Roy Lewallen wrote in message ...

The observation that changing line length changes the measured SWR is
regularly reported in this newsgroup, and the explanation is as
regularly provided.

There are at least three ways this can happen.

1. The SWR meter is designed for an SWR that's different from the line
impedance. It's easy to show that this will result in different readings
for different line lengths. I've measured RG-58 at over 60 ohms
characteristic impedance, so this can happen even with a perfect 50 ohm
SWR meter and "50 ohm" line. In this case, changing line length isn't
really changing the line's SWR, just the meter reading.


But a 60 ohm transmission line transformation from a non-50 ohm
load will certainly change the SWR, as it won't be on the constant
VSWR circle anymore.



2. There's significant loss in the cable. In that case, the longer the
distance between the meter and the load, the better the SWR.


That's the obvious one.


3. There's current on the outside of the coax. This means that the
outside of the transmission line is actually part of the antenna. When
you change its length, it changes the effective length of the antenna,
which really does change the SWR. Current on the outside of the cable
can also get into a poorly shielded SWR meter and modify its reading.

And this is all in agreement with established theory. So you see, theory
does say you can change the SWR reading, and in some cases, the actual
SWR, by changing the coax length. But only under very specific
circumstances. When observations don't match theory, chances are
overwhelming high that either the observation is erroneous or
misinterpreted, or theory is being misapplied.

Roy Lewallen, W7EL



I believe the source of confusion is he


PA----+----50 ohm line----+SWR meter+----50 ohm line----+50 ohm dummy
load
1 2 3



The "+" are connector points. You folks are saying that as you
change the PA (source) impedance, that the SWR you read will remain
the same, even if the incident power changes due to the change in
reflected power at point 1.

This i can agree with, as no matter how much incident power makes
it past the impedance discontinuity at point 1, the system after this
point will theoretically always reflect the same ratio of reflected
power to incident.
But, if you place an SWR meter of the same impedance as the output
of the PA at point 1, you will definitely see a change in SWR at point
1 as you change the PA impedance, as you are changing the reference
impedance (center of Smith re-normalized). This is what i thought you
meant when you said "change the source impedance", but you meant to
say "change the source, but keep the reference impedance the same".

Understood, assuming this is what you guys mean.


Slick