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