"Amos Keag" wrote
My 60 meter mobile antenna is 90 degrees long, 1/4 wavelength resonant at
18 +j0 ohms [MFJ analyzer]. etc
_____________

But how much of that resistance term is the true radiation resistance of
your short radiator, and how much is contributed by the coil and "ground
plane" losses?

Resonating an electrically short radiator with a loading coil doesn't change
the radiation resistance of the short vertical antenna (whip) itself -- and
the r-f current able to flow in that whip is the source of practically all
of the useful radiation that system can produce.

The loading coil makes it possible for a practical transmitter to deliver
r-f power into that short antenna system, but doesn't change the fact that
it is only the short radiator itself that provides the useful radiation.
The system may have the net reactance of a resonant, 1/4-wave vertical, but
it will still have the radiation resistance of the original, short radiator,
and that radiation resistance is the primary determinant of system radiation
efficiency when using these coil-loaded antennas.

RF