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.

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

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

Best regards, Richard Harrison, KB5WZI