Cecil,

I thought that we were considering steady state single frequency sine waves.
The whole thing becomes so much more straight forward when talking about
pulses.....

Tam/WB2TT
"Cecil Moore" wrote in message
...
Tarmo Tammaru wrote:
There are models for both lossy and non lossy transmission line. I have
not
used them, so it might take some learning. I can tell you though that
given
a load and transmission line, if you find the Z at the meter with an HP
vector impedance meter, and then put a lumped impedance of that same
value
at the meter, you will get the same results.

Not in reality, you won't. Any TV ghosting that exists because of
reflections will
disappear when you go to a lumped impedance. And the noise across the
lumped
impedance will not be identical to the noise associated with a long
transmission line.
--
73, Cecil http://www.qsl.net/w5dxp



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"Jim Kelley" wrote in message
...

Tarmo's simulation results seem to conflict with Richard's
interpretation of Chipman. Lately you've been leaning toward Richard's
point of view. How does the story end? ;-)

73, Jim AC6XG

Jim,
It probably can't be proven, unless somebody comes up with an alternative
definition for SWR. If you look at my simulator equations of a few listings
back, I proved that what my model (and the Bird wattmeter) call SWR is
RL/Z0. So, unless I screwed up, running any number of simulations is not
going to disprove that.

If you look at textbook examples where they transmit pulses, The source
impedance determines what V+ is, and whether there is a second reflection
from the source, but NOT what the reflection at the load end is. Lets delve
on this for a second. It seems fair to say that if the source impedance
determines V+, clearly it has an effect on V-. But, that does not mean it
has anything to do with rho.

Tam/WB2TT

Tarmo Tammaru wrote:
Jim,
It probably can't be proven, unless somebody comes up with an alternative
definition for SWR. If you look at my simulator equations of a few listings
back, I proved that what my model (and the Bird wattmeter) call SWR is
RL/Z0. So, unless I screwed up, running any number of simulations is not
going to disprove that.

It's hard to imagine how Rs (Zs) could have any effect on that ratio.

If you look at textbook examples where they transmit pulses, The source
impedance determines what V+ is, and whether there is a second reflection
from the source, but NOT what the reflection at the load end is.

Unless I = 0, source impedance should certainly have an effect on source
voltage. My car battery this morning comes to mind. Seemed to have
developed a high internal resistance. It's doing some serious current
limiting.

Chipmans explanation re-reflection was eloquent I thought.

Lets delve
on this for a second. It seems fair to say that if the source impedance
determines V+, clearly it has an effect on V-. But, that does not mean it
has anything to do with rho.

I don't know how else to look at it.

The question that comes to mind is whether the argument is about the
effect source impedance has on actual SWR, or the effect it has on
measured SWR - considering the real world limitations of metering
instruments. Perhaps people are talking about different things.

73, Jim AC6XG

Tarmo Tammaru wrote:
It seems fair to say that if the source impedance
determines V+, clearly it has an effect on V-. But, that does not mean it
has anything to do with rho.

Chipman seems to say that an SWR meter can be disturbed by a localized energy
exchange between reactive values with opposite signs. The impedance of the
source has an effect upon where in the transmission line those localized
energy exchanges occur.
--
73, Cecil http://www.qsl.net/w5dxp



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Jim Kelley wrote:
It's hard to imagine how Rs (Zs) could have any effect on that ratio.

Consider a reactive load where energy can be locally exchanged between
the load reactance and the impedance looking back into the feedline.
Zs can certainly affect the impedance looking back into the feedline.
--
73, Cecil http://www.qsl.net/w5dxp



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Jim Kelley wrote:
It's hard to imagine how Rs (Zs) could have any effect on that ratio.

Here's an interesting quote from _Transmission_Lines_, by Chipman, page 175:

"Equation (8.27) demonstrates explicitly that the shape of a standing wave
pattern representing |V(d)| as a function of d on a transmission line is in
no way affected by the quantities, Vs, Zs, and rho(s) at the source."
--
73, Cecil http://www.qsl.net/w5dxp



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Source impedance DOES affect the amount of energy moving in and sloshing
around in a transmission line. It DOESN'T affect the ratio of forward to
reflected waves, and therefore DOESN'T affect the SWR.

