Cecil Moore wrote:

For that statement, the length of the feedline is unknown
and the load is unknown.

I was commenting on this, the subject of our conversation:

"Consider the following:

Source---50 ohm feedline---+---1/2WL 150 ohm---50 ohm load"

As I was saying, for the two boundaries as a network, and we call rho at
the first boundary r12 and rho at the second boundary r23 then
rho(network) = (r12 + r23)/(1 + r12*r23) = 0. Note that if we use your
value for r12, the network generates a reflection. I note the utility
of negative rho in this example.

Seems to me, rho
cannot both cause a voltage and be caused by a (voltage divided
by a current) which is an impedance upon which rho is dependent.

Voltages on a transmission line do not determine reflection
coefficients.
Reflection coefficients are determined by characteristic impedances, not
virtual ones.

73, Jim AC6XG

Jim Kelley wrote:
. . .
Voltages on a transmission line do not determine reflection
coefficients.
Reflection coefficients are determined by characteristic impedances, not
virtual ones.

73, Jim AC6XG

I disagree with this. When applied to transmission lines, the (voltage)
reflection coefficient is, as far as I can tell, universally defined as
the ratio of reflected to forward voltage to reverse voltage at a point.
So a reflection coefficient can be, and often is, calculated for every
point along a line, not just at discontinuities or points of actual
reflection. This can be done with nothing more than the knowledge of the
values of forward and reflected voltages at the point of calculation.

As it turns out, the value of the reflection coefficient at any point
will be equal to (Z - Z0) / (Z + Z0), where Z is the impedance seen
looking down the line toward the load at the point of calculation. I'm
very leery of the use of "virtual" anything, since it often adds an
unnecessary level of confusion. But if I were to calculate a reflection
coefficient at some point along a continuous line, I could replace the
remainder of the line and the load with a lumped load of impedance Z,
and maintain exactly the same reflection coefficient and forward and
reverse traveling waves in the remaining line. I wouldn't object, then,
if someone would say that there was a "virtual impedance" of Z at that
point when the line was intact, since all properties prior to that point
are unchanged if a lumped Z of that value is substituted for the
remainder. (I personally wouldn't call it "virtual" -- I'd just call it
the Z at that point, since it's the ratio of V to I there.) The point is
that the reflection coefficient was the same before and after the
substitution of the remaining line with a real lumped Z. Before the
substitution, reflection was occurring at the load. After, the
reflection is occurring at the new, substituted Z load. Yet the
reflection coefficient and traveling waves remain the same on the
remaining line.

A reflection coefficient isn't the cause of anything. It's simply a
calculated quantity used for computational and conceptual convenience.
Only an impedance discontinuity causes reflections, but we can calculate
a reflection coefficient at any point we choose, with its value being
well defined and unambiguous.

Roy Lewallen, W7EL

Jim Kelley wrote:


Cecil Moore wrote:
For that statement, the length of the feedline is unknown
and the load is unknown.

I was commenting on this, the subject of our conversation:

My statement that you objected to didn't have anything to
do with the following.

"Consider the following:

Source---50 ohm feedline---+---1/2WL 150 ohm---50 ohm load"

Reflection coefficients are determined by characteristic impedances, not
virtual ones.

On the contrary, the reflection coefficient, rho, at '+' in
the example above, is NOT determined by the characteristic
impedances. Above, rho (looking at '+' from the left) is determined
by taking the square root of (Pref/Pfwd) = 0. (150-50)/(150+50) is
NOT rho. I was mistaken to call that quantity "rho" in my article.
That quantity that I called "rho" is actually 's11' and I need to
update my article.
--
73, Cecil, W5DXP

Roy Lewallen wrote:

Jim Kelley wrote:
. . .
Voltages on a transmission line do not determine reflection
coefficients.
Reflection coefficients are determined by characteristic impedances, not
virtual ones.

73, Jim AC6XG

I disagree with this. When applied to transmission lines, the (voltage)
reflection coefficient is, as far as I can tell, universally defined as
the ratio of reflected to forward voltage to reverse voltage at a point.

That rho is equivalent to that ratio of voltages is not in dispute. I
might dispute that it's 'defined' by that ratio. We agree the
reflection is caused by an impedance discontinuity. It is the
relationship of those impedances that determines how much of an incident
voltage will be reflected. From my perspective, one builds a network of
impedances in order to achieve the desired voltage relationships. But
one cannot build voltage relationships in order to obtain a network of
impedances.

Maybe it's another chicken and egg argument.

Only an impedance discontinuity causes reflections, but we can calculate
a reflection coefficient at any point we choose, with its value being
well defined and unambiguous.

Wouldn't the most well defined and unabiguous be at a point of
reflection? ;-)

73, Jim AC6XG

Cecil Moore wrote:

Jim Kelley wrote:
Reflection coefficients are determined by characteristic impedances, not
virtual ones.

On the contrary, the reflection coefficient, rho, at '+' in
the example above, is NOT determined by the characteristic
impedances.

I just showed you how characteristic impedances are used to calculate
the reflection coefficient at '+'. But you can wish it into the
cornfield if you like, Anthony. :-)

(150-50)/(150+50) is
NOT rho.

Is it the reflection coefficient for a 50 ohm to 150 ohm impedance
discontinuity?

I was mistaken to call that quantity "rho" in my article.
That quantity that I called "rho" is actually 's11' and I need to
update my article.

