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Richard Clark · April 3rd 09, 02:34 AM posted to rec.radio.amateur.antenna

On Thu, 2 Apr 2009 16:42:32 -0700 (PDT), wrote:

Did you ever try to model a resonator where the coil required is
actually longer than the insulator, so that the vertical part of the
resonator is actually inside the coil

Hi Brett,

After following Cecil's pet theory, and your statement of finding it
useful, your question above becomes a remarkably perplexing
application of that knowledge.

The theory should have answered your question before hand. The
typical shorthand explanation for coils' length equivalency should
have too. The fact of the matter is that for a given height radiator,
there are probably an infinite variety of values for inductance along
the length where the radiator is broken for that coil's insertion. One
inductance value (however it is wound) is not equivalent everywhere -
hence the proposition of a coil having an equivalent length is rather
preposterous.

Most loaded vertical design proceeds along very few lines. The first
consideration is coil placement which is largely dominated with one
consideration: radiation resistance. The optimal point (for an
optimal size of the coil) is somewhere between 40% and 60% the way up
(adding a top hat lends to the variability). Sub-optimal designs
abound and are freely offered everywhere without explanation (but long
on unsupported claims).

Knowing the insertion point, the second consideration is overall
height in relation to wavelength. If the vertical is a loaded one, we
here in the group can well anticipate that it is going to be a very
short one seeking "special sauce" enhancement. Height and wavelength
will dominate the coil inductance at the given insertion point.

Knowing the insertion point and the height/wavelength, the coil's
inductance is fairly well defined. The third consideration is keeping
its loss low. So far I haven't seen that pursued to its logical
conclusion to instead see discussion wander in the poppy fields of
electrical length (that was solved with inductance). Loss drives you
to make the inductor large, with large separations in adjacent
windings. Tightly wound coils, or toroids in the basement band might
be useful at night for an aviation beacon light.

Worry about filling the coil's axis with the radiator is focusing on
the wrong problem, remove the excess, or fold it into the turns of the
coil. This may alter the inductance slightly, but that can be taken
care of with trimming at the top - trivial stuff. Stacked tubing
makes this less than trivial. You go into this expecting that kind of
flexibility because antennas are not designed straight from paper to
elevated tube the first time.

73's
Richard Clark, KB7QHC