Analyzing Stub Matching with Reflection Coefficients
 

On Apr 14, 6:01 pm, Roy Lewallen wrote:
Walter Maxwell wrote:

Consider my two explanations, or definitions of what I consider a virtual short--perhaps it should have a
different name, because of course 'virtual' implies non-existence. The short circuit evident at the input of
the two line examples I presented---do you agree that short circuits appear at the input of the two lines? If
so, what would you call them?

I'd call them "virtual shorts". If they were short circuits, we should
be able to connect a wire across the transmission line at that point
with no change in transmission line operation. But we can't. While
things will look the same on the generator side, they won't be the same
beyond the real short. So they aren't short circuits.

I want to bring up another reason to be very careful to even call them
"virtual shorts." They are virtual shorts only at certain
frequencies. To me, that is a very important distinction. Keeping
that frequency dependence in mind helps me be ever aware that they are
not anything like a real short.

Cheers,
Tom

Analyzing Stub Matching with Reflection Coefficients
 

Roy Lewallen, W7EL wrote:
"If you`ll read what I`ve written, you`ll hopefully see that my only
point of contention is with your claim that waves reflect from a
"virtual short". They do not."

Seems to me they do.

If you are lucky enough to have a copy of Terman`s 1955 opus, we can
reason together.
On page 91 is found Fig. 4-3 Vector (phasor) diagrams showing manner in
which incident and reflected waves combined to produce a voltage
distribution on the transmission line.

At an open circuit, the voltage phasors are in-phase.

E2, the reflected phasor, rotates clockwise as it travels back toward
the source.

E1, the incident phasor, rotates counter-clockwise as we look back
toward the source.

Looking 1/4-wavelength back from the open-circuit, E2 and E1, each
having rotated 90-degrees, but in opposite directions, are now
180-degrees out-of-phase.

On page 92, Fig. 4-4 shows the current, which summed to zero at the open
circuit, has risen to its maximum value at 1/4-wavelength back from the
open-circuit while the voltage dropped to its minimum, nearly zero,
maybe close enough to declare a "virtual short-circuit", 1/4-wavelength
back from the open-circuit.

What`s a short-circuit? Little voltage and much current.

What`s the difference between a physical short and the virtual short?
Nothing except the shunting conductor.

Is there current flowing at the open-circuit end of the 1/4-wave line
segment? No, the open-circuit won`t support current.

If a high-impedance generator of the same frequency were connected to
the virtual short point on the line, would it also be shorted? Yes.
Where? At the virtual short, not the open-circuit at the end of the
line.

Best regards, Richard Harrison, KB5WZI

Analyzing Stub Matching with Reflection Coefficients
 

K7ITM wrote:

I want to bring up another reason to be very careful to even call them
"virtual shorts." They are virtual shorts only at certain
frequencies. To me, that is a very important distinction. Keeping
that frequency dependence in mind helps me be ever aware that they are
not anything like a real short.

And only in steady state. And only in one direction. Yes, care in needed.

Roy Lewallen, W7EL

Analyzing Stub Matching with Reflection Coefficients
 

Roy Lewallen wrote in news:1323ikbgpa8cfb6
@corp.supernews.com:

K7ITM wrote:

I want to bring up another reason to be very careful to even call them
"virtual shorts." They are virtual shorts only at certain
frequencies. To me, that is a very important distinction. Keeping
that frequency dependence in mind helps me be ever aware that they are
not anything like a real short.

And only in steady state. And only in one direction. Yes, care in
needed.

It is simple inadequate models that lead to the thinking:

In the presence of mismatch, there is a reflected wave.

The anode glows red under mismatch, it obviously is caused by the power
reflected from the antenna mismatch. (The observation is only made when
the anode is red, so since the anode being red is always associated with
a mismatch, then it is believed that mismatch always causes the anode to
glow red, even though that is not a logical conclusion. The element of
danger to equipment reinforces this, and elevates it to the status of a
law.)

One solution is to insert an ATU near the transmitter, it works by re-
reflecting the power in the reflected wave so it is all goes to antenna
and totally radiated, thats what it is all about, getting all the
transmitter power up the stick, how else could it work, the reflected
power doesn't reach the transmitter any more. The anodes run cooler,
clear proof that the explanation is sound.

ATU is really a misnomer, it doesn't tune the antenna at all (we all knew
that), it is really a total-re-reflector when you have the true insight.

We have to remember that in the absence of good models of transmission
line behaviour (eg quantitative models), people will try to fit models
that they can understand, good or bad. If the path from mismatch to red
anodes is too complicated, simplify it, leave all the intermediate
explanation and conditions out, cut to the chase, what is the outcome,
make it a rule.

I agree with you Roy. I think that inventing explanations that are based
on things that aren't or don't happen is satisfying the learner's quest
for knowledge with potentially false information that must be unlearned
to move forward. Worse is that these kearners seize upon these inadequate
models and propagate them, the new experts of ham radio.

One of the risks to ham radio of the new six-hour hams is our feeding
them with inappropriate and inadeqate dumbed down models. I suppose it is
not new, this is probably the root of most of the myths of ham radio (eg
resonant antennas always work markedly better... make it resonant and it
will improve out of sight).

Owen

Analyzing Stub Matching with Reflection Coefficients
 

I wrote:
"Where? At the virtual short, not the open-circuit at the end of the
line."

If the virtual short were replaced with a real short, would anything
change? Not a thing except the line voltage distribution diagram would
lose its final 1/4-wavelength.

Best regards, Richard Harrison, KB5WZI

Analyzing Stub Matching with Reflection Coefficients
 

Richard Harrison wrote:
I wrote:
"Where? At the virtual short, not the open-circuit at the end of the
line."

