Jim Kelley wrote:
"Fig. 7-19 is certainly interesting."

Jim`s cryptic statement sent me on a search.

Eureka! My 1950 version of Kraus has that Fig. 7-19 on page 193. It
shows propagarion velocities found by several researchers as a function
of helix circumference.

Phase velocity is the velocity at which a point of constant phase is
propagated in a progressive (traveling) sinusoidal wave.

Best regards, Richard Harrison, KB5WZI