You must not have seen my posting yesterday on this thread, where I gave
the definition. Do I need to post it again?

Roy Lewallen, W7EL

============

No I saw it right *after* I posted. DUH

Jack K9CUN

On 14 Aug 2003 18:46:56 -0700, (Dr. Slick) wrote:

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.

Reading a SWR at this point 1 (or 2 or 3), as specified above, would
simply be zero deflection confirming the remainder of the circuit has
no return.

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.

It will reflect nothing, by specification of the circuit above.

But, if you place an SWR meter of the SAME IMPEDANCE AS THE OUTPUT

But? What has departed will not turn around after the fact of
changing something as immaterial as this presumed meter Z (which is no
more than the minutia of the scale unless circuit gross dimensions
exceed wavelength considerations). If this were significant, the
meter circuit itself would be the point of discontinuity and thus
reflective, but this would be wholly unobserved because
1. The meter pick-up (as it were) follows the discontinuity
2. The power flowing from the meter encounters no other discontinuity
as specified by the circuit above.

Given 2., it follows that zero deflection will still resolve to zero
deflection irrespective of the meter's guts.

To force a reading would require reflection from the output of the
meter's bridge/coupler which simply casts the issue back to a lousy
meter design (but one with apparent high sensitivity - in other words,
it is sucking out a huge amount of power to do the same chore others
do better with less). One can certainly conspire to screw up by
simply ignoring circuit dimension and wavelength - or taking a chance
with selecting an anonymous instrument from the bottom of the drawer
(which is the same thing). Any reading other than zero deflection
from the circuit above would indeed indicate a system failure (the
instrument being part of the system). Take care that failure here is
measured by degree - any reading forces other tests and in and of it
self is not conclusive of anything.

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

Serious SWR measurement at high frequency is far more simply achieved
with greater precision, accuracy, and less impact on the system
employing the rather more creaky methods of a slotted line. With this
approach you can at least relate to the serious considerations of
characteristic Z and wavelength specific issues (and often answered
with simple plumbing components hand crafted once to last decades).
Calibration is as simple as replacing the ruler if it falls off (if
you first confirm that the connections are not reflective, but then we
return to the discussion above).

Once again, it is a chicken and egg problem. You need a better method
of SWR determination to validate a poorer one. There is no such thing
as self validating this particular instrument above HF. A poor Bird
Wattmeter is especially prone to reflective connections (notorious in
fact) and given those connections do this before the reading, many
users are quite satisfied in their state of ignorance. But again,
this hardly amounts to much, unless you wish to achieve an accurate
Power determination. Often the system reflections will eclipse those
of the connectors and still be perfectly useful in an industrial
setting. The user of the Bird in that community doesn't really care
about absolute Power, only relative Power with mediocre absolute
accuracy. Quite often they are simply tasked to confirm levels within
2 or 3dB; the instrument's slop of 1dB is often accounted for within
the methods of measuring those specified levels. The universality of
the Bird is due only to robustness.

Your circuit above is simply the first step in
selecting/calibrating/verifying an instrument. Throw a mismatch into
the game and lights will begin to flash.

73's
Richard Clark, KB7QHC

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

Dr. Slick wrote:
Your vocabulary is very confusing here, Cecil. "Incident"
usually refers to the forward power, so when you say "incident
reflected" it's extremely confusing.

"Forward power incident upon the load" or "Reflected power
incident upon the source" makes sense to me. Given the
definition of "incident", the reflected power has to
be incident upon something. Simply replace "incident upon"
with "arriving at".
--
73, Cecil, W5DXP

Dr. Slick wrote:

W5DXP wrote:
It seems pretty
obvious that not all PA's are Z0-matched so they will always re-reflect
100% of the incident reflected power. But that is exactly what that
definition implies.

Your vocabulary is very confusing here, Cecil. "Incident"
usually refers to the forward power, so when you say "incident
reflected" it's extremely confusing.

The point was not to confuse. So allow me to re-word it.

Some say there is zero power reflected from a PA. For that to
be true, all PA's must exhibit an impedance of Z0, i.e. all PA's
must be Z0-matched. Doesn't that seem a little far-fetched?

Some say that 100% of the reflected power is re-reflected by
the PA. For that to be true, all PA's must exhibit an open,
short, or pure reactance to the reflected waves. Doesn't that
seem a little far-fetched?

The problem lies in the definition of "generated power" which
is forward power minus reflected power. A mental exercise will
illustrate.

XMTR---one second long feedline-----mismatched load

For the first two seconds, a directional wattmeter at the
XMTR reads 100W forward, zero watts reflected.

During steady-state, the directional wattmeter reads 100W
forward, 25 watts reflected.

If the XMTR is a 100W signal generator equipped with a circulator
and load resistor, we have no problem deciding that the signal
generator is generating a continuous 100W and dissipating whatever
reflected energy arrives.

If the XMTR is a ham transmitter, we say it generates 100W for
two seconds, and after that, it generates 75 watts, by definition.
This seems to me to be just a useful shortcut that avoids opening
Pandora's Box (in which the source is located). :-)

If a transmitter only generates (forward power minus reflected
power), it follows that the transmitter always re-reflects 100%
of the reflected power arriving at its terminals. Does that
sound reasonable?

