Before the advent of transmitters with fixed impedance output circuits the vast
majority of amateur radio operators didn't worry about SWR. Having an SWR
bridge was not high on the list of items which were found in an amateur radio
station.
Basically, if your transmitter would "load" the transmission line and if you
could make contacts, you didn't worry about SWR. If you had problems "loading"
the feedline or if you had problems "getting out", then you worked on the
antenna until you started making contacts.
At least one of my transmitters can "handle" an SWR of at least 40:1 and I have
several that can "handle" an SWR of at least 24:1. The result is that if I
happen to use one of those transmitters I just don't worry about SWR. Yes, the
higher the SWR the less efficient the system generally is. But, like in the
"goode olde dayes", the "proof is in the pudding" in that if I can easily make
contacts then, frankly, I couldn't care less about improving the SWR,
especially to the extent that some amateur radio operators believe that you
have to achieve.
Glen, K9STH
Website: http://k9sth.com
--- On Thu, 4/23/09, Paul Christensen <w9ac@arrl.net> wrote:
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.
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