Once again, here's a way to see why. I'll restrict the discussion to a
lossless line for simplicity. When you first turn the source on, a
forward wave (voltage and current) travels toward the load. The source
impedance does play a role in determining the size of this wave; it can
be determined by analysis of a simple voltage divider circuit, with the
source voltage dividing between the source impedance and the line Z0. A
portion of the forward wave is reflected from the load unless the line
is perfectly matched. The fraction which is reflected has nothing to do
with the source impedance, and in fact it can easily be calculated from
only the line and load impedances. That fraction (magnitude and angle)
is known as the reflection coefficient -- you can find the formula in
any transmission line text, or derive it yourself very easily.

Take a look at the system just before the reflected wave returns to the
source. At each point along the line we have a forward wave and a
reflected wave, which vectorially add. These create standing waves and,
if the line is long enough, we can calculate the SWR directly as the
ratio of maximum to minimum voltage along the line. A little bit of
algebra will show that the SWR is determined entirely by the ratio of
forward to reflected waves -- their absolute values don't matter
(except, of course, as it affects their ratio). Given a reflection
coefficient, you can calculate the SWR.

Ok, now suppose that some fraction of the returning wave reflects from
the source and heads back toward the load. Say, X percent of it. When it
reaches the load, exactly the same fraction of it is reflected as was
the case for the original forward wave. That is, if the new forward wave
is X percent of the original, then the new reflected wave is also X
percent of the original reflected wave. If we add the new forward and
reflected waves to the original ones, and take the ratio of forward to
reverse, we find that the ratio of the new, combined forward wave to the
new, combined reflected wave is exactly the same as it was for the first
forward and reflected waves. It doesn't matter what X is -- no matter
what fraction of the reflected wave bounces off the source, the same
fraction of that new forward wave is reflected from the load. The SWR is
the same as it was for the original pair of waves. Eventually, we build
up a large number of pairs of forward and reflected waves. And the ratio
of each forward wave to its corresponding reflected wave is always the
same -- it's the reflection coefficient of the load. So when we add all
the forward waves into a single forward wave and all the reflected waves
into a single reflected wave, we get the same ratio. And that ratio
doesn't depend in any way on the source impedance or what fraction of
each returning wave is re-reflected from the source.

One of the nice things about this way of looking at it is that it's
entirely supported by the theory and equations describing transmission
line operation which engineers have used to design working systems for
the past hundred years or so.

Roy Lewallen, W7EL

Cecil Moore wrote:
Jim Kelley wrote:

It's hard to imagine how Rs (Zs) could have any effect on that ratio.


Consider a reactive load where energy can be locally exchanged between
the load reactance and the impedance looking back into the feedline.
Zs can certainly affect the impedance looking back into the feedline.

Holy mackrel, Mr. Science!

But gee, you can see that from even a casual glance at the equations
you'll find in virtually any transmission line text. It's kind of like
saying that wow, Terman concludes that resistance is voltage divided by
current, so now we can believe it too.

People who believe that SWR is affected by source impedance have either
rejected established theory, or don't have the background or interest to
read and understand what we consider to be very simple equations. So I'd
hardly expect them to be impressed by someone pointing out what the
equations and established theory say clearly and unambigously. You can't
fight Ouija boards with math.

Roy Lewallen, W7EL

Cecil Moore wrote:
Jim Kelley wrote:

It's hard to imagine how Rs (Zs) could have any effect on that ratio.


Here's an interesting quote from _Transmission_Lines_, by Chipman, page
175:

"Equation (8.27) demonstrates explicitly that the shape of a standing wave
pattern representing |V(d)| as a function of d on a transmission line is in
no way affected by the quantities, Vs, Zs, and rho(s) at the source."

Source impedance DOES affect the amount of energy moving in and sloshing
around in a transmission line. It DOESN'T affect the ratio of forward to
reflected waves, and therefore DOESN'T affect the SWR.

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

But it DOES affect the indicated SWR and so the indicated SWR is incorrect.

It is the meter which is at fault ! It is designed to indicate correctly
only when the source is 50 ohms.

Here's the proof - Rho = (50-Zt) / (50+Zt) - which you may have seen
before.

SWR, of course, is calculated from Rho and the meter scale is calibrated
accordingly.

If the source is not what the meter expects then it gives the wrong answers.
And its faithful worshippers believe it!
---
Reg, G4FGQ

Tarmo Tammaru wrote:
I thought that we were considering steady state single frequency sine waves.
The whole thing becomes so much more straight forward when talking about
pulses.....

It is still straight forward when we take reality into account. :-)
Pure steady state single frequency sine waves, sans noise and/or
jitter, exist only in the human mind.
--
73, Cecil, W5DXP