Since S-parameters were never even mentioned in your article, updating
it seems an understatement.

73, Jim AC6XG

On Thu, 02 Oct 2003 14:00:26 -0700, Jim Kelley
wrote:
updating it seems an understatement.
:-)

Jim Kelley wrote:
Roy Lewallen wrote:

Jim Kelley wrote:
. . .
Voltages on a transmission line do not determine reflection
coefficients.
Reflection coefficients are determined by characteristic impedances, not
virtual ones.

73, Jim AC6XG

I disagree with this. When applied to transmission lines, the (voltage)
reflection coefficient is, as far as I can tell, universally defined as
the ratio of reflected to forward voltage to reverse voltage at a point.


That rho is equivalent to that ratio of voltages is not in dispute. I
might dispute that it's 'defined' by that ratio.

I stand corrected on that point. I did a quick scan of ten
electromagnetics references. Seven quite clearly defined it that way.
One (Magnusson) was vague, and one (Jordan & Balmain) gave pretty equal
weight to both V and Z ratios as a definition. Holt, whom I consider one
my best references, clearly defines reflection coefficient in terms of
impedances. Regardless of definition, virtually all give both the V and
Z relationships which are, of course, mathematically equivalent.

We agree the
reflection is caused by an impedance discontinuity. It is the
relationship of those impedances that determines how much of an incident
voltage will be reflected. From my perspective, one builds a network of
impedances in order to achieve the desired voltage relationships. But
one cannot build voltage relationships in order to obtain a network of
impedances.

Maybe it's another chicken and egg argument.

I think so. One sees different impedances, i.e., V/I ratios, along a
transmission line, so it could be argued that impedances have been
created from voltages and currents. But I'll leave the arguing to those
people more interested in philosophy than engineering. As long as the
principles are understood and communicated, the point of view is largely
a matter of taste.


Only an impedance discontinuity causes reflections, but we can calculate
a reflection coefficient at any point we choose, with its value being
well defined and unambiguous.


Wouldn't the most well defined and unabiguous be at a point of
reflection? ;-)


I don't think so. It's completely well defined and unambiguous at any
point along the line. At any point, there's only one value of Vf and Vr,
and only one value of Z and Z0, so there can only be one value of
reflection coefficient. Knowing the reflection coefficient and Z0, for
example, one knows for sure what the value of Z is at that point. I find
that a number of the text authors freely use the concept of reflection
coefficient at any point along a line, not necessarily a point of
reflection. I don't have any problem with it, either. I see requiring a
reflection in order to have a reflection coefficient as sort of like
requiring dissipation in order to have a resistance. It's not really
necessary, and the calculated value can still have meaning.

Please don't let this detract from the ongoing discussion. It's really a
minor point.

Roy Lewallen, W7EL

I disagree with this. When applied to transmission lines, the (voltage)
reflection coefficient is, as far as I can tell, universally defined as
the ratio of reflected to forward voltage to reverse voltage at a point.
So a reflection coefficient can be, and often is, calculated for every
point along a line, not just at discontinuities or points of actual
reflection.

This can be done with nothing more than the knowledge of the
values of forward and reflected voltages at the point of calculation.

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

Sorry! Just to continue and further confuse the haggling, the forward
voltages are unknown because one does not know, in the case of amateur
systems, what is the internal voltage and internal impedance of the
transmitter.

It is this unknown voltage and internal impedance which the so-called SWR
(Rho) meter merely ASSUMES.

Are you disagreeing with something I said, or just adding a note? If
disagreeing, what again was it that you disagree with?

The source voltage and internal impedance have nothing to do with the
reflection coefficient at any point. The forward and reverse voltages
are indeed known if one knows, for example, the line Z0 and the load
impedance and the load power, voltage, or current. It's not necessary to
know the source impedance to find these values, the forward and reverse
voltages, or the reflection coefficient.

Roy Lewallen, W7EL

Reg Edwards wrote:
I disagree with this. When applied to transmission lines, the (voltage)
reflection coefficient is, as far as I can tell, universally defined as
the ratio of reflected to forward voltage to reverse voltage at a point.
So a reflection coefficient can be, and often is, calculated for every
point along a line, not just at discontinuities or points of actual
reflection.


This can be done with nothing more than the knowledge of the
values of forward and reflected voltages at the point of calculation.


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

Sorry! Just to continue and further confuse the haggling, the forward
voltages are unknown because one does not know, in the case of amateur
systems, what is the internal voltage and internal impedance of the
transmitter.

It is this unknown voltage and internal impedance which the so-called SWR
(Rho) meter merely ASSUMES.

Jim Kelley wrote:
I just showed you how characteristic impedances are used to calculate
the reflection coefficient at '+'. But you can wish it into the
cornfield if you like, Anthony. :-)

Absolutely no chance that you are simply wrong?

(150-50)/(150+50) is NOT rho.

Is it the reflection coefficient for a 50 ohm to 150 ohm impedance
discontinuity?

It is the 's11' reflection coefficient for that impedance discontinuity.
It is NOT the 'rho' at '+' unless the signals are orthogonal to each
other at '+'. Chances are they are not orthogonal.

I was mistaken to call that quantity "rho" in my article.
That quantity that I called "rho" is actually 's11' and I need to
update my article.

Since S-parameters were never even mentioned in your article, updating
it seems an understatement.

