Dr. Slick wrote:
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.

Once again, you're confusing the SWR meter reading with the SWR on the
line. When the line and SWR impedances are different, the two are *not*
the same. When I speak of the SWR on the line, I mean the SWR on the
line, not the meter reading.

Now look at the sentence you wrote. It's not very clear to me, but it
would certainly make more sense if you replaced "the SWR" with "the SWR
meter reading". Do *not* confuse the two.




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.

I'm saying that the line SWR doesn't change when you change the source
impedance. I didn't say anything about incident or reflected power
anywhere. And I won't. Cecil probably said something about the incident
and reflected power, but explanations in those terms are strictly up to him.

If you were to change the impedance of the left hand line (the one
between the PA and meter), then the SWR on the left hand line would
change, but the SWR on the right hand line wouldn't, and the SWR meter
reading would remain the same. For that matter, you can do anything you
want between the SWR meter and the PA -- add an impedance of any kind in
series or parallel, change the left hand transmission line length and/or
Z0, change the power, whatever you want, and it won't change either the
meter's indicated SWR or the actual SWR on the right hand line. The rule
is that whatever you change, it won't affect the SWR on any line that's
"downstream" (toward the load) from the change you made.

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.

Be really, really careful when you start talking about forward and
reflected power. It can very easily lead you to wrong conclusions about
what's going on. Just check the postings on this group for the past few
months for evidence. All the phenomena you can observe and measure can
be fully explained by looking at forward and reverse voltage and current
waves, and it's a whole lot less hazardous.

One of the several problems with thinking in terms of forward and
reflected power is that it's universally meant to refer to average
power. So you've lost all time and phase information, making it
impossible to clearly see how the traveling waves interact. If you must
deal with "forward power" and "reverse power", do your thinking and
calculations with voltage and current waves, then calculate the power
when you're all done.

As I said before, the ratio of forward to reflected voltage or current
is independent of the source impedance. That ratio, when measured at the
load, is simply the reflection coefficient at that point.

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).

No, you won't see a change in the SWR at point 1 as you change the PA
impedance. All the fiddling you do with your Smith chart just won't make
it happen. Sorry.

The SWR, voltage, current, impedance, power, reflection coefficient,
waves, or anything else don't change in response to your Smith chart
exercises.

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".

No. When I said change the source impedance, I meant change the source
impedance. Surely we don't need a discussion about what "impedance"
means? When you get out your grease pencil and change the reference
value of your Smith chart, it doesn't magically change the waves on the
line on your workbench.


Understood, assuming this is what you guys mean.


It's time for me to leave this discussion. I've tried to make my
statement as clearly and simply as I know how, but somehow people have
decided that I really meant something else, or that there's this
condition or that condition that cause exceptions to it, or that it all
depends on what you scribble on your Smith chart with a grease pencil.
It bears a striking resemblance to a political science (what an
oxymoron!) course I took, in which we could make up any definition for
anything, or any interpretation of anything anyone said or wrote (and
were encouraged to do so), and all were equally valid. I've spent too
much time interacting with engineers and not nearly enough with
politicians and philosophers to know how to deal well with this
fuzziness. Anyone who really cares can look up the equations in a couple
of minutes. I'm sure they're on the web, if you have an aversion to
paper media.

Look up the equations, study them, understand them. If you don't believe
them, make up your own equations. Then set up a couple of simple
experiments to test them, and see which are right. That's how science
and engineering are done.

Roy Lewallen, W7EL