If the virtual short were replaced with a real short, would anything
change? Not a thing except the line voltage distribution diagram would
lose its final 1/4-wavelength.

Don't you consider it a significant difference that no voltage, current,
or power would reach the load?

Roy Lewallen, W7EL

Analyzing Stub Matching with Reflection Coefficients
 

Owen Duffy wrote:

One of the risks to ham radio of the new six-hour hams is our feeding
them with inappropriate and inadeqate dumbed down models. I suppose it
is not new, this is probably the root of most of the myths of ham radio

Not new at all... There's a huge amount of new stuff for beginners to
learn, so they need simplified ideas to get them started. But it
shouldn't ever have to be about unlearning. We shouldn't be feeding them
false ideas that they will need to throw away completely.

Our local radio club does a lot of teaching, and at all levels we try to
say: "Learn this to check the right box in the exam, but remember
something else about it: it isn't a hard fact. It's actually an onion."

At the next level, we peel away a few more of the skins. The aim is
always to show them how last year's simplified information fits into a
bigger and deeper picture. We don't want them to throw the old
information away; at the next level we want them to keep it, understand
what was right about it, and also see its limitations.

At least, that's what we are aiming for. The challenge for the teacher
to live up to it.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Analyzing Stub Matching with Reflection Coefficients
 

Ian White GM3SEK wrote in news:xG9HBmG1ReIGFAV3
@ifwtech.co.uk:

Owen Duffy wrote:

One of the risks to ham radio of the new six-hour hams is our feeding
them with inappropriate and inadeqate dumbed down models. I suppose it
is not new, this is probably the root of most of the myths of ham radio

Not new at all... There's a huge amount of new stuff for beginners
to
learn, so they need simplified ideas to get them started. But it
shouldn't ever have to be about unlearning. We shouldn't be feeding
them
false ideas that they will need to throw away completely.

Our local radio club does a lot of teaching, and at all levels we try
to
say: "Learn this to check the right box in the exam, but remember
something else about it: it isn't a hard fact. It's actually an onion."

At the next level, we peel away a few more of the skins. The aim is
always to show them how last year's simplified information fits into a
bigger and deeper picture. We don't want them to throw the old
information away; at the next level we want them to keep it, understand
what was right about it, and also see its limitations.

At least, that's what we are aiming for. The challenge for the teacher
to live up to it.

Ian,

That sounds a good approach, and it doesn't betray the trust that
learners should have in their trainers.

When ham radio is being reduced to a "communicator" hobby, it is worth
emphasising that there is great opportunity for personal development and
satisfaction in working through those layers. Some of us think that is
what ham radio is about, the ITU does, ITU-R RR Article 1 says "1.56
Amateur service: A radiocommunication service for the purpose of self-
training, intercommunication and technical investigations carried out by
amateurs, that is, by duly authorised persons interested in radio
technique solely with a personal aim and without pecuniary interest".

Owen

Analyzing Stub Matching with Reflection Coefficients
 

K7ITM wrote:
. . .
The analogy may not be prefect, but I think it's a lot like the
usefulness of the idea of a "virtual ground" at the inverting input of
an op amp. But it's a virtual ground only under specific conditions:
strong negative feedback is active, and the non-inverting input is at
(AC, at least) ground potential. For it to be a useful concept
without too many pitfalls, the person using it has to be aware that
the conditions that make it a good approximation don't always hold.
Similarly for a "virtual short" on a line.
. . .

Let me relate a story. . .

Years ago at Tektronix, I transferred to a different group. Across the
aisle was a very bright engineer, fresh from school with a Masters or
PhD degree -- I don't recall which. I recall that his advanced education
had specialized in nonlinear control systems, very much a mathematically
complex and challenging field. His entirely academic background had been
very different from mine, so I was often enthralled by his attempts to
reconcile reality with the idealized world he had, until very recently,
occupied.

One day I found him muttering, trying this and that, until he asked for
some help with his test setup. He was driving an inverting op amp
circuit with a square wave, and he was seeing sharp pulses at the
"virtual ground" summing junction with his 'scope. He had tried moving
his probe grounds, replacing the op amp, bypassing, and everything else
he could think of, to rid his display of this obvious erroneous
artifact. The voltage at the summing junction, he explained, should
always be zero, since it's a virtual ground. Those spikes shouldn't be
there.

I tried to explain to him how a "virtual ground" was created: An input
signal initially generates a voltage at the op amp input. The op amp
responds by sending an inverted signal back to the summing junction
which adds to the initial voltage to produce very nearly zero at the
input. I explained that it can never be zero, but at best is the op amp
output voltage divided by its open loop gain. But more to the point, the
op amp isn't infinitely fast, so it takes some time for it to respond to
that initial voltage or any changes. And during that lag, the summing
junction voltage can move a great distance from zero. So the spikes are
occurring during the time it takes the op amp to respond to changes in
the input stimulus.

Well, he didn't get it. He just couldn't make the transition from the
idealized, infinitely fast and infinite gain op amps of his academic
models to the real things he had to work with. And looking back on it, I
think the basic problem was that he never really fully understood just
how that virtual ground came about even in an idealized world.

After a number more frustrating and unresolved collisions with reality,
he wisely quit and got a teaching job. I'm sure he did well in the
academic world.

Those many models we use daily to keep calculations, concepts, and
analyses manageable are just that -- models. It's imperative to
constantly be aware of the range over which those models are valid, and
alert to any situation which might make the model invalid. People
solving routine problems can, unfortunately, often get along for a long
time without realizing the limitations of their models, and can be
lulled into a belief that they're not models at all but reality. But in
the environment where I've spent most of my time, this carelessness
leads you very quickly into places which can be very difficult to get
out of.

