Ian,

The equivalent shunt self-capacitance of a coil obviously affects the
magnitude and phase of the current which flows in it. This particularly
applies when a high-value loading coil is near to the top of a vertical
antenna which is terminated with a very short rod or whip.

(The self-capacitance of an isolated coil is calculable and can be easily
checked by using one of these small hand-held antenna analysers to measure a
coil's self-resonant frequency extremely accurately.)

In the extreme case, when there is no whip, the only capacitance across the
coil is its own self-capacitance. Yet to behave as a loading coil and draw
current up the antenna below it, it is required to have a low impedance.

A circuit analysis becomes quite involved. The coil impedance has to be in
the form of a series resonance with the length of antenna wire below it. So
we have a series resonance in the presence of the coil's shunt capacitance.

From ordinary lumped circuit theory the equivalent coil Q drastically falls,
a very large voltage appears across the coil, and a very large circulating
current flows around the coil and its own self-capacitance.

Efficiency goes for a Burton and with a high power transmitter either the
coil melts or collapses due to voltage-breakdown between turns.

The moral of this story is never to locate a loading coil near the top of an
antenna. It also explains why maximum efficiency usually occurs between
half-way and 2/3 of the way up.

With an exceptionally good ground maximum efficiency occurs with bottom
loading. In which case you don't need a coil in the antenna at all. You can
include it in the tuner.

To see how radiating efficiency of a short or long vertical changes with
coil height and how coil loss increases extremely rapidly as the coil nears
the top of the antenna, download in a few seconds program LOADCOIL from
website below. Its quite safe to use the program - there's no danger of
setting the coil on fire.

You can slide the coil up and down the antenna from the keyboard and
immediately observe how a variety of parameters change. Also copious notes.
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Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
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