Owen Duffy wrote:
"Jeff" wrote in
e.com:

If you used a TDR, for example, to look at the set-up you would see 2
points of discontinuity, firstly at the 100 ohm source to 50 ohm cable
interface, and secondly at the cable to 200 ohm load. BOTH of these
discontinuities add to the overall mismatch as seen by the 100 ohm
load.

Your TDR does not work in the steady state frequency domain space, and is
misleading you.

In the steady state, the (complex) ratio of forward voltage to reflected
voltage is determined solely by the load impedance and characteristic
impedance of the line.

In crude terms, during establishement of steady state, you can view that
a load end reflected wave which is then partially reflected at a
mismatched source end, will reach the load end and be reflected in the
same ratio as the earlier passes. The subsequent round trips as steady
state is approached do not change the (complex) ratio of forward voltage
to reflected voltage in the steady state.

I know you have support here for the assertion that source end mismatch
affects VSWR in the steady state, but you won't find it in reputable text
books.

Owen and Cecil are right: the source (transmitter) has no effect
whatever on the VSWR on the line.

That isn't just an assertion - it is part of the bedrock transmission
line theory. Owen referred to "reputable textbooks", one of which would
surely be 'Theory and Problems of Transmission Lines' by R A Chipman
[1]. This book gains a lot of its reputation from its very complete
mathematical development of the theory, showing all the detailed
working.

Chipman treats standing wave patterns in two different ways: first by
assuming the final steady-state conditions, and then in much more detail
by considering multiple reflections between the load and the source.
Given a sufficient number of reflections, the multiple-reflection model
converges on exactly the same results as the steady-state analysis -
just as it does in the physical world.

VSWR on the line is determined by the ratio |Vmax|/|Vmin|. The complex
impedance that the source sees at the input terminals of the line is the
ratio V/I at that point (where V and I are both vector quantities which
include phase information). An alternative way of calculating either
VSWR or Zin is through the ratio Vforward/Vreflected (again vector
quantities).

All of these approaches are alternative pathways through the same body
of theory. They are all consistent with one another, and there is no
contradiction between any of them.

You will notice that all these standing wave relationships involve
ratios. Chipman's detailed analysis confirms that these ratios are
determined EXCLUSIVELY by the properties of the line and the load -
never the source.

The source properties do determine the magnitudes of all of the
individual voltages and currents - but when you change the source
properties (output voltage and/or impedance) all the individual voltages
and currents on the line and at the load are changed by the same factor.
So when you take the ratio, the source properties cancel right out
again.

All this confirms that, if you sweat out the math in all the different
levels of detail that Chipman did, the source (transmitter) still has no
effect whatever on the VSWR on the line.




[1] Out of print, but well worth searching for: ISBN 0-07-010747-5.
The web bookstores currently have eight copies on offer, at a range of
prices.



--

73 from Ian GM3SEK

Owen and Cecil are right: the source (transmitter) has no effect whatever
on the VSWR on the line.

That isn't just an assertion - it is part of the bedrock transmission line
theory. Owen referred to "reputable textbooks", one of which would surely
be 'Theory and Problems of Transmission Lines' by R A Chipman [1]. This
book gains a lot of its reputation from its very complete mathematical
development of the theory, showing all the detailed working.

I am sorry but you are not correct, I have not read Chipman so I cannot
comment on his analysis or your interpretation of his results, but my
understanding , practical experiments and CAD analysis would lead me to
disagree.

If we take the situation where the source is matched (50ohms) to the 5.35
wavelength transmission line (lossless to simplify things) with a 100ohm
load, I agree that the vswr is 4:1, unchanging with frequency.

Plotted on a Smith Chart when swept against frequency this gives a circle
centred on 1 (50ohms) with a radius of 4. i.e. on a constant VSWR circle.

Now if we change the source impedance to 100ohms and repeat the same sweep
and re-plot, keeping the chart normalized to 50 ohms, the circle moves on
the resistance axis, still with a radius of 4 and now passing though 2 (100
ohms) resistive. The centre moves to about 0.6 (30ohms). It then becomes
obvious that the locus of the circle is NOT a constant VSWR against
frequency.

You will come to the same conclusion if you normalize the chart to 100 ohms,
the new source impedance and re-plot.

The coax is acting as an impedance transformer, causing a shift along the
resistance axis.