Roy Lewallen, W7EL

Analyzing Stub Matching with Reflection Coefficients
 

Walter Maxwell wrote:
..
..


We have thus proved that the virtual short circuit established at the stub point is actually performing as a
real short circuit.
..
..

Walt, W2DU

It is interesting to look at a single short pulse propagating along the
TL. At the stub point, the pulse must encounter a discontinuity in
impedance and therefore there will be a reflection. This can been seen
on a TDR. So there is a real reflection from a stub regardless of
whether or not it is a virtual short.
Alan
VK2ADB

Analyzing Stub Matching with Reflection Coefficients
 

I'm not sure how many times it's worthwhile to keep repeating this, but
I guess I'll give it another couple of tries before giving up.

Richard Harrison wrote:
Roy Lewallen, W7EL wrote:
"If you`ll read what I`ve written, you`ll hopefully see that my only
point of contention is with your claim that waves reflect from a
"virtual short". They do not."

Seems to me they do.

If you are lucky enough to have a copy of Terman`s 1955 opus, we can
reason together.

Sorry, I'm not. All I have is a 1947 Third Edition of _Radio
Engineering_. I'm unfortunately stuck with having to think for myself.
But I trust you to quote him accurately, and Terman is to be trusted.

On page 91 is found Fig. 4-3 Vector (phasor) diagrams showing manner in
which incident and reflected waves combined to produce a voltage
distribution on the transmission line.

I'm sure they're correct, and similar diagrams can be found in many of
my other texts.

At an open circuit, the voltage phasors are in-phase.

Yes. And the current phasors are out of phase.

E2, the reflected phasor, rotates clockwise as it travels back toward
the source.

E1, the incident phasor, rotates counter-clockwise as we look back
toward the source.

Yes. These of course follows from the mathematical analysis of
transmission lines, found in many texts, and with which I'm very familiar.

Looking 1/4-wavelength back from the open-circuit, E2 and E1, each
having rotated 90-degrees, but in opposite directions, are now
180-degrees out-of-phase.

On page 92, Fig. 4-4 shows the current, which summed to zero at the open
circuit, has risen to its maximum value at 1/4-wavelength back from the
open-circuit while the voltage dropped to its minimum, nearly zero,
maybe close enough to declare a "virtual short-circuit", 1/4-wavelength
back from the open-circuit.

Yes, this is universally known.

What`s a short-circuit? Little voltage and much current.

Well, at a short circuit you'll find zero volts and any current. You'll
also find this at other places which aren't short circuits, such as
where multiple voltage waves add to zero and at the summing junction of
a perfect op amp. These aren't short circuits, but they are points of
zero voltage. Saying they are all the same is like saying that because
you find water in a creek, any place you find water must be a creek.
What sort of logic is that?

What`s the difference between a physical short and the virtual short?
Nothing except the shunting conductor.

Well, yes. For one thing, waves won't reflect from a virtual short. They
will, from a real short. Another difference is that a real short will
prevent any waves from proceeding beyond it; they pass right through a
virtual short. Good thing, too, or you wouldn't get any power to your
load. Another is behavior at other frequencies and with other
waveshapes. Walt has mentioned another, that a virtual short acts like a
real short only in one direction, even when all the other conditions for
similarity are met.

Is there current flowing at the open-circuit end of the 1/4-wave line
segment? No, the open-circuit won`t support current.

Correct, of course.

If a high-impedance generator of the same frequency were connected to
the virtual short point on the line, would it also be shorted? Yes.
Where? At the virtual short, not the open-circuit at the end of the
line.

Well, yes and no. When you first hook it to the virtual short, it won't
be shorted -- it'll see just the Z0 of the cable. Only when its output
reaches the end of the stub, reflects back, and adds to the forward wave
will it be short circuited. So it's the open end of the line which is
essential to creating the apparent short at the generator.

Now let us, as you say, reason together.

You're pointing out some similarities between a virtual short and a real
one, and giving that as evidence that waves reflect from a virtual
short. So consider a point on a 50 ohm line at which the forward and
reverse waves add to a V/I of, say, 10 ohms, purely resistive to keep it
simple. If you connect a generator (of the correct frequency) at that
point, it will see 10 ohms after things settle down to steady state,
just like your generator saw a short circuit at the "virtual short" in
steady state. So can we conclude that a traveling wave will partially
reflect when it encounters the 10 ohm point? The effective or "virtual"
reflection coefficient can be calculated as -2/3, from which the
reflected wave can be calculated. And, in fact, if we assume that such a
reflection takes place, we can calculate the magnitude and phase of the
resulting wave and, sure enough, there it will really be.

But if the wave does really reflect from this "virtual discontinuity",
we might have a problem. That point is a ways away from the "virtual
short" point (check your Terman diagram if you don't follow), so we have
a partial reflection occurring at this point as well as the full
reflection from the "virtual short". In fact, unless the line is
matched, we'll have reflections from every point along the line, or at
best everywhere except an infinitesimally short spot every half
wavelength! What a mess! Does Terman describe this problem in his book?
A diagram, perhaps, showing the infinite number of partial reflections
taking place all along the line?

No? Well, then, maybe it takes a perfect "virtual short" to get a
reflection, and even a tiny, tiny imperfection will prevent it. So that
would mean that you'd get no reflection at all from a "virtual
almost-short" on even a very slightly lossy line, right? The whole idea
goes to pot when you add even a tiny amount of loss? Or is a little loss
ok? Then we get a full reflection from a good or pretty good "virtual
short", but nothing if it gets too far from perfection. Do me a favor
and check your Terman for an equation or graph which shows just where
this abrupt transition point is (that is, at what "virtual resistance"
the reflection ceases), and why it exists.

