Michael Coslo wrote:
Any relation to the loosely wrapped "coils" of shortened verticals like
bug catchers?

Take a look at the geometry. Assuming that the current on one
side of a turn on the coil is equal to the current on the
opposite side of the coil but traveling in the opposite direction,
one can see why those two currents per turn resemble transmission
line currents (differential) rather than antenna currents (common-
mode). (A 10" coil wound with 1/2WL of wire radiates roughly the
same amount of energy as a 10" straight wire.)

Physically large air-core loading coils can be modeled as a
transmission line with a Z0 and VF (delay).

http://hamwaves.com/antennas/inductance.html

The delay through the coil can be calculated by knowing the
Beta = _____ rad/m "Axial propagation factor of n=0 sheath
helix waveguide mode at the design frequency"

The VF of W8JI's 100T, 10TPI, 2" dia test coil calculates
out to be ~0.03 resulting in a ~25 nS (~37 deg) delay
through the coil at 4 MHz.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com