Looking at it another way, the vswr changes sinusoidally with frequency, in
our example, between 2:1 and 8:1. (The same as the Smith chart plot with a
circle of radius 4 centred at about 0.6).

73
Jeff

Jeff wrote:
You will come to the same conclusion if you normalize the chart to 100 ohms,
the new source impedance and re-plot.

The Z0 of the transmission line has not changed to 100
ohms so normalizing the chart to 100 ohms is not valid.
--
73, Cecil, http://www.qsl.net/w5dxp

" You will come to the same conclusion if you normalize the chart to 100
ohms,
the new source impedance and re-plot.

The Z0 of the transmission line has not changed to 100
ohms so normalizing the chart to 100 ohms is not valid.
--
73, Cecil, http://www.qsl.net/w5dxp

It is just as valid as using 50 ohms, and the result is the same, a changing
vswr.

I see you have not commented on the main point of my post, that being that
the smith chart shows a changing vswr when you change the source impedance.

Hint: transmission line transformers would not work if the vswr did not
change.

73
Jeff

Jeff wrote:
w5dxp wrote:
The Z0 of the transmission line has not changed to 100
ohms so normalizing the chart to 100 ohms is not valid.

It is just as valid as using 50 ohms, and the result is the same, a changing
vswr.

No, the center of the Smith Chart is the Z0 of the transmission
line (when used on a transmission line). One cannot willy nilly
change the reference Z0. The confusion from doing such is obvious.

I see you have not commented on the main point of my post, that being that
the smith chart shows a changing vswr when you change the source impedance.

I think I see the problem. It is an *error* to change the
Smith Chart reference point when the source impedance
changes while the T-line Z0 and load remain the same.

Hint: transmission line transformers would not work if the vswr did not
change.

Hint: A lossless series-section transmission line transformer
has a *constant SWR*. It is the *constant SWR circle* that
causes the impedance transformation.

A fixed-constant SWR on 300 ohm line looks like it changes
when measured with a 50 ohm SWR meter but that is an illusion.
The SWR meter *must* be calibrated to the Z0 of the
transmission line in order to obtain a valid SWR reading.
The impedance is indeed being transformed all around the
constant SWR circle.

With your software, you are conceptually doing the same thing
as using a 50 ohm SWR meter on a 300 ohm transmission line.
The meter reading is invalid when taken at face value. The
meter reading does NOT indicate a valid SWR on the 300 ohm
feedline and neither does your software.
--
73, Cecil, http://www.qsl.net/w5dxp

"Cecil Moore" wrote in message
. ..
Jeff wrote:
w5dxp wrote:
The Z0 of the transmission line has not changed to 100
ohms so normalizing the chart to 100 ohms is not valid.

It is just as valid as using 50 ohms, and the result is the same, a
changing vswr.

No, the center of the Smith Chart is the Z0 of the transmission
line (when used on a transmission line). One cannot willy nilly
change the reference Z0. The confusion from doing such is obvious.

You are mis-representing what I said; which was that you can plot the
problem using with the chart normalized to EITHER 50 or 100 ohms (the
impedance of the generator or that of the line) and the net result will be
the same answer.

The chart does not necessarily be normalized to the impedance of a
transmission line that you are trying add, otherwise you would never be able
to include a series line of an impedance other than that of the chart in a
matching network.

Jeff

Jeff wrote:
The chart does not necessarily be normalized to the impedance of a
transmission line that you are trying add, otherwise you would never be able
to include a series line of an impedance other than that of the chart in a
matching network.

The point is that it is best to use one Smith Chart
for each Z0. Trying to plot multiple Z0's on the
same chart leads to the present confusion. The
fact that a piece of transmission line with an
SWR1 transforms impedances is NOT proof that
the SWR is changing. It is *only* proof the
the impedance is changing and that happens
with constant SWR.
--
73, Cecil, http://www.qsl.net/w5dxp

"Jeff" wrote in
.com:

Owen and Cecil are right: the source (transmitter) has no effect
whatever on the VSWR on the line.

That isn't just an assertion - it is part of the bedrock transmission
line theory. Owen referred to "reputable textbooks", one of which
would surely be 'Theory and Problems of Transmission Lines' by R A
Chipman [1]. This book gains a lot of its reputation from its very
complete mathematical development of the theory, showing all the
detailed working.

I am sorry but you are not correct, I have not read Chipman so I
cannot comment on his analysis or your interpretation of his results,
but my understanding , practical experiments and CAD analysis would
lead me to disagree.

