Richard Harrison wrote:

Keith wrote:
"Why the resistance to explaining this case?"

Because it should be obvious to anyone who knows enough to ask the
question.

Nevertheless, I assume the questioner is sincere so here is a simple
answer. It`s in Terman if you want more details.

The question was, indeed, sincere.

Zo = sq rt Z/Y

Z = R + j omega L = line series impedance per unit length, ohms.

Y = G + j omega C = line shunt admittance per unit length, ohms.

For r-f in good lines, this reduces to:

Zo = sq rt L/C

For d-c, this reduces to:

Zo = sq rt R/G

But we were discussing ideal lines, for which, if I recall correctly,
R and G are both 0.

This leaves us with Z0 = jwL/jwC (w being my font challenged excuse
for Omega)
which should, in the limit as w approaches 0, leave us with the same
answer as for RF. Or have I forgotten how to do math (which is quite
possible).

In any case, a slight modification to the experiment can get around
this difficulty. We'll just perform the experiment at a frequency
sufficiently low as to be indistinguishable from 0 given the duration
of the experiment.

Obviously line immpedance at d-c is likely quite different from line
impedance at r-f.

This would certainly be true for real world lines where the luxury of
R=G=0 does not exist.

....Keith