In my article, I called (Z1-Z0)/(Z1+Z0) the RHO(fv) and said it was
equal to S11. I should just have called it 'S11'. And I just checked
my web page. S11 is definitely mentioned in my article.
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards wrote:
Sorry! Just to continue and further confuse the haggling, the forward
voltages are unknown because one does not know, in the case of amateur
systems, what is the internal voltage and internal impedance of the
transmitter.

There are an infinite number of internal voltages and internal impedances
that will give the same voltage on the line. All you need to know is the
forward power and reflected power.

It is this unknown voltage and internal impedance which the so-called SWR
(Rho) meter merely ASSUMES.

Pretty easy to measure the forward and reflected powers and take the
square root of Pref/Pfwd to find 'rho' based on those assumptions.
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards wrote:

I disagree with this. When applied to transmission lines, the (voltage)
reflection coefficient is, as far as I can tell, universally defined as
the ratio of reflected to forward voltage to reverse voltage at a point.
So a reflection coefficient can be, and often is, calculated for every
point along a line, not just at discontinuities or points of actual
reflection.

This can be done with nothing more than the knowledge of the
values of forward and reflected voltages at the point of calculation.

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

Sorry! Just to continue and further confuse the haggling, the forward
voltages are unknown because one does not know, in the case of amateur
systems, what is the internal voltage and internal impedance of the
transmitter.

It is this unknown voltage and internal impedance which the so-called SWR
(Rho) meter merely ASSUMES.


Reg, that can't possibly be you. Someone has hijacked your e-mail.

Where is either of those assumptions required? Those are transmitter
properties, and they only affect the overall level of
power/voltage/current on the line. Reflection coefficient (rho) and SWR
are properties exclusively of the line and its load, not the
transmitter.

If you change anything at the transmitter, all forward and reflected
quantities change by the same scaling factor so their ratio stays the
same.

The SWR/rho meter measures reflection coefficient as a ratio of forward
and reflected signals. Either you yourself calculate the ratio of the
forward and reverse readings, or else you adjust the meter for
full-scale on the forward setting (which amounts to the same thing).

If you believe that rho has anything to do with the transmitter, you'd
expect to find some transmitter properties in the fundamental
definitions of what rho *is*. But they ain't there.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

On Fri, 3 Oct 2003 07:39:15 +0100, "Ian White, G3SEK"
wrote:

Sorry! Just to continue and further confuse the haggling, the forward
voltages are unknown because one does not know, in the case of amateur
systems, what is the internal voltage and internal impedance of the
transmitter.

It is this unknown voltage and internal impedance which the so-called SWR
(Rho) meter merely ASSUMES.


Reg, that can't possibly be you. Someone has hijacked your e-mail.

Where is either of those assumptions required? Those are transmitter
properties, and they only affect the overall level of
power/voltage/current on the line. Reflection coefficient (rho) and SWR
are properties exclusively of the line and its load, not the
transmitter.


Hi Ian,

There are one of two possible explanations for your posting:
1. You have not obtained that copy of Chipman that you ordered.
2. You have not read it.
Of course, you can add a third, fourth or fifth... in complete absence
of Chipman's discussion if his material does not agree with your
interpretations.

This is not an unexplored topic, and in fact dates back to earlier
discussions whose citations to Chipman were offered by me to no refute
- merely denial and the general wholesale abandonment of learned
posters who preferred to chase after specious claims (simpler game).

73's
Richard Clark, KB7QHC

Cecil Moore wrote:

Jim Kelley wrote:
I just showed you how characteristic impedances are used to calculate
the reflection coefficient at '+'. But you can wish it into the
cornfield if you like, Anthony. :-)

Absolutely no chance that you are simply wrong?

Only if Born and Wolfe are wrong. Since people use these formulas every
day, proving them wrong might be quite a challenge. I should think the
fact that their formula produces the correct answer to your problem
should lend Born and Wolfe at least some credence.

(150-50)/(150+50) is NOT rho.

Is it the reflection coefficient for a 50 ohm to 150 ohm impedance
discontinuity?

It is the 's11' reflection coefficient for that impedance discontinuity.
It is NOT the 'rho' at '+' unless the signals are orthogonal to each
other at '+'. Chances are they are not orthogonal.

If the 150 ohm line was terminated in 150 ohms or was infinitely long
would
Vr = Vf * (150-50)/(150+50)?

73, Jim AC6XG

Richard Clark wrote:
There are one of two possible explanations for your posting:
1. You have not obtained that copy of Chipman that you ordered.
2. You have not read it.
Of course, you can add a third, fourth or fifth... in complete absence
of Chipman's discussion if his material does not agree with your
interpretations.

Richard, you might be interested to know that HP's s-parameter ap note,
AN 95-1, page 22 under Transducer Power Gain, lists the power available
from the source as the (square of the magnitude of the source voltage)
divided by [one minus the (square of the magnitude of the source's complex
reflection coefficient)], i.e. |Vs|^2/(1-|rho|^2)=power available from
the source where presumably source-rho = (Zs-Z0)/(Zs+Z0)
--
73, Cecil, W5DXP

Jim Kelley wrote:
(150-50)/(150+50) is NOT rho.

Is it the reflection coefficient for a 50 ohm to 150 ohm impedance
discontinuity?

