Cecil, W5DXP wrote:
"I`m assuming that the "relative phase velocity" is the same thing as
the VF of the coil."

Back a couple of pages on 251, Kraus defines v/c as equal to "relative
phase velocity of the wave propagating along the helical conductor, v
being the phase velocity along the helical conductor and c being the
velocity of light in free space."

Repetition of "along the helical conductor" implies to me, thal like
Terman, Kraus says the signal follows the actual wire, not sprinting
across the coil as if it were a straight rod.

Best regards, Richard Harrison, KB5WZI