On Mon, 28 Nov 2005 17:54:35 GMT, "Jerry Martes"
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.

Impedance is a complex quantity (ie with real and imaginary parts).
There are an infinite number of different impedances (being
combinations of the real and imaginary parts) that could cause a given
SWR on a given line in most cases. (The exception is the case when
VSWR=1, you do know the impedance, it is the nominal Zo for which the
instrument was calibrated.)

Whilst you can work out the SWR that will result from a specific
impedance on a specific Zo line, you cannot do the inverse, you don't
have enough information.

Owen
--

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

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
--

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

On Mon, 28 Nov 2005 23:52:40 GMT, Cecil Moore wrote:

Owen Duffy wrote:

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

To do that, you need to determine the position of the standing wave
pattern with respect to the load, and a typical reflectometer style
SWR meter does not do that. You could put a ruler to the line, but you
are using another instrument to make a another measurement that the
reflectometer could not make.

It is misleading to suggest that a reflectometer style SWR meter alone
is useful for determining the impede dance of a load connected to the
meter by a length of transmission line, save possibly the case when
VSWR=1 and the line is low loss and Zo is the same as the calibration
Z of the SWR meter.

Owen
--

Owen Duffy wrote:
To do that, you need to determine the position of the standing wave
pattern with respect to the load, and a typical reflectometer style
SWR meter does not do that.

Yes it does, if one has the ability to vary the length of the feedline
until a current maximum point (minimum SWR) is known to be located at
the balun/choke. I do it everytime I get on the air. That's how I tune
my antenna system and I don't use any conventional tuner at all.

It is misleading to suggest that a reflectometer style SWR meter alone
is useful for determining the impedance of a load connected to the
meter by a length of transmission line, save possibly the case when
VSWR=1 and the line is low loss and Zo is the same as the calibration
Z of the SWR meter.

Not misleading at all. I do it all the time. I know the exact length,
velocity factor, and Z0 of my feedline. I know an SWR current maximum
point is located at my choke. I know if it is greater than, less than,
or equal to 50 ohms. It is a rather simple-minded process to accurately
estimate the antenna feedpoint impedance given everything I know. You
should try it sometime. Even if I didn't know if the current maximum
impedance was lower than or higher than 50 ohms, there would only be
two possible antenna impedances. EZNEC has a perfect track record in
predicting which of those two antenna impedances actually exists.
--
73, Cecil http://www.qsl.net/w5dxp

Have it your way Cecil...

Owen
--

Cecil, there's no need to rack your brains for hours trying to deduce
the antenna input impedance from the input impedance of the
transmission line. There's a computer program which will tell you the
exact answer in milliseconds. Download program ZL_Zin from website
below.
----
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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