Roy Lewallen wrote:

A single conductor doesn't have a characteristic impedance -- it's the
impedance between the two conductors of a transmission line. You can
measure a characteristic impedance between, say, a coil and ground, but
its value depends on the spacing between the two. If the coil is tilted
with respect to the ground, the impedance of this two-conductor system
will change with the position along the coil.

Roy: I understand what you are saying. But the derivation of
Characteristic Impedance in the Corum Bros. paper depends only on the
coil dimensions and number of turns; it is independent of any
relationship to other conductors or groundplanes. I also note that
ON4AA's inductance calculator predicts the "Characteristic impedance
of n=0 sheath helix waveguide mode at design frequency" based purely
on the coil geometry. The maths is a bit beyond me (trying to solve
Maxwell's equations for a solenoidal helix), but seems to bear analogy
to the derivation of the characteristic impedance of a waveguide.

I'm inclined to try to understand it better, because it's this derived
Characteristic Impedance, along with the axial Velocity Factor, that
generates the reactance values which seem such a good match to
experimental and modelled results.

Regards,
Steve G3TXQ