Relating this question to the s-parameters as explained in HP's
ap note AN 95-1, s11 is the input reflection coefficient when the
load reflection coefficient is zero. s'11 is the input reflection
coefficient with an arbitrary load reflection coefficient.

s'11 = s11 + [s12*s21(ZL-Z0)/(ZL+Z0)]/[1-s22(ZL-Z0)/(ZL+Z0)]

So s'11 is the rho to the left of '+' in the previous diagram.
That's the rho on the 50 ohm line.

s'11 = rho

s11 equals rho only when Z-Load = Z0, i.e. (ZL-Z0)/(ZL+Z0)=0
--
73, Cecil, W5DXP

Jim Kelley wrote:

Cecil Moore wrote:
Absolutely no chance that you are simply wrong?

Only if Born and Wolfe are wrong. Since people use these formulas every
day, proving them wrong might be quite a challenge. I should think the
fact that their formula produces the correct answer to your problem
should lend Born and Wolfe at least some credence.

Absolutely no chance that there is a small detail that you do not
understand and are therefore misinterpreting something?

If the 150 ohm line was terminated in 150 ohms or was infinitely long
would
Vr = Vf * (150-50)/(150+50)?

Yes, it would, and all three reflection coefficients, rho, s11, and
s'11 would be equal in that particular case. Please see my other posting
on this subject and you will comprehend the small mental error you are
making. In short,

s'11 = rho but s11 doesn't equal rho unless the load-rho equals zero.
--
73, Cecil, W5DXP

Cecil Moore wrote:

Jim Kelley wrote:

Cecil Moore wrote:
Absolutely no chance that you are simply wrong?

Only if Born and Wolfe are wrong. Since people use these formulas every
day, proving them wrong might be quite a challenge. I should think the
fact that their formula produces the correct answer to your problem
should lend Born and Wolfe at least some credence.

Absolutely no chance that there is a small detail that you do not
understand and are therefore misinterpreting something?

In this instance: It's no greater than, and probably less than the
chance that you do not understand and are therefore misinterpreting
something.

Here's an idea, Cecil. Instead of simply trying to discredit your
correspondent, why don't actually find something wrong with the
equations?

s'11 = rho but s11 doesn't equal rho unless the load-rho equals zero.

Hence the utiltity of the Born and Wolfe equations.

73, Jim AC6XG

On Fri, 03 Oct 2003 12:01:08 -0500, Cecil Moore
wrote:

Richard Clark wrote:
There are one of two possible explanations for your posting:
1. You have not obtained that copy of Chipman that you ordered.
2. You have not read it.
Of course, you can add a third, fourth or fifth... in complete absence
of Chipman's discussion if his material does not agree with your
interpretations.

Richard, you might be interested to know that HP's s-parameter ap note,
AN 95-1, page 22 under Transducer Power Gain, lists the power available
from the source as the (square of the magnitude of the source voltage)
divided by [one minus the (square of the magnitude of the source's complex
reflection coefficient)], i.e. |Vs|^2/(1-|rho|^2)=power available from
the source where presumably source-rho = (Zs-Z0)/(Zs+Z0)

Hi Cecil,

-sigh- even when you offer confirmatory recitations you still miss the
details. There are only 11 pages in Application Note 95-1 and the
material you describe appears on page 4 not 22.

The voltage from the generator is also portrayed in Fig. 3 entitled
"Flow graph of network of Fig. 2." Figure 2, of course, shows the
generator complete with Zs which most here deny exists, or dismiss as
immaterial to any discussion. This is due entirely to their speed
reading past their own sources' discussion that ALL DISCUSSION OF SWR
assumes the source matches the line it feeds. Such an explicit or
implicit relationship is fundamentally required, or the entire text
that they cite is rendered useless gibberish. The most garbled of
those proclamations is that the source Z has no bearing on line SWR.

This same flowgraph is present in many similar works (AN 95-1 is
hardly unique) and being presented early in the work (like Chipman's
similar observation of requiring source-line matching) is skipped so
that the reader (sic) can scrounge their favorite snippet of math and
remove it from its required context. Chipman also presents much the
same treatment in non S-Parameter discussion, but that is quite
obviously from the part unread by the great mass of so called
adherents to his discussion.

However, to give some flexibility to the discussion; such shortfalls
of understanding how SWR works is simply through lack of experience in
the matter. It is understandable when the usual approach to this
topic is taken by employing a transmitter that both specifies its
output at a Z of 50 Ohms and exhibits a Z of 50 Ohms. Given such a
source, the casual debater is lulled into the comfortable illusion of
having been born on third base thinking they hit a triple in the
debate against source Z (no, the count is three strikes).

Simply because they encounter no ill consequence of source mismatch is
NOT evidence of the source Z being immaterial to the process of
measuring SWR. Luck counts for nothing in debate - unless it is
admitted to. None here count themselves lucky - it would diminish
their sense of erudition.

I don't expect there will be any substantive discussion following this
that will change physics to conform to those illusions (my comments
here will not "change their minds").

73's
Richard Clark, KB7QHC

Sour grapes, or what?

As I said, the reflection coefficient at '+' can be calculated
accurately using just the characteristic impedances, as shown by Born
and Wolf.

Or you could use s-parameters. Why does it bother you so much that
there might be another way to do it? I suspect they're really one in
the same.


Cecil Moore wrote:

Jim Kelley wrote:
Here's an idea, Cecil. Instead of simply trying to discredit your
correspondent, why don't actually find something wrong with the
equations?

