Cecil, W5DXP wrote:
"The measured velocity factor as a function of the ratio of
coil-circumference/wavelength is presented by Kraus, Figure 8-34 in the
3rd edition."

Yes. The older edition, posted briefly on the web, has a Fig. 7-19 which
also has VF`s found by Chu & Jackson, and by Bagby. They also found
propagation faster than c for some coil circumferences / wavelength.
Photons, massless at rest, can do that.

Best regards, Richard Harrison, KB5WZI