Help me out here with my reasoning.

Roy Lewallen, W7EL

Analyzing Stub Matching with Reflection Coefficients
 

Roy Lewallen wrote:
Richard Harrison wrote:
If the virtual short were replaced with a real short, would anything
change? Not a thing except the line voltage distribution diagram would
lose its final 1/4-wavelength.

Don't you consider it a significant difference that no voltage, current,
or power would reach the load?

:-) :-) That's what I asked when you said the
virtual load on a transmitter could be replaced
with a lumped circuit "without changing anything".
--
73, Cecil http://www.w5dxp.com

Analyzing Stub Matching with Reflection Coefficients
 

Roy Lewallen wrote:
It's imperative to
constantly be aware of the range over which those models are valid, and
alert to any situation which might make the model invalid.

Roy, if your model prohibits interaction of coherent
waves, it is seriously flawed.
--
73, Cecil http://www.w5dxp.com

Analyzing Stub Matching with Reflection Coefficients
 

Richard Harrison wrote:
Roy Lewallen, W7EL wrote:
"If you`ll read what I`ve written, you`ll hopefully see that my only
point of contention is with your claim that waves reflect from a
"virtual short". They do not."

Seems to me they do.

I wonder if we can agree that if there is no physical
impedance discontinuity, there can be no reflections?

For instance, given a piece of 50 ohm open-ended coax
with a driving source:

source-----50 ohm coax-----+---1/4WL 50 ohm coax--open

There is a virtual short at point '+' and that virtual
short exists at a point where there is no physical
impedance discontinuity. Can we agree that the forward
wave is unaffected by that virtual short? Can we agree
that the reflected wave is unaffected by that virtual
short? After all, there is absolutely nothing there
that can physically disrupt any waves.

Or given one wavelength of coax being driven by a signal
generator equipped with a circulator load.

SGCL---1/4 WL---x---1/4WL---y---1/4WL---x---1/4WL---open

There are obviously reflections at the open. Are there
any reflections at the virtual open at 'y'? Are there
any reflections at the virtual shorts at 'x'? I would
submit that in the above example, the *only* reflections
in the entire system are happening at the open end of
of the coax and that the virtual shorts and opens are
themselves effects and not the cause of anything (except
maybe arguments) :-)
--
73, Cecil http://www.w5dxp.com

Analyzing Stub Matching with Reflection Coefficients
 

Roy Lewallen wrote:
"Don`t you consider it a significant difference that no voltage,
current, or power would reach the load?"

No, because the example`s load is a perfect open-circuit despite Richard
Clark`s disdain for good insulators. From the shorting point all the way
back to the generator, the voltage distribution is unchanged, real short
or virtual short. (I did not say unchanging.)

Best regards, Richard Harrison, KB5WZI

Analyzing Stub Matching with Reflection Coefficients
 

On Sun, 15 Apr 2007 19:23:56 +1000, Alan Peake wrote:



Walter Maxwell wrote:
.
.


We have thus proved that the virtual short circuit established at the stub point is actually performing as a
real short circuit.
.
.

Walt, W2DU

It is interesting to look at a single short pulse propagating along the
TL. At the stub point, the pulse must encounter a discontinuity in
impedance and therefore there will be a reflection. This can been seen
on a TDR. So there is a real reflection from a stub regardless of
whether or not it is a virtual short.
Alan
VK2ADB

I thank you for that, Alan, because, to continue, when the pulse is replaced with a sine wave, there is also a
reflection from the stub. And when going still further, since the stub presents a susceptance equal to the
line susceptance of the opposite sign at the stub point, some of the sine wave continues along the line and
reaches the mismatched termination, which also produces a real reflection. When the stub is placed at the
proper place on the line relative to the SWR (mismatch), the phase of the waves (voltage and current)
reflected from the load are opposite, respectively, to those of the waves reflected from the stub. The sum of
the voltage waves then yield a resultant reflection coefficient of 180° and the sum of the current waves yield
a resultant of 0°, establishing a short circuit to both sets of reflected waves, but an open circuit to the
source waves.

For people who understand that fields (voltage and current) radiating from two vertical radiators that are of
the same magnitude and of opposite sign result in a null in their radiation pattern in a particular direction
must also understand that for the null to be established there must also be interaction, or interference, or
summing between the fields to cause the travel to cease in the null direction. These people also understand
that energy that was traveling in the null direction has been re-directed in another direction, raising the
level of the energy in that direction from the original level.

IMHO, these people can't have it both ways. If the fields interact, or interfere in space, such as in those
radiated from two radiators, then coherent fields traveling in a transmission line must also interact,
interfere, or sum. This is the concept on which I'm basing my impedance matching analogy using the summation
of reflection coefficients.

Walt

Analyzing Stub Matching with Reflection Coefficients
 

Walter Maxwell wrote:
... then coherent fields traveling in a transmission line must also interact,
interfere, or sum.

There is no doubt that Roy is absolutely wrong when he
asserts that coherent EM waves do not interact. Every
time we tune our antenna tuners to zero reflected energy,
we are causing EM waves to interact following the rules
of *linear* interference. All those waves, inductors,
and capacitors within the antenna tuner are operating
within a linear environment. If they weren't, we would
generate lots of harmonics.

