"John Smith" wrote
Think, "One-to-one isolation transformer."

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If it was a fixed 1-to-1 transformer and one end was terminated with
Zx, then the input impedance at the other end would also be fixed at
Zx.

But it isn't!

By no stretch of the imagination is it a fixed voltage or current
transformer of any sort.

By the way, a "Longitudinal current" is that which flows along the
feedline when both wires are considered to be one wire. ie., when
the wires are effectively connected in parallel with each other. It
is that current which is measured by a clamp-on ammeter when clamped
around both wires. The two wires can be coaxial or balanced-twin or
open-wires. I've temporarily forgotten what US citizens call it.
----
Reg, G4FGQ.