It's obvious you didn't comprehend the difference between
s11=(Z1-Z0)/(Z1+Z0) and rho=s'11=Vref/Vfwd, at least for
awhile. Anyone can copy equations out of a book while maintaining
a misunderstanding of a definition. There is no problem with the
equations. The only problem is with the correspondent's definitions
and it is a minor one that is easy to fix just by getting the
definitions correct.
--
73, Cecil, W5DXP

Jim Kelley wrote:
Here's an idea, Cecil. Instead of simply trying to discredit your
correspondent, why don't actually find something wrong with the
equations?

It's obvious you didn't comprehend the difference between
s11=(Z1-Z0)/(Z1+Z0) and rho=s'11=Vref/Vfwd, at least for
awhile. Anyone can copy equations out of a book while maintaining
a misunderstanding of a definition. There is no problem with the
equations. The only problem is with the correspondent's definitions
and it is a minor one that is easy to fix just by getting the
definitions correct.
--
73, Cecil, W5DXP

Richard Clark wrote:

wrote:
-sigh- even when you offer confirmatory recitations you still miss the
details. There are only 11 pages in Application Note 95-1 and the
material you describe appears on page 4 not 22.

-sigh- The PDF version of HP ap note AN 95-1 contains 79 pages.

Simply because they encounter no ill consequence of source mismatch is
NOT evidence of the source Z being immaterial to the process of
measuring SWR.

A source mismatch affects the power available from the source. The
SWR does not depend upon the power available from the source. The
SWR is the same whether the source is 1% efficient or 99% efficient.
Efficiency depends upon Zs. SWR does not.
--
73, Cecil, W5DXP

On Fri, 03 Oct 2003 13:49:15 -0500, Cecil Moore
wrote:

A source mismatch affects the power available from the source. The
SWR does not depend upon the power available from the source. The
SWR is the same whether the source is 1% efficient or 99% efficient.
Efficiency depends upon Zs. SWR does not.

Hi Cecil,

So, a source exhibiting 200 Ohms Resistive feeding a 50 Ohm line that
is then terminated in a load of 200 Ohms Resistive exhibits what SWR?

The absence of a numeric answer is par for the course here. The
answer, of course, can be found in Chipman's text but that requires
the act of reading, not snipping (which would still be available to
the literate). Many here stumble when it comes to measuring SWR
employing (in this case) 2 resistors and a hank of line - how they
could imagine they respond faithfully to more elaborate enquiries is
quite amusing, especially when they argue the Source Z has nothing to
do with it.

My mental image of that assemblage of pundits is that of them crowded
on a small desert isle, each proclaiming it to be a vast, lush
continent. Another SWR Don added to that bunch will teeter someone
into the brine. ;-)

Do any of you know how to tread water? Seems to be the perfect
Darwinian thinning mechanism; but in fact most already tread water at
high tide.

73's
Richard Clark, KB7QHC

Jim Kelley wrote:
As I said, the reflection coefficient at '+' can be calculated
accurately using just the characteristic impedances, as shown by Born
and Wolf.

Why is this so hard for you to understand? What is the rho
of the following?

source---50 ohm feedline---+---150 ohm feedline---load150

"Just the characteristic impedances" are given. You say you
can "calculate rho accurately" from just that. So prove your
statement. (load150 means the load is not equal to 150 ohms)

If you say rho = (150-50)/(150+50) then you are mistaken. If
Born and Wolf say rho = (150-50)/(150+50) then Born and Wolf
are mistaken. rho = Vref/Vfwd and, contrary to what you say,
there is *NOT* enough information given to calculate rho.
--
73, Cecil, W5DXP

Cecil Moore wrote:

Jim Kelley wrote:
As I said, the reflection coefficient at '+' can be calculated
accurately using just the characteristic impedances, as shown by Born
and Wolf.

Why is this so hard for you to understand?

Answer: it isn't hard for me to understand.

Question: Why must you inevitably get personal in these technical
discussions?

What is the rho
of the following?

source---50 ohm feedline---+---150 ohm feedline---load150

"Just the characteristic impedances" are given. You say you
can "calculate rho accurately" from just that. So prove your
statement. (load150 means the load is not equal to 150 ohms)

That's a different problem, isn't it.

If you say rho = (150-50)/(150+50) then you are mistaken.

Let the record show that I didn't say it. ;-)

If
Born and Wolf say rho = (150-50)/(150+50) then Born and Wolf
are mistaken.

You mean to tell me you don't even know what you're arguing about here?
If you understood and had paid any attention at all you'd have known
what Born and Wolf would say, and you wouldn't be speculating so wildly
about it.

rho = Vref/Vfwd and, contrary to what you say,
there is *NOT* enough information given to calculate rho.

That's _exactly_ what I would say about it.

It wouldn't be possible to evaluate r23 without knowing the load
impedance. And as I pointed out before, the 150 ohm feedline must be
some known number of quarter wavelengths in order to know which form of
the equation to use.

73, Jim AC6XG

On Fri, 03 Oct 2003 14:48:38 -0500, Cecil Moore
wrote:

Jim Kelley wrote:
As I said, the reflection coefficient at '+' can be calculated
accurately using just the characteristic impedances, as shown by Born
and Wolf.