Seems to me, the only valid point of argument is whether
a purely virtual impedance is a cause or an effect.
--
73, Cecil http://www.w5dxp.com

Analyzing Stub Matching with Reflection Coefficients
 

Cecil, W5DXP wrote:
"I wonder if we can agree that if there is no physical impedance
discontinuity, there can be no reflection?"

Terman says on page 118 of his 1955 opus:
"When a wave traveling along a transmission line encounters an isolated
discontinuity, it is partially reflected; i.e., while a portion of the
wave continues to travel down the line, another portion of the wave is
reflected backwards."

On page 119 Terman says:
"A traveling wave passing through such a section (a tapered transmission
line to gradually and continuously change its impedance from one value
to another) will have its ratio of voltage to current transformed in
accordance with the ratio of the characteristic impedances involved."

Abrupt changes in impedance are discontinuities which produce
reflections. These are secondary energy sources.

Total reflection produced the wave distribution diagrammed in Fig. 4-3
on page 91, from an open-circuit on the line.

I see that a virtual short-circuit results 1/4-wave back from the open
circuit and the "short" is repeated 1/2-wave back from the first virtual
short. Through all the virtual shorts and opens, Terman shows the
incident and reflected waves progressing unimpeded.

I believe that if a generator is connected through 1/4-wave of
transmission line to a real short, 360-degrees of phase rotation
presents the generator with a voltage which is almost of the same phase
and magnitude as the generator`s output. Almost no current flows either
into the short or back into the generator. It is similar to connecting
nearly identical transformer windings in parallel.

Hybrid ring isolators are also constructed of 1/4-wave transmission line
sections, and whether a port accepts or rejects energy results from
voltage distributions at the ports, I think.

Best regards, Richard Harrison, KB5WZI

Analyzing Stub Matching with Reflection Coefficients
 

Richard Harrison wrote:
Through all the virtual shorts and opens, Terman shows the
incident and reflected waves progressing unimpeded.

Suggesting that a virtual short or virtual open, by itself,
doesn't cause reflections which is what I think others are
trying to say. Reflections associated with virtual shorts
and virtual opens always occur with some extra ingredients,
like physical impedance discontinuities, the very existence
of which makes one wonder if they might somehow be
associated with the reflections. Reflections always occur
at physical impedance discontinuities.
--
73, Cecil http://www.w5dxp.com

Analyzing Stub Matching with Reflection Coefficients
 

On Apr 14, 9:31 pm, Cecil Moore wrote:
Roy Lewallen wrote:
K7ITM wrote:
. . .
It's a useful visualization tool and design aid; it's a poor analysis
tool at best. At worst, it will lull you into building something that
just won't work, wasting time and resources.

In my opinion, the potential harm can be much worse. If it causes you to
buy into the notion that traveling waves interact in a linear medium,
that opens the door to a whole universe of invalid conclusions.

Here is how Hecht described interference in "Optics":
"... interference corresponds to the *interaction* of two or
more lightwaves yielding a resultant irradiance that deviates
from the sum of the component irradiances."

If traveling waves cannot interact in a linear medium, why
does Hecht say they do indeed interact?

It is exactly that kind of misleading terminology that has caused his
text to fall out of favor among many physics faculty.

To deny the body of laws of physics regarding EM waves from
the field of optics is an example of extreme ignorance.

You really aren't qualified to speak on behalf of the field of optics,
Cecil. You aren't quaified to speak on behalf of Eugene Hecht either,
for that matter. However, I think Dr. Hecht is still around so
perhaps you can persuade him to back you up. Be sure to ask him what
he thinks about the 4th mechanism of reflection.

ac6xg

Analyzing Stub Matching with Reflection Coefficients
 

Jim Kelley wrote:
If traveling waves cannot interact in a linear medium, why
does Hecht say they do indeed interact?

It is exactly that kind of misleading terminology that has caused his
text to fall out of favor among many physics faculty.

:-) Please argue with Hecht if he is still alive or with
his ghost if he is not.

You really aren't qualified to speak on behalf of the field of optics,
Cecil.

Oh yes, instead of anything technical, we get an argumentum
ad verecundiam from you, the delusionary diversion gurus use
when they are sure they already know everything. Jim, it is
obvious that you don't even understand the intensity equations
in Born and Wolf. Wade through the S-Parameter analysis that
I have started and I will prove it to you.
--
73, Cecil http://www.w5dxp.com

Analyzing Stub Matching with Reflection Coefficients
 

On Apr 15, 2:03 am, Roy Lewallen wrote:

After a number more frustrating and unresolved collisions with reality,
he wisely quit and got a teaching job. I'm sure he did well in the
academic world.

He doesn't sound like anyone I know that does "well" in the academic
world.

ac6xg

Analyzing Stub Matching with Reflection Coefficients
 

Jim Kelley wrote:
On Apr 15, 2:03 am, Roy Lewallen wrote:
After a number more frustrating and unresolved collisions with reality,
he wisely quit and got a teaching job. I'm sure he did well in the
academic world.

He doesn't sound like anyone I know that does "well" in the academic
world.

:-) You must be an academic. :-)
--
73, Cecil http://www.w5dxp.com

Analyzing Stub Matching with Reflection Coefficients
 

On Sun, 15 Apr 2007 09:18:28 -0500, Cecil Moore wrote:

Walter Maxwell wrote:
... then coherent fields traveling in a transmission line must also interact,
interfere, or sum.

There is no doubt that Roy is absolutely wrong when he
asserts that coherent EM waves do not interact. Every
time we tune our antenna tuners to zero reflected energy,
we are causing EM waves to interact following the rules
of *linear* interference. All those waves, inductors,
and capacitors within the antenna tuner are operating
within a linear environment. If they weren't, we would
generate lots of harmonics.

