Richard Harrison wrote:

"Since the velocity with which the signal propagates along the
helix wire approximates the velocity of light if the frequency is not
too low (caveat is unimportant, see footnote in book), the axial field
due to the signal advances with a velocity that is very closely the
velocity of light multiplied by the ratio of helix pitch to helix
circumference."

Well done, sir.

Kraus details this in his
section on helical antennas.

Fig. 7-19 is certainly interesting.

Cecil owes you a fruit basket I think. ;-)

73, Jim AC6XG