Ian, if I ever understood, I have long ago lost track of the raging
arguments and consequences.

You are quite correct, of course, in your analysis of what goes on in and
around a loading coil. But in the face of such rigid minds and attitudes
your attempts to convince people of the errors in their ways by simple logic
is doomed to failure.

A solenoidal coil of wire, a loading coil of any proprtions, can be
considered to be a continuously loaded, fat, relatively short, single wire,
transmission line.

Because of the inductive loading it will have a much higher Zo than a solid
cylinder of the same length and diameter.

Its inductance per unit length will be that of the coil.

Its capacitance per unit length will be largely unchanged. For close-wound
turns tt will be the same as the solid cylinder. For spaced turns
capacitance will only be slightly reduced and calculable.

Zo = Sqrt(L/C) and R is the wire HF resistance including proximity effect.

To simplify, for a first approximation R can be neglected and the line
becomes loss-less. If the length of coil is long enough then its radiation
resistance Rrad may be high enough to be taken into account alongside R.

The propagation velocity V = 1/Sqrt(L*C) from which phase-shift per unit
length of coil can be calculated. (Phase shift appears to be a sore point
in the arguments)

If necessary, attenuation per unit length can be calculated from R+Rrad.

The properties of this line, the coil, is now amenable to normal
transmission line analysis with a fair degree of accuracy. Accuracy is
limited by the accuracy of determining coil dimensions. Such things as the
increase in overall diameter by wire diameter matter.

Input impedance can be calculated from the terminating impedance. The
terminating impedance is the remainder of the antenna (another transmission
line) but for the purpose of settling arguments arbitrary values can be
chosen.

The phase shifts relative to feedpoint at each junction along the loaded
antenna can be calculated. Some of my programs use the above-described
calculating method. But none of them have relative-phase outputs for the
simple reason that nobody has yet found any practical use for such useless
data and in any case there's usually not enough space on the screen.

All phases are relative. I've a feeling arguments have arisen because of
confusion about what phases are relative to. You've been arguing about
different things and you can't ALL be that stupid. Unfortunately,
communication via newsgroups cannot make use of body-language. ;o) ;o)
----
Reg, G4FGQ

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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