Seems to me, the only valid point of argument is whether
a purely virtual impedance is a cause or an effect.

I quote from Terman, 1947 ed. page 109:
"The distance L1 from the load, and the length 'a' of the stub, are so chosen that the reflected wave produced
by the shunting impedance of the stub is equal in magnitude and opposite in phase to the reflected wave
existing on the line at this point as a result of the reflection from the load impedance ZL. Thus, although a
reflected wave is present in the length L1 because of the reflection from ZL, there is no reflected wave on
the generator side of the stub line as a result of the cancellation of the two reflected waves."

I maintain that Terman's statements above agree precisely with my statements introducing this thread, in which
I sum the reflection coefficients of the load and stub reflections to determine the cancellation of the two
reflected waves that result in the impedance match. Call it what you like, the condition at the stub point
totally re-reflects the two sets of reflected waves, but allows total passage of the source waves through the
junction of the main line and the stub.

Seems to me that the only disagreement with my original posting is whether the condition at the stub point can
be called a 'virtual' short circuit. If I'm outvoted on that term how about selecting another term for it that
everyone can agree on. I'm sure there is no disagreement on the wave analysis of the stub matching circuitry.

Walt, W2DU

Analyzing Stub Matching with Reflection Coefficients
 

Walter Maxwell wrote:
I quote from Terman, 1947 ed. page 109:
"The distance L1 from the load, and the length 'a' of the stub, are so chosen that the reflected wave produced
by the shunting impedance of the stub is equal in magnitude and opposite in phase to the reflected wave
existing on the line at this point as a result of the reflection from the load impedance ZL. Thus, although a
reflected wave is present in the length L1 because of the reflection from ZL, there is no reflected wave on
the generator side of the stub line as a result of the cancellation of the two reflected waves."

Wouldn't a short-circuit at the mouth of the stub keep
current from flowing in the stub?
--
73, Cecil http://www.w5dxp.com

Analyzing Stub Matching with Reflection Coefficients
 

Jim Kelley wrote:
On Apr 15, 2:03 am, Roy Lewallen wrote:

After a number more frustrating and unresolved collisions with reality,
he wisely quit and got a teaching job. I'm sure he did well in the
academic world.

He doesn't sound like anyone I know that does "well" in the academic
world.

You were lucky. He closely resembled the majority of my college professors.

Roy Lewallen, W7EL

Analyzing Stub Matching with Reflection Coefficients
 

Cecil Moore wrote:

...
:-) You must be an academic. :-)

There is a saying in the world, it exists for good reason; Those who can
DO, those who can't TEACH.

Since I am only a part-time instructor, I can do a little bit.

JS

Analyzing Stub Matching with Reflection Coefficients
 

On Apr 14, 9:52 pm, Cecil Moore wrote:

Does energy being redistributed in new directions really look
like a lack of interaction to you?

Roy is absolutely right, Cecil. Interact is a very poor choice of
terms in this discussion. Would you assert that photons can have an
effect on each other? The fact is, waves and photons can only
interact with matter. If the superposition of waves actually had an
effect on the waves themselves then interference patterns wouldn't
look the way they do. Think about it.

ac6xg

Analyzing Stub Matching with Reflection Coefficients
 

I've mentioned before that readers, like a group of triaged medical
patients, fall into three general categories:

1. Those who have made up their minds and won't have them changed no
matter what you say or what evidence you present;
2. Those who already agree with what you're saying;
3. Those who are willing to read what you say and can be convinced.

Posting for the benefit of groups 1 and 2 is a waste of time, because
there's no difference in anyone's belief or knowledge from the beginning
to the end of the discussion. The third group, however, is worth while.
Unfortunately, the active posters often are composed of the first two
groups, and I see in this discussion that's been entirely the case. So
we're left to hope that the lurkers are taking something away from this.
To the lurkers out the I hope you've read the postings, looked at the
evidence, and reached some conclusions. Better yet, I hope some of you
have been spurred to learn more about the topic, do some investigation
of your own from reputable sources, and gain a deeper understanding of
the fundamentals involved.

I see that my statements that waves don't interact with or reflect from
each other in a linear medium is already being morphed into claims that
I've denied that superposition happens, even though I've been careful to
distinguish the two. So one final request to the lurkers: Read what I
wrote, not interpretations of what I wrote.

I've tried to explain my point in about every way I know how, and
further postings would just become more repetitive. So I'll bow out at
this point, disappointed because I've been totally ineffectual at
communicating my point to the active posters, but with hope that some of
the lurkers have understood. And Walt, I'm especially disappointed that
I've been unable to explain to you what I mean, because I fear that the
interpretive error will detract from and reduce the credibility of your
otherwise exceptional and wonderful works.

Roy Lewallen, W7EL

Analyzing Stub Matching with Reflection Coefficients
 

On Apr 15, 6:53 am, Walter Maxwell wrote:
On Sun, 15 Apr 2007 19:23:56 +1000, Alan Peake
It is interesting to look at a single short pulse propagating along the
TL. At the stub point, the pulse must encounter a discontinuity in
impedance and therefore there will be a reflection. This can been seen
on a TDR. So there is a real reflection from a stub regardless of
whether or not it is a virtual short.
Alan
VK2ADB

I thank you for that, Alan, because, to continue, when the pulse is replaced with a sine wave, there is also a
reflection from the stub.

Hi Walt -

Begging your pardon, but don't TDR's examine the transient response of
a system, rather the steady state response?

ac6xg

Analyzing Stub Matching with Reflection Coefficients
 

Jim Kelley wrote:
Cecil Moore wrote:
Does energy being redistributed in new directions really look
like a lack of interaction to you?