Why is this so hard for you to understand? What is the rho
of the following?

source---50 ohm feedline---+---150 ohm feedline---load150

"Just the characteristic impedances" are given. You say you
can "calculate rho accurately" from just that. So prove your
statement. (load150 means the load is not equal to 150 ohms)


Howzabout (as a variation of another posting):
source50---50 ohm feedline---+---150 ohm feedline---load150

If it is so easy for you, and difficult for Jim, this should be a
slam-dunk.... But I won't hold my breath for either of my posts to
find a literal, numeric answer.

I also promise no more follow-up responses to either thread where no
solution (a numeric one, not a philosophical treatise) is presented.
;-)

73's
Richard Clark, KB7QHC

Richard Clark wrote:
So, a source exhibiting 200 Ohms Resistive feeding a 50 Ohm line that
is then terminated in a load of 200 Ohms Resistive exhibits what SWR?

Assuming a lossless 50 ohm line, the steady-state SWR is 4:1 no
matter what the source impedance (assuming there exists a source
voltage not equal to zero volts). Steady-state SWR in a lossless
feedline is a constant fixed voltage ratio from end to end.
--
73, Cecil, W5DXP

Richard Clark wrote:
Howzabout (as a variation of another posting):
source50---50 ohm feedline---+---150 ohm feedline---load150

If it is so easy for you, and difficult for Jim, this should be a
slam-dunk....

It is not easy for me or anyone else. It is impossible to accurately
calculate rho just from the above information. Jim is the one who
says it's easy. I say it's impossible. Did you read Roy's thoughts
on the subject?
--
73, Cecil, W5DXP

On Fri, 03 Oct 2003 14:58:15 -0500, Cecil Moore
wrote:

Richard Clark wrote:
So, a source exhibiting 200 Ohms Resistive feeding a 50 Ohm line that
is then terminated in a load of 200 Ohms Resistive exhibits what SWR?

Assuming a lossless 50 ohm line, the steady-state SWR is 4:1 no
matter what the source impedance (assuming there exists a source
voltage not equal to zero volts). Steady-state SWR in a lossless
feedline is a constant fixed voltage ratio from end to end.

Hi Cecil,

WRONG!

The method of computation you employ violates Chipman and any other of
a host of authoritative sources on the topic. If you actually
attempted to verify this at the bench you would "perhaps" find it at
only one point, or at harmonic distances (wavelength specific) along
the line. That means you would stand only a couple of percent chance
of that with a random choice. Guesses are not responsive to the
intent and no points are awarded.

I will leave you to discover Chipman's means to find SWR any where
along any line for yourself. You might enjoy the celebrity of being
the second to do so. ;-)

73's
Richard Clark, KB7QHC

Richard Clark wrote:
The method of computation you employ violates Chipman and any other of
a host of authoritative sources on the topic. If you actually
attempted to verify this at the bench you would "perhaps" find it at
only one point, or at harmonic distances (wavelength specific) along
the line.

-sigh- One cannot verify anything about lossless lines on the bench.
The only verifications about lossless lines that are possible have to
be done in one's head because that's the only place lossless lines
exist. One can come close with open-wire line and extrapolate the
results to lossless lines. You see the effects more on coax than on
open-wire line simply because coax is lossier than open-wire line.

I will leave you to discover Chipman's means to find SWR any where
along any line for yourself.

You told me to reference page 139 which I did. All that page talks
about is lossy feedline with a complex Z0. The purely resistive feedline,
given by you in your example, cannot have a complex Z0. So what Chipman
has to say is irrelevant to the problem you posed, i.e. purely resistive
Z0, purely resistive source impedance, and purely resistive load. You
apparently should have posed a complex Z0.

Chipman explains perfectly why the measured SWR may vary with a
lossy line, i.e. with a complex Z0. There are points of conjugate
matching up and down the line where an oscillation takes place. The
oscillation causes extra reflections and re-reflections at the
conjugate match point, an exchange of a third energy between the
capacitive reactance and the inductive reactance at that point,
that affects the SWR readings. But such is not possible with the
purely resistive Z0 that you posed.
--
73, Cecil http://www.qsl.net/w5dxp



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On Fri, 03 Oct 2003 18:11:14 GMT, Richard Clark wrote:

On Fri, 03 Oct 2003 12:01:08 -0500, Cecil Moore
wrote:

Richard Clark wrote:
There are one of two possible explanations for your posting:
1. You have not obtained that copy of Chipman that you ordered.
2. You have not read it.
Of course, you can add a third, fourth or fifth... in complete absence
of Chipman's discussion if his material does not agree with your
interpretations.

Richard, you might be interested to know that HP's s-parameter ap note,
AN 95-1, page 22 under Transducer Power Gain, lists the power available
from the source as the (square of the magnitude of the source voltage)
divided by [one minus the (square of the magnitude of the source's complex
reflection coefficient)], i.e. |Vs|^2/(1-|rho|^2)=power available from
the source where presumably source-rho = (Zs-Z0)/(Zs+Z0)

Hi Cecil,

-sigh- even when you offer confirmatory recitations you still miss the
details. There are only 11 pages in Application Note 95-1 and the
material you describe appears on page 4 not 22.

The voltage from the generator is also portrayed in Fig. 3 entitled
"Flow graph of network of Fig. 2." Figure 2, of course, shows the
generator complete with Zs which most here deny exists, or dismiss as
immaterial to any discussion. This is due entirely to their speed
reading past their own sources' discussion that ALL DISCUSSION OF SWR
assumes the source matches the line it feeds. Such an explicit or
implicit relationship is fundamentally required, or the entire text
that they cite is rendered useless gibberish. The most garbled of
those proclamations is that the source Z has no bearing on line SWR.

