Gee, Reg. Until now I've always gotten the impression that you already
learned everything there was to know about this stuff. :-)

73, AC6XG

=============================

Jim, if you promise, cross-your-heart, not to tell anybody I'll let you into
a long-hidden secret.

When it comes to the the distribution of current along a loading coil on a
vehicle-mounted whip I am severely handicapped in that I have never been
mobile in a motor car with a radio transmitter, never owned a motor car,
never even held a driver's licence.

Otherwise I am quite a normal person who takes an interest in electric
currents flowing along wires, rods, through coils and around the surfaces of
such things as vehicle bodies. Normallity extends to world-wide objections
against being choked by petrol fumes and the now common practice of
financing pirate expeditions to obtain the diminishing raw material from
which the poisonous liquid is refined.

It seems Yuri is interested in modelling short, coil loaded antennas. He
refers to controversy. I can set his mind at rest and assure him there is
none.

When the length of a loading coil is short in comparison with the overall
height of the antenna, certainly in comparison with a wavelength, the
current into one end can be assumed, with negligible error, to be equal to
that which comes out of the other end as with any other coil in an L,C,R
network analysis. Its stray capacitance can be ignored except for
investigating its self-resonant frequency.

It is a lump of inductance effectively concentrated at its midpoint. For
estimating antenna behaviour and performance it is necessary only to add
half the length of the coil to the length of the lower portion of the
antenna, and to do likewise to the upper length. The antenna's distributed
radiation and wire loss resistance can be sufficiently accurately estimated
from these dimensions, all being transformed to the feedpoint according to
the normal transforming action of the lengths of transmission line (the
antenna parts) involved.

================================

When coil length is nearly as long as the antenna, ie., a close-wound
helical for the lower frequencies, in which coil loss for a high inductance
is minimised by using thick wire rather than an inconvenient, very large
diameter coil, the antenna is best considered as a continuously loaded
1/4-wavelength transmission line in which its uniformly-distributed
capacitance, loss resistance and radiation resistance per unit length is
taken into account.

================================

For practical purpose, these different-proportioned sorts of short vertical
antennas all have the same, simple, well known radiation pattern. Any slight
differences are overwhelmed by variations, entirely out-of-human-control, in
the local environment and along the propagation path. What matters is
radiating efficiency. The standard of radiating efficiency is that of a very
high 1/2-wave dipole of any orientation and there's no need to be concerned
here where the radiation disappears to.

There is only one question of consequence. At what height up a short
vertical is a coil of given intrinsic Q to be located to maximise radiating
efficiency? It is never at or very near the top! As coil height increases
the required inductance and number of turns increases rapidly. Coil loss
resistance always overtakes the improvement in radiation resistance due to
the change in distribution of current along the antenna. A high resistance,
self resonant coil of many turns of fine wire right at the top of the
antenna eventually fails.

================================

There are various special cases which are dealt with by simple programs
available from the website below.

There is one program which covers from helicals, via screwdrivers, to lumped
coils. The coil can slide up and down the antenna to find the location of
maximum efficiency for given coil length and diameter. The number of coil
turns and wire gauge are automatically recalculated to maintain the same
required antenna resonant frequency. Program name is LOADCOIL.exe
Download and run in a few seconds.
----
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Regards from Reg, G4FGQ
For Free Radio Design Software
go to http://www.g4fgq.com
=======================