"Jerry Martes" wrote
I'm writing this to represent the "other side" of an arguement
that states
that VSWR *cant* be measured. I claim that VSWR *can* be measured
and that
VSWR can be used to identify the impedance terminating the
transmission
line.
========================================

The only way to measure SWR on a transmission line is to run a
voltmeter along it. At least TWO measurements are needed. Not ONE.
And line length is involved.

The voltmeter readings will indeed tell you what the SWR is. But
nothing else. It will be possible to calculate from the readings and
the distance between max and min what the velocity of propagation is.

But it is essential to add extra critical information before anything
else can be deduced. Without this EXTRA information knowledge of the
SWR (if it can be obtained) is useless. The so-called SWR meter does
not and cannot provide this information.

To calculate the terminating impedances from the SWR it is neccsary
also to know the line impedance, its velocity and the exact locations
of the max-volts and min-volts relative to the ends. The meter will
not tell you.

And the foregoing is on a line which exists only in one's imagination.

I am sorry to repeat, the indications of the SWR meter apply only to
the input impedance of the line from the transmitter to the antenna.
The meter, in itself, tells you nothing about what is happening to
conditions along the line. It certainly tells you nothing about the
antenna's input impedance which is of primary interest.

IF, BY SOME MEANS, YOU CAN MEASURE SWR, then there is much more
information needed before the performance of the system can be
predicted.

The funny thing is - the performance of the system can be deduced
from the extra information without reference to the SWR. The whole
business is laughable.

Just change the name of the meter and all will become clear.
----
Reg, G4FGQ.

Owen Duffy wrote:

wrote:
that VSWR *cant* be measured. I claim that VSWR *can* be measured and that
VSWR can be used to identify the impedance terminating the transmission
line.

Jerry, unless you redefine the term "impedance", you cannot generally
measure impedance with a typical reflectometer style SWR meter.

Here's how I do it, Owen. I modified my SWR meter to tell me
if the voltage sample is leading or lagging the current sample
and if the voltage sample is greater or less than the current
sample. Given the SWR is ratioed to 50 ohms, that's all I need.
I adjust the length of my feedline until I find a current maximum
point and the rest is easy.
--
73, Cecil http://www.qsl.net/w5dxp

Reg Edwards wrote:
The only way to measure SWR on a transmission line is to run a
voltmeter along it.

Reg, what about Rho = SQRT(P-/P+) and SWR=(1+Rho)/(1-Rho)?
--
73, Cecil http://www.qsl.net/w5dxp

"Cecil Moore" wrote
The tuner SWR meter only indicates the SWR on the 50 ohm coax
between the transmitter and the tuner. However, I have an SWR
meter on the antenna side of my tuner and it does indeed indicate
the SWR on my transmission line.

XMTR--SWR meter#1--tuner--SWR meter#2--50 ohm coax to a G5RV

SWR meter#2 does indeed indicate the SWR on the coax feed to
my G5RV. It obviously does not indicate the SWR at the antenna.
======================================

Cec, you are not telling the whole truth.

The meter tells you nothing about the important main G5RV transmission
line. ie., the SWR on the ladder line between the end of the coax and
the antenna. Neither can it tell you what the antenna input impedance
is.

There are other ways of finding the input impedance of the G5RV
antenna and its effect on line SWR if you should ever be sufficiently
interested. You could use a computer program. ;o)
----
Reg.

On Mon, 28 Nov 2005 20:45:37 GMT, Cecil Moore wrote:

Owen Duffy wrote:

wrote:
that VSWR *cant* be measured. I claim that VSWR *can* be measured and that
VSWR can be used to identify the impedance terminating the transmission
line.

Jerry, unless you redefine the term "impedance", you cannot generally
measure impedance with a typical reflectometer style SWR meter.

Here's how I do it, Owen. I modified my SWR meter to tell me
if the voltage sample is leading or lagging the current sample
and if the voltage sample is greater or less than the current
sample. Given the SWR is ratioed to 50 ohms, that's all I need.
I adjust the length of my feedline until I find a current maximum
point and the rest is easy.

Nice try Cecil.

Note Jerry's statement: "VSWR can be used to identify the impedance
terminating the transmission line".

Your method, impractical as it is, means you have found a point where
the impedance at the instrument terminals is purely resistive, and a
minimum, and you are correct that you could calculate the value of
that resistance. That measurement does not tell you the "impedance
terminating the transmission line" unless the line is of zero length.
You could make another measurement with another instrument (line
length using a ruler) and using other knowledge, calculate the
"impedance terminating the transmission line".