This same flowgraph is present in many similar works (AN 95-1 is
hardly unique) and being presented early in the work (like Chipman's
similar observation of requiring source-line matching) is skipped so
that the reader (sic) can scrounge their favorite snippet of math and
remove it from its required context. Chipman also presents much the
same treatment in non S-Parameter discussion, but that is quite
obviously from the part unread by the great mass of so called
adherents to his discussion.

However, to give some flexibility to the discussion; such shortfalls
of understanding how SWR works is simply through lack of experience in
the matter. It is understandable when the usual approach to this
topic is taken by employing a transmitter that both specifies its
output at a Z of 50 Ohms and exhibits a Z of 50 Ohms. Given such a
source, the casual debater is lulled into the comfortable illusion of
having been born on third base thinking they hit a triple in the
debate against source Z (no, the count is three strikes).

Simply because they encounter no ill consequence of source mismatch is
NOT evidence of the source Z being immaterial to the process of
measuring SWR. Luck counts for nothing in debate - unless it is
admitted to. None here count themselves lucky - it would diminish
their sense of erudition.

I don't expect there will be any substantive discussion following this
that will change physics to conform to those illusions (my comments
here will not "change their minds").

73's
Richard Clark, KB7QHC

Richard, I'm dismayed with your statements above. Are you really serious? Or are
you just giving Cecil a bad time?

I've been grappling with your last email to me concerning the nature of the
source resistance of RF amps, and as with your statements above, I'm at a loss
as to how to respond, because we are 180 degrees apart on the source resistance
issue. I'm still going to respond to it, but right now I want to address the SWR
issue.

Richard, how can you possibly believe that the output impedance of the source
has any effect on the SWR on a transmission line? The only conditions
responsible for SWR are the Zo of the line and the ZL of the load--nothing else.
I've been bench measuring SWR for more than 50 years, beginning with using the
slotted line before more sophisticated machinery was available. It didn't matter
what the source impedance was, the SWR remained the same, whatever the source.
Ian told it like it is, and so does Walter C. Johnson in his "Transmission Lines
and Networks, Page 100, where he says:

"The steady state ratio Eplus/Eminus was determined in Eq 11 as the reflection
coefficient k...This ratio is determined only by the load and the line, not by
the generator. It is completely unaffected by the quantity kg = (Zg - Zo)/(Zg +
Zo), which is the reflection coefficient seen by an individual
backward-traveling wave as it reaches the generator terminals. ...the latter
affects the steady state solution only on its influence on the sending-end
voltage, i.e., through its influence on the magnitude of the entire solution."

Your reply comments, please.

Walt, W2DU

Just a comment.

Whereas it is true the SWR remains relatively constant along a relatively
low loss line, it is NOT true that the reflection coefficint remains
constant. Its phase angle (1/2 of the information it contains) varies in
proportion the distance from the termination.

On Fri, 3 Oct 2003 22:57:54 +0000 (UTC), "Reg Edwards"
wrote:

Just a comment.

Whereas it is true the SWR remains relatively constant along a relatively
low loss line, it is NOT true that the reflection coefficint remains
constant. Its phase angle (1/2 of the information it contains) varies in
proportion the distance from the termination.


A little further comment.

The magnitude of rho is always the ratio of reflected to forward voltage at
whatever whatever point it is measured on any lossless or non-lossless line. On
lossless line the magnitude of rho is constant along the entire line, but
decreases logarithmically with distance from the load,depending on the
attenuation of the line.

However, the phase angle of rho equals the angular difference between the
forward and reflected waves. Because they are traveling in opposite directions
the rate of angular change is twice the change in electrcal distance along the
line. Thus the phase angle of rho changes at twice the change in electrical
distance along the line, varying between zero and 180 degrees for every 1/4wl.

Walt, W2DU

On Fri, 03 Oct 2003 16:12:48 -0500, Cecil Moore
wrote:
One cannot verify anything about lossless lines on the bench.

You can't, that's for sure, and not even with a line so short you
couldn't measure its loss - so what's the point of arguing lossless?

So what Chipman
has to say is irrelevant to the problem you posed

Hi Cecil,

I have already provided quotes, chapter and verse that refute your
statement. No point in going further.

73's
Richard Clark, KB7QHC

On Fri, 03 Oct 2003 21:41:15 GMT, Walter Maxwell wrote:


Richard, I'm dismayed with your statements above. Are you really serious? Or are
you just giving Cecil a bad time?

That wouldn't take much to push Cecil off dead center.


I've been grappling with your last email to me concerning the nature of the
source resistance of RF amps, and as with your statements above, I'm at a loss
as to how to respond, because we are 180 degrees apart on the source resistance
issue. I'm still going to respond to it, but right now I want to address the SWR
issue.

Richard, how can you possibly believe that the output impedance of the source
has any effect on the SWR on a transmission line?

Stephen Adam of HP using Beatty describes it quite well. The data you
have by email and has been posted here demonstrates it equally well.
It takes no more than two resistors and a length of line to confirm or
deny. My data confirms it, absolutely no one has offered negative
evidence, simply denials.