Or, if reflected energy is never re-reflected from a PA, then
the PA must be dissipating the reflected power, which it previously
generated, just like a signal generator with circulator load does.
Does that sound reasonable?

Or, if there is a circulator load, the reflected waves contain
energy, but if there's not a circulator load, the reflected waves
don't contain any energy. Does that sound reasonable?

What sounds reasonable to me is that the reflected waves arriving
back at the source obey the rules of the wave reflection model
as described by Ramo and Whinnery. But since the impedance presented
to the reflected waves by the transmitter is usually unknown, we
are again up that proverbial creek without a paddle.

So, by all means, use the shortcuts, but recognize that they are
definitional shortcuts which may or may not represent reality.
--
73, Cecil, W5DXP



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W5DXP wrote in message ...
Dr. Slick wrote:
Your vocabulary is very confusing here, Cecil. "Incident"
usually refers to the forward power, so when you say "incident
reflected" it's extremely confusing.

"Forward power incident upon the load" or "Reflected power
incident upon the source" makes sense to me. Given the
definition of "incident", the reflected power has to
be incident upon something. Simply replace "incident upon"
with "arriving at".


"Forward Incident" a bit redundant in my opinion. "Incident"
usually refers to the power moving towards the load, away from the
generator.

And your previous quote was: "It seems pretty
obvious that not all PA's are Z0-matched so they will always
re-reflect
100% of the incident reflected power."

So here, you don't say where it is "incident" upon, of where it
is arriving at, athough I assume you mean simply the reflected power.

I'm just trying to make your vocabulary less confusing.


Slick

Dr. Slick wrote:
"Forward Incident" a bit redundant in my opinion. "Incident"
usually refers to the power moving towards the load, away from the
generator.

I don't think that is true. The HP ap note, AN 95-1, refers to
"the voltage wave incident on port 1" and "the voltage wave
incident on port 2". Those two waves are moving in opposite
directions, one toward the load and one toward the source.
--
73, Cecil, W5DXP



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On Fri, 15 Aug 2003 11:39:51 -0700, W5DXP
wrote:

You can't measure phase or magnitude?

I simply cannot differentiate what part of the forward wave has been
reflected back from the source Vs what part is actually generated
by the source. That's the crux of the problem.

Hi Cecil,

But since the impedance presented
to the reflected waves by the transmitter is usually unknown
You have a forward part from the transmitter,
a reverse part from the reflection.

What more do you need unless you are discarding phase? If so, recover
it (if this is all about a SWR meter, then take out the diodes and use
a comparator).

73's
Richard Clark, KB7QHC

Richard Clark wrote:
You have a forward part from the transmitter,
a reverse part from the reflection.

We are talking about re-reflection FROM THE SOURCE!
The forward part and re-reflected part from the source
are coherent and traveling in the same direction so they
cannot be separated for measurement purposes. That's
why Bruene tried to determine the source impedance by
bouncing another separate signal off the source.
--
73, Cecil, W5DXP



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Dr. Slick wrote:
Roy Lewallen wrote in message ...

This is typical of you, Roy, for you to back out of a discussion
when you don't want to admit that maybe someone else has a logical
point. I've agreed with you on many things (antennas as transducers
and such), but like many intelligent but close-minded people, you
cannot accept someone elses points.

I guess you think other people can't teach you anything, eh?

NO ONE knows it all, even about a specific topic as impedance
matching.


Slick

Let me explain why I leave these discussions.

I certainly don't know everything, and am constantly learning. But not
from threads like this one. I can change the SWR on a transmission line
by renormalizing my Smith chart? Is that a "logical point" I'm running
away from?

The reason I post in the first place isn't to try and convince the party
I'm directing my posting to. Nor is it an ego trip. What I hope to
accomplish is to provide a counterpoint to what I see as incorrect
information. This group is read by a very large number of "lurkers" who
seldom or never post. I know this for a fact, because many of them
introduce themselves to me at Dayton and other places I appear publicly.
When someone posts misinformation on this group, I try to present what I
consider to be correct.

There are people, some of whom post here, who won't be convinced
regardless of the evidence. It's as much a total waste of time to argue
with those people as it is to try and convince someone his religion is
wrong. When I encounter a person like that, I'll post my point of view,
present what evidence I can, then withdraw. I have much better things to
do than continue flogging a dead horse. I feel that the lurkers, who are
really the people I'm addressing, should be able to make up their minds
on the basis of what's been presented.

There's more than ample evidence to back up what I've said that's easily
available to anyone with a real interest in learning. Anyone who really
cares and is willing to invest even a modicum of effort can search out
the information and reach a conclusion. You've chosen not to go to that
effort(*), but rather interpret what I say in a way that suits your
preconception. Sorry, I just won't waste more of my time trying to talk
you out of it. If lurkers are convinced by your arguments and find them
more compelling than the ones I've made, then so be it. I've done what I
can. To continue posting over and over again the same thing isn't my
choice of a way to use my time.

There are people who feel that the person who posts the last message
"wins", and so anyone who withdraws has "lost". You can see the result
of this philosophy in the threads that have run to literally hundreds of
postings without ever resolving the issue. It's simply not a game I play.

Roy Lewallen, W7EL

(*) For example, have you ever looked up the equations for calculation
of SWR? Noticed that there's no term for the source impedance? And no
term for your Smith chart normalization factor?