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
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