Art Unwin wrote:
I suspect that Corum made some approximations.

Of course, they are approximations. The wire
diameter doesn't even appear in the equation.
Quoting the Corum paper:

"A useful engineering *approximation* has
been found for the fundamental resonance of
helices ...".

"... an *approximation* for M has been determined
by Kandoian and Sichak which is appropriate for
quarter-wave resonance ... for helices with
diameters considerably less than a free-space
wavelength".

"We have found that this expression gives acceptable
results (errors less than 10%) for most practical
applications that involve wave propagation on
helical resonators ...".
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com