line, we can hardly know the difference
between the effects of a load impedance and those of another - and this
is why the specifications are usually given in terms of generic mismatch.
As for the difference in behaviour between solid state and tube amps,
the latter are notoriously more tolerant because failures in tubes are
usually rather "slow" (causes are mainly overheating, but also
evaporation of electrodes due to excessive current or disruptive
discharge due to overvoltage, and each of these causes requires a
certain amount of time to reach "final damage"), while the fusion of a
part of a semiconductor device (due to excessive current or power
dissipation, or formation of hot spots) or the disruptive discharge in a
junction (usually due to avalanche effect) are very fast phenomena,
which kill the device in very little time, or even in almost no time at
all... but all this is true at more or less any working frequency, not
only in RF.
Hoping to have shed some little light...
--
73 es 51 de i3hev, op. mario
Non è Radioamatore, se non gli fuma il saldatore!
- Campagna 2006 "Il Radioamatore non è uno che ascolta la radio"
it.hobby.radioamatori.moderato
http://digilander.libero.it/hamweb
http://digilander.libero.it/esperantovenezia