If we take the situation where the source is matched (50ohms) to the
5.35 wavelength transmission line (lossless to simplify things) with a
100ohm load, I agree that the vswr is 4:1, unchanging with frequency.

Plotted on a Smith Chart when swept against frequency this gives a
circle centred on 1 (50ohms) with a radius of 4. i.e. on a constant
VSWR circle.

Now if we change the source impedance to 100ohms and repeat the same
sweep and re-plot, keeping the chart normalized to 50 ohms, the circle
moves on the resistance axis, still with a radius of 4 and now passing
though 2 (100 ohms) resistive. The centre moves to about 0.6 (30ohms).
It then becomes obvious that the locus of the circle is NOT a constant
VSWR against frequency.

You will come to the same conclusion if you normalize the chart to 100
ohms, the new source impedance and re-plot.

The coax is acting as an impedance transformer, causing a shift along
the resistance axis.

Looking at it another way, the vswr changes sinusoidally with
frequency, in our example, between 2:1 and 8:1. (The same as the Smith
chart plot with a circle of radius 4 centred at about 0.6).


If you are asserting that VSWR on a real or even theoretical line varies
sinudoidally with displacement, it is time to go back to basics. You need
some time with a reputable text book.

Owen

Owen Duffy wrote:
"Jeff" wrote in
e.com:

Owen and Cecil are right: the source (transmitter) has no effect
whatever on the VSWR on the line.

That isn't just an assertion - it is part of the bedrock transmission
line theory. Owen referred to "reputable textbooks", one of which
would surely be 'Theory and Problems of Transmission Lines' by R A
Chipman [1]. This book gains a lot of its reputation from its very
complete mathematical development of the theory, showing all the
detailed working.

I am sorry but you are not correct, I have not read Chipman so I
cannot comment on his analysis or your interpretation of his results,
but my understanding , practical experiments and CAD analysis would
lead me to disagree.


[...]

Looking at it another way, the vswr changes sinusoidally with
frequency, in our example, between 2:1 and 8:1. (The same as the Smith
chart plot with a circle of radius 4 centred at about 0.6).


If you are asserting that VSWR on a real or even theoretical line varies
sinudoidally with displacement, it is time to go back to basics. You need
some time with a reputable text book.

Agreed, but make that a textbook that specifically deals with the
subject in enough detail. Chipman was highly recommended by contributors
to earlier rounds of this debate. It isn't an easy read, but it's
certainly thorough.

I ordered the book from the other side of the world because I wanted to
be very sure of my answers next time around. We don't know where you
are, Jeff, but it would probably be easier and cheaper for you to do the
same.



--

73 from Ian GM3SEK

On Thu, 1 Mar 2007 20:13:32 +0000, Ian White GM3SEK
wrote:

I ordered the book from the other side of the world because I wanted to
be very sure of my answers next time around.

Hi Ian,

It will contain much of interest. For instance, it relates to Owen's
moribund thread "the power explanation." Page 205, third paragraph
from the bottom conforms to one of my recent posts the
"Although the power delivered by the source to the line is thus
shown to be reduced by the amount of the reflected power returning
to the input terminals ... the implication of the latter reasoning
that the reflected wave power is entirely absorbed in the source
impedance without affecting the total output of the signal source
generator, is incorrect."

Contrary to that teaching, is discussion on page 203, last paragraph.
It relates to figure 9-26, clearly illustrating a mismatched line fed
by a source with a source resistance. This may be upsetting to many:
"At the signal source end of the line ... none of the power
reflected by the terminal load impedance is re-reflected on
returning to the input end of the line."
The ellipsis reveals that the source Z matches the line Z.

To begin at the beginning of multiple reflection coverage, go to the
same named section (8.8) on page 174. It is not his complete say on
the topic, but it starts here formally. To add insult to someone's
injury, his math includes source Z. However, by the same token
Chipman explicitly states:
"... the shape of the standing wave pattern ... is in no way
affected by the quantities Vs, Zs and Rho-s at the source."

I would also note the irony in that Chipman expresses reflections in
lines in terms of power. To subdue that irony, I would also admit he
is quick to shift to energy when the usage of power is to lead to
problematic solutions (so, using power as an expression in this
context is allowable by precedent as being informal).

Of course, Chipman must be accepted as an authority for any of these
issues to be considered valid.

73's
Richard Clark, KB7QHC