Cecil Moore wrote:

For anyone interested in an in-depth look at the
subject of loading coils, here is an excellent
reference.

http://www.g3ynh.info/zdocs/magnetics/part_1.html

"When modeling and using inductive devices, it is
important to be aware that the concept of lumped
inductance is only strictly applicable at low
frequencies."

"In the high-frequency region, it is no longer possible
to treat the coil as though its reactance is purely
inductive; the reason being that a wave emerging from
the coil is now significantly delayed, and therefore
has a phase which differs from its phase on entry."
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com