> Short answer is the only thing SWR is good for is determining at what
> point the RF voltages induced by the mismatch will damage your
> transmitter system. This is why modern rigs fold back transmitter
> power when excessive SWR is detected, This keeps the voltage developed
> at a low enough level to prevent damaging the finals.
I think the importance VSWR in HF multiband operation lies somewhere in
between extreme views. I need to know three things before I become
worried about VSWR: (1) line characteristic Z; (2); the matched line loss
spec in dB/ft.; and (3) antenna input terminal Z range. If I know these
answers, I can then compute VSWR and determine systematic loss due to a
combination of VSWR and the manufactures specified matched line loss in
dB/ft. Also, when I know these answers, I can compute an optimum line
length for maximum power transfer from the transmitter into the line when
the line characteristic Z is not equal to the antenna input Z.
What bothers me about the "who cares about VSWR" crowd is that they are
often ignoring total system losses as a result of unknown VSWR, unknown
matched line loss, and transfer loss between the Tx and line. Even if my
line had zero loss, the resulting Z seen at the TX end of the line can make
power transfer inefficient from the rig into the line.
It doesn't take much to get close to reasonable values of line VSWR (and
only then say "who cares about VSWR") and input Z at the Tx side of the line
to make the system reasonably efficient. But I would not want to hide my
head in the sand and use some random length of line, and some random length
of antenna -- and expect my transmatch to efficiently create a conjugate
match. The system may be tuned and resonated for reactance cancellation,
but wouldn't it also be nice to know how much was lost in the process? If
the answer is yes, then you need to know more about the antenna system
especially the line Z to antenna Z mis(match) and specified matched line
loss.
Paul, W9AC
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