Roy is absolutely right, Cecil. Interact is a very poor choice of
terms in this discussion.

Roy did NOT say "interact" was a poor choice of terms. He
chose to use it as did Hecht. Hecht says waves interact.
Roy says they don't interact. I'm more likely to trust
Hecht over someone who says an S-Parameter analysis is
"gobbledigook" (sic) and doesn't even know how to
spell the word.

Would you assert that photons can have an
effect on each other?

Of course. Coherent photons emitted from different phased
antenna elements obviously have an effect on each other.
I suspect in addition to not understanding coherent
interference, you also don't understand coherency. When
coherent photons are traveling in the same path in the
same direction, they do not pass like ships in the night.
They affect each other. If they interfere destructively,
they redistribute some of their photonic energy in a
different direction. Every reference in the world says
that is what happens.

The fact is, waves and photons can only
interact with matter. If the superposition of waves actually had an
effect on the waves themselves then interference patterns wouldn't
look the way they do. Think about it.

I have thought long, hard, and deep about it. Wave cancellation
is prima facie evidence that coherent waves can have a permanent
effect on each other, even in free space. I don't know anything
more permanent than wave cancellation bringing both the net
E-field and the net H-field to zero.

No, not all coherent waves interfere. No, not all waves that
interfere cancel each other. Some of them do just the opposite.
But sometimes they do cancel. If you would wade through
the S-Parameter analysis with me, you would understand. An S-
parameter analysis of a Z0-match will expose the wave cancellation
toward the source for all to see. But it's rather obvious that
your mind is already made up, you think you know everything, and
will stop at nothing, including character assassination, to avoid
learning anything new.

How about taking the S-Parameter analysis step by step? Then
you can point out the very step where I wander astray of the
laws of physics.
--
73, Cecil http://www.w5dxp.com

Analyzing Stub Matching with Reflection Coefficients
 

Roy Lewallen wrote:
I've mentioned before that readers, like a group of triaged medical
patients, fall into three general categories:

1. Those who have made up their minds and won't have them changed no
matter what you say or what evidence you present;
2. Those who already agree with what you're saying;
3. Those who are willing to read what you say and can be convinced.

:-) As is usual for omniscient gurus, Roy doesn't even
comprehend that there is a number 4.

4. Those who question what you say and can prove that you are
wrong.

Is it possible for Roy to be wrong? How about using standing-
wave current with its unchanging phase to try to measure the
phase shift through a loading coil? Roy actually did exactly
that and, at last assertion, defends those ignorant
measurements. He still hasn't comprehended what he did wrong.

The day a guru forgets that 4th possibility above is the
day that he becomes an obsolete historical artifact. Two
waves redistribute their energy components in different
directions. How in the world is that not interaction?

Quotes from two web pages from the field of optical engineering:

www.mellesgriot.com/products/optics/oc_2_1.htm

"Clearly, if the wavelength of the incident light and the thickness
of the film are such that a phase difference exists between reflections
of p, then reflected wavefronts interfere destructively, and overall
reflected intensity is a minimum. If the two reflections are of equal
amplitude, then this amplitude (and hence intensity) minimum will be
zero." (Referring to 1/4 wavelength ideal thin films.)

"In the absence of absorption or scatter, the principle of conservation
of energy indicates all 'lost' reflected intensity will appear as
enhanced intensity in the transmitted beam. The sum of the reflected and
transmitted beam intensities is always equal to the incident intensity.
This important fact has been confirmed experimentally."

micro.magnet.fsu.edu/primer/java/scienceopticsu/interference/waveinteractions/index.html

"... when two waves of equal amplitude and wavelength that are
180-degrees ... out of phase with each other meet, they are not actually
annihilated, ... All of the photon energy present in these waves must
somehow be recovered or *redistributed* in a new direction, according to
the law of energy conservation ... Instead, upon meeting, the photons
are *redistributed* to regions that permit constructive interference, so
the effect should be considered as a *redistribution* of light waves and
photon energy rather than the spontaneous construction or destruction of
light."

Here is a question for Roy: Lurkers with inquiring minds want to know:
How can a redistribution of energy in affected waves occur without
interaction between the waves? Magic? Divine intervention? What?
--
73, Cecil http://www.w5dxp.com

Analyzing Stub Matching with Reflection Coefficients
 

On 15 Apr 2007 14:33:40 -0700, "Jim Kelley" wrote:

On Apr 15, 6:53 am, Walter Maxwell wrote:
On Sun, 15 Apr 2007 19:23:56 +1000, Alan Peake
It is interesting to look at a single short pulse propagating along the
TL. At the stub point, the pulse must encounter a discontinuity in
impedance and therefore there will be a reflection. This can been seen
on a TDR. So there is a real reflection from a stub regardless of
whether or not it is a virtual short.
Alan
VK2ADB

I thank you for that, Alan, because, to continue, when the pulse is replaced with a sine wave, there is also a
reflection from the stub.

Hi Walt -

Begging your pardon, but don't TDR's examine the transient response of
a system, rather the steady state response?

ac6xg

You're correct, of course, Jim, but I was intuitively assuming we'd not be continuing the use of the TDR with
the sine wave signal. I'm sure my intuition wasn't communiated, sorry.

Walt

Analyzing Stub Matching with Reflection Coefficients
 

On Apr 15, 12:50 pm, Walter Maxwell wrote:

Seems to me that the only disagreement with my original posting is whether the condition at the stub point can
be called a 'virtual' short circuit.

