OK, Cec, I have a better idea of what you mean by '50-ohm
environment'.

You refer to: -

Vfor/Ifor = Vref/Iref = Z0

with which I agree.

But Zo is a pure resistance and the relationship can be true only when
there is no phase shift between Vfor/Ifor and Vref/Iref.

Which means, in the ideal meter, the voltage tap-off and current
tap-off points must be identically located within the instrument.

I would now like to go off in a slightly different tack to these
extensive threads.

There is a length of conductor between the input and output terminals
of the meter. It cannot be avoided. For obvious reasons related to the
high-frequency response and accuracy, the Zo environment along this
conductor has to be maintained as good as is reasonably possible. (
With the common or garden SWR meter nobody bothers very much. It
hardly matters anyway.)

The location of the voltage and current tapping points along the
conductor doesn't matter two hoots. What matters is the distance
beween them because of the phase difference. Small errors due to
misplacement are unavoidable and have to be lived with.

Getting to the point of this message: - The length of transmission
line inside the meter and the unknowns regarding 'Zo environment' play
no part either in operation or analysis of the meter.

The story that the length of line inside the meter is used to detect
and measure standing wave ratios is just another old-wives' tale which
confuses CB-ers, novices and professional engineers alike.

( By the way, if there is a short transmission line of any sort inside
the instrument, its length can be used to set the measuring
sensitivity. The longer the line the greater the sensitivity. But the
longer the line the greater the measuring errors and the worse the
frequency response.)
----
Reg, G4FGQ.

===========================================
"Cecil Moore" wrote

The transmission line reflection model tells us that the Z0
of a transmission lines forces the following relationship.

Vfor/Ifor = Vref/Iref = Z0

A "50 ohm environment" used in the context of the previous
discussion would be one in which the above relationship
is forced on the system at certain points in the system.

600 ohm transmission line going from a tuner/balun to an
antenna establishes a 600 ohm environment for the signals
on the transmission line. An SWR meter calibrated for 600
ohms will indicate the actual SWR.

About a year ago, based on a discussion that you and I were
having, I asked the sci.physics.electromag newsgroup how
long a piece of RG-213 coax has to be to establish the
above relationship. The answer was that the non-TEM product
terms decrease at about 1/e every two inches for RG-213.

The RG-400 coax leads going to and from my SWR meter are
two feet each. So I asserted to you that my SWR meter
was reading the actual SWR in the middle of that run of
RG-400. I think you disagreed with my assertion but I
cannot remember for sure.
--
73, Cecil http://www.qsl.net/w5dxp