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