You are leading yourselves astray again.

Anything which produces a velocity factor which varies significantly
with frequency is INCORRECT.

The propagation delay (in nano-seconds) along a coil is a constant and
is independent of frequency.

It depends only on the physical dimensions of the coil, ie., on L and
C, which are fixed in a given situation.

This is just the same as on an ordinary transmission line.
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Reg.