In a practical situation, there is a risk that there is current
flowing on the outer of a coaxial line or unbalance current in the
case of an open wire line. In that case, changing the feedline length
may affect the load impedance at the end of the line, so your proposed
method may alter the very thing you are "measuring".

Knowledge of the SWR AND the position of the standing wave pattern wrt
the load AND the loss characteristics of the line is enough
information to determine the "impedance terminating the transmission
line"... but a typical reflectometer SWR meter does not measure all of
those things.

Owen
--

Cecil Moore wrote:
Reg Edwards wrote:

The only way to measure SWR on a transmission line is to run a
voltmeter along it.


Reg, what about Rho = SQRT(P-/P+) and SWR=(1+Rho)/(1-Rho)?


If you know the forward and reflected power, VSWR can be calculated as
follows:

VSWR=(1+sqrt Pr/Pi) / (1-sqrt Pr/Pi)

I believe.

--
Over The Hill
__________________________________________________ ___________________________

The question of whether computers can think is like the question of
whether submarines can swim.

***Edsgar Dijkstra***

Reg, what about Rho = SQRT(P-/P+) and SWR=(1+Rho)/(1-Rho)?
==================================
Cec, That's a calculation, merely arithmetic, not a measurement.

It applies only to a long line lossless line which does not exist but
Zo must be 50 ohms. And nobody has the foggiest idea where max-volts
and min-volts are located. And so the calculated information is
useless except for trolling on newsgroups. ;o)
----
Reg.

Reg Edwards wrote:
Reg, what about Rho = SQRT(P-/P+) and SWR=(1+Rho)/(1-Rho)?

==================================
Cec, That's a calculation, merely arithmetic, not a measurement.

It applies only to a long line lossless line which does not exist but
Zo must be 50 ohms. And nobody has the foggiest idea where max-volts
and min-volts are located. And so the calculated information is
useless except for trolling on newsgroups. ;o)
----
Reg.


Here are several VSWR measurement techniques. They are real they are
valid and I used them for years. VSWR is a *very* important parameter,
especially when dealing with high power systems.

http://people.deas.harvard.edu/~jone...ssign_3_98.pdf

--
Over The Hill
__________________________________________________ ___________________________

The question of whether computers can think is like the question of
whether submarines can swim.

***Edsgar Dijkstra***

Reg Edwards wrote:
Reg, what about Rho = SQRT(P-/P+) and SWR=(1+Rho)/(1-Rho)?
==================================
Cec, That's a calculation, merely arithmetic, not a measurement.

It applies only to a long line lossless line which does not exist but
Zo must be 50 ohms. And nobody has the foggiest idea where max-volts
and min-volts are located. And so the calculated information is
useless except for trolling on newsgroups. ;o)
----
Reg.
Hi Reg, Do you advocate changing the measurement and calaculation that
all our handy little Antenna Analyzers report as SWR to TLI? That
would cost a lot to re-tool. My MFJ-259, and I think you have stated
that you have one also, reports lots of useful info, and uses a bridge,
a PIC, and a neat display. SWR as well as many other parameters are
measured indirectly, and calculated with the Micro. You could make the
agrument that all the other stuff the Analyzer reports needs to be more
descriptive, but it would cost too much to change. If you read the
manual, and know what the displays on any meter really means, then why
change anything?
Gary N4AST

Owen Duffy wrote:
Your method, impractical as it is, ...

"My method" was in widespread use before I was born. I learned
it from my Elmer in the early 50's. He was always looking for
that "magic" current maximum point to feed from his link
coupled tank circuit.

... means you have found a point where
the impedance at the instrument terminals is purely resistive, and a
minimum, and you are correct that you could calculate the value of
that resistance. That measurement does not tell you the "impedance
terminating the transmission line" unless the line is of zero length.

A transmission line transforms the impedance in a predictable
manner given the transmission line specifications. One can
backtrack the SWR spiral on a Smith Chart to get a reasonable
estimate for the antenna impedance. The impedances for my dipole
calculated in such a manner are pretty close to the ones predicted
by EZNEC.

In that case, changing the feedline length
may affect the load impedance at the end of the line, ...

The load impedance is what it is, virtually unaffected by
feedline length.

... so your proposed
method may alter the very thing you are "measuring".

Virtually every time one makes a measurement, one alters the
very thing that one is measuring. That's just a fact of life
and not a valid reason to give up trying to make measurements.
--
73, Cecil http://www.qsl.net/w5dxp