Hi Walt,

Most everyone has directly expressed complete agreement with that
idea. Here's the recurring theme:


*******Virtual impedance discontinuities do not cause
reflections.********


73, Jim AC6XG

Analyzing Stub Matching with Reflection Coefficients
 

On Apr 15, 1:29 pm, Roy Lewallen wrote:
Jim Kelley wrote:
On Apr 15, 2:03 am, Roy Lewallen wrote:

After a number more frustrating and unresolved collisions with reality,
he wisely quit and got a teaching job. I'm sure he did well in the
academic world.

He doesn't sound like anyone I know that does "well" in the academic
world.

You were lucky. He closely resembled the majority of my college professors.

Roy Lewallen, W7EL

Hi Roy -

I didn't say that I don't know any like that, I just wouldn't say
they're doing 'well' at it. ;-)

ac6xg

Analyzing Stub Matching with Reflection Coefficients
 

On Sun, 15 Apr 2007 13:51:44 -0700, Roy Lewallen wrote:

I've mentioned before that readers, like a group of triaged medical
patients, fall into three general categories:

1. Those who have made up their minds and won't have them changed no
matter what you say or what evidence you present;
2. Those who already agree with what you're saying;
3. Those who are willing to read what you say and can be convinced.

Posting for the benefit of groups 1 and 2 is a waste of time, because
there's no difference in anyone's belief or knowledge from the beginning
to the end of the discussion. The third group, however, is worth while.
Unfortunately, the active posters often are composed of the first two
groups, and I see in this discussion that's been entirely the case. So
we're left to hope that the lurkers are taking something away from this.
To the lurkers out the I hope you've read the postings, looked at the
evidence, and reached some conclusions. Better yet, I hope some of you
have been spurred to learn more about the topic, do some investigation
of your own from reputable sources, and gain a deeper understanding of
the fundamentals involved.

I see that my statements that waves don't interact with or reflect from
each other in a linear medium is already being morphed into claims that
I've denied that superposition happens, even though I've been careful to
distinguish the two. So one final request to the lurkers: Read what I
wrote, not interpretations of what I wrote.

I've tried to explain my point in about every way I know how, and
further postings would just become more repetitive. So I'll bow out at
this point, disappointed because I've been totally ineffectual at
communicating my point to the active posters, but with hope that some of
the lurkers have understood. And Walt, I'm especially disappointed that
I've been unable to explain to you what I mean, because I fear that the
interpretive error will detract from and reduce the credibility of your
otherwise exceptional and wonderful works.

Roy Lewallen, W7EL

Hi Roy,

I thank you for the time and effort you've put in in expressing your position, but for all the attempts I've
made to appreciate your position, I'm now more confused than before the thread began. I don't know what else
to say other than it puts me squarely in the first of the three categories, doesn't it?

Walt

Analyzing Stub Matching with Reflection Coefficients
 

Walter, W2DU wrote:
"Thank you for the time and effort you`ve put in in expressing your
position-----."

Yes, Thank you very much and please add me to your catagory no. 3.

Best regards, Richard Harrison, KB5WZI

Analyzing Stub Matching with Reflection Coefficients
 

On 15 Apr 2007 15:10:11 -0700, "Jim Kelley" wrote:

On Apr 15, 12:50 pm, Walter Maxwell wrote:

Seems to me that the only disagreement with my original posting is whether the condition at the stub point can
be called a 'virtual' short circuit.

Hi Walt,

Most everyone has directly expressed complete agreement with that
idea.

Here's the recurring theme:


*******Virtual impedance discontinuities do not cause
reflections.********


73, Jim AC6XG

OK Jim, if that's so, then I've got to figure out a new way to explain how antenna radiation patterns are
modified by changing the relative phase of the signals fed to multiple radiators, and by changing the spacing
between the radiators. Looks like I've had it all wrong for lo these many years. I thought I've been reading
the same references as all the other posters.

Walt

Analyzing Stub Matching with Reflection Coefficients
 

On Apr 15, 2:39 pm, Cecil Moore wrote:
Jim Kelley wrote:
Cecil Moore wrote:
Does energy being redistributed in new directions really look
like a lack of interaction to you?

Roy is absolutely right, Cecil. Interact is a very poor choice of
terms in this discussion.

Roy did NOT say "interact" was a poor choice of terms.

That's correct. I said that interact is a poor choice of terms.

He
chose to use it as did Hecht. Hecht says waves interact.
Roy says they don't interact.

As I said, Roy is correct.

Would you assert that photons can have an
effect on each other?

Of course. Coherent photons emitted from different phased
antenna elements obviously have an effect on each other.

That is incorrect. Charged particles and photons interact in antennas.

I have thought long, hard, and deep about it. Wave cancellation
is prima facie evidence that coherent waves can have a permanent
effect on each other, even in free space. I don't know anything
more permanent than wave cancellation bringing both the net
E-field and the net H-field to zero.

And the funny thing is, you say that even you know of instances in
which the net fields are zero, and yet the waves propagate beyond that
point. You've at least seen a picture of an interference pattern,
right?

If you would wade through
the S-Parameter analysis with me, you would understand.

I think you just like to argue.

How about taking the S-Parameter analysis step by step? Then
you can point out the very step where I wander astray of the
laws of physics.

If the S parameter analysis addressed where you are going wrong, then
that might be worthwhile. The problem as I said is with your idea
about waves and energy. Obviously we all get the same answer at the
end of the problem.

73, Jim AC6XG

Analyzing Stub Matching with Reflection Coefficients
 

Richard Harrison wrote:
Yes, Thank you very much and please add me to your catagory no. 3.

And please add the 4th distinct possibility, that any mortal
human being can be proved to be wrong about something. :-)
--
73, Cecil http://www.w5dxp.com