The only conditions
responsible for SWR are the Zo of the line and the ZL of the load--nothing else.
I've been bench measuring SWR for more than 50 years, beginning with using the
slotted line before more sophisticated machinery was available. It didn't matter
what the source impedance was, the SWR remained the same, whatever the source.
Ian told it like it is, and so does Walter C. Johnson in his "Transmission Lines
and Networks, Page 100, where he says:

If Walter Johnson was not explicit about it, he was certainly implicit
about the requirement that the source match the line it is driving for
any discussion of SWR. This is so commonplace that no one ever
examines the situation where the source is a mismatch.

Too many here simply flip to the section in their favorite book about
SWR and wholly neglect the fundamentals that present this simple
requirement. I have presented quotes, chapter and verse from Chipman
where he explicitly says as much, and those who hold Chipman have
abandoned discussion rather than refute those quotes or accept their
error. As one scribbler put it I was not going to "change his mind."
I have no doubt of that, such a statement paints one into an extremely
embarrassing corner once having uttered it. One thing I learned as a
Metrologist is that I am always wrong, the significance is in the
degree of error, not the philosophy of sin and the rejection in
ignorance.

Any number of correspondents here "might" have the capacity to simply
repeat my methods and report their data; but absolutely none
demonstrate it. I might be so far in error the meter is pegged, but
the quality of "sneer review" absolves me of sin. ;-)

Hi Walt,

I await your response by email for our last round of discussion. What
is presented above is old material already discussed. There is
nothing new presented by me in it that has not found its way to your
mailbox.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
I have already provided quotes, chapter and verse that refute your
statement.

Here's a quote from Chipman. "These large reflection coefficients are an
example of the phenomenon of 'resonant rise of voltage' in series resonant
circuits... The large reflection coefficients are obtained only when the
reactance of the load impedance is of opposite sign to the reactance
component of the characteristic impedance." You posed a purely resistive Z0.
The "resonant rise of voltage" cannot happen with a purely resistive Z0.
Would you like to pose a complex Z0?
--
73, Cecil http://www.qsl.net/w5dxp



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Richard Clark wrote:
I have presented quotes, chapter and verse from Chipman
where he explicitly says as much, ...

And I have presented quotes, chapter and verse from Chipman,
that disprove your interpretations of what he said.
--
73, Cecil http://www.qsl.net/w5dxp



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Reg, that can't possibly be you. Someone has hijacked your e-mail.

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

Ian, it IS me! Please calm yourself.

Let me put what I said into somewhat different words.

SWR meters are designed to operate and provide indications of SWR, Rho, Fwd
Power, Refl.Power, on the ASSUMPTION that the internal impedance of the
transmitter is 50 ohms. It makes the same INCORRECT assumption as a lot of
people do. This should not be surprising because it was people who designed
it.

So SWR meters nearly always give FALSE indications about what actually
exists.

Perfectionists may be upset at the repercussions of this alarming statement.

PS: In the whole of his excellent 236-page exceedingly comprehensive
volume, Chipman, in 1969, makes not the slightest mention of SWR meters.
----
Reg, G4FGQ

Reg Edwards wrote:

SWR meters are designed to operate and provide indications of SWR, Rho, Fwd
Power, Refl.Power, on the ASSUMPTION that the internal impedance of the
transmitter is 50 ohms.

I believe that to be an incorrect statement, Reg. The assumption is that
a Z0 of 50 ohms exists and the transmission line is long enough to force
the ratio of V/I to be 50 ohms for the forward wave and the reflected wave.
The phase between the forward voltage and current is assumed to be zero.
The phase between the reflected voltage and current is assumed to be zero.
Given all those assumptions, the internal impedance of the transmitter is
irrelevant. I'm not saying all those assumptions are always met.

Put a 50 ohm dummy load on an SWR meter and feed it with a transmitter
of unknown source impedance. The SWR meter will always read 1:1 because
the dummy load forces the V/I ratio to be 50 no matter what the source
impedance. That's what the 50 ohm characteristic impedance of the
transmission is supposed to do to make the source impedance irrelevant.

PS: In the whole of his excellent 236-page exceedingly comprehensive
volume, Chipman, in 1969, makes not the slightest mention of SWR meters.

In 1969, virtually all ham transmitters had an adjustable pi-net output
so an SWR meter was not needed. When I started out as a ham in the 1950's,
just as many hams used 75 ohm coax as used 50 ohm coax, maybe more. The
pi-net output of a typical ham transmitter back then didn't care what the
Z0 was. I didn't own an SWR meter until the 1980's when I bought an IC-745.

In 1969, the "antenna tuner" was built into the transmitter. If wide-
range antenna tuners were built into transmitters today, there would be
little need for the SWR meter. I don't know of anyone who puts an SWR
meter between an SGC-230 and the antenna.
--
73, Cecil http://www.qsl.net/w5dxp



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--

SWR meters are designed to operate and provide indications of SWR, Rho,
Fwd
Power, Refl.Power, on the ASSUMPTION that the internal impedance of the
transmitter is 50 ohms.
=================================
I believe that to be an incorrect statement, Reg. The assumption is that
a Z0 of 50 ohms exists and the transmission line is long enough to force
the ratio of V/I to be 50 ohms for the forward wave and the reflected
wave.

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

But where have you hidden this remarkable transmission line which is long
enough to mug and hoodwink so-called SWR meters?

It does not exist!

Your argument falls flat at the start.

Reg, G4FGQ