Folks,
----- Original Message -----
From: "Earl W Cunningham" <k6se@juno.com>
> I've never modeled an "upside-down Christmas tree" like that, but I
agree
> that the difference will be even more pronounced in that case.
And likely what the correspondent is describing that you refer to as
"upside-down Christmas tree" is not actually a complete description,
and unintentionally leaves out some critical details.
There is at least one reason why a tower may not behave in the manner
suggested by beams as tophats or as a model suggests:
Insulated conductors (as in coax running up the tower to a floating
driven element, or much more interesting, to an inverted vee for low
bands, which are not grounded to the tower at top and bottom, and
possibly have a balun or choking device that is weak or unfortunately
placed for 160 meters.
The shield of the coax is closely coupled to the tower, and may be
thought of as an opportunistic transformer secondary, with the tower
as the primary winding. Further the "primary" is in some sense in
series with the tower impedance because there is no way current can
flow in the tower without trying to make current flow in the shield
(secondary).
If someone has an 80m or 40M inverted vee using a coil of coax as a
choke at the top, that MAY actually become the CONTROLLING 160m
characteristic of the tower and its complex of metallic
attachments/surrounds.
If any conductor going up the tower extends away from the tower at the
base without a low impedance RF connection to the tower at the base,
the wire on or buried in the ground has to be evaluated by the model
to really see what the vertical AND horizontal part of the wire will
do. Which then gets into the problem that some models are limited in
predicting the behavior of insulated wires on or in the ground.
ALL the metal ON AND NEAR the tower (not just the beams) needs to be
in the model so that an opportunistic resonance is detected and its
effect can be estimated. If the radial system is not sufficiently
extensive, the impedance of the tower base connection to earth ground
has to be estimated (or measured) and put in the model.
One helpful trick to guess what may be going on: in your mind's eye,
work BACKWARD on the conduction paths from the various tips of
conductors up the tower to where they join and become common. At the
joining points the path closest in length to an ODD multiple of a
quarter wave will exhibit the lowest impedance. By carrying the most
current they will control more of the result. Those paths close to an
EVEN multiple of a quarter wave (including zero) will become very
high.
Remember that this cannot be an absolute rule because to SOME degree,
current on one conductor is inductively coupled to every other
conductor in the vicinity.
Remember that unconnected paths (rotator wires, control wires coax to
and including other antennas) have to be evaluated separately as if a
transformer secondary with the direct conducting path as a primary.
And then try going back to the idea that the 160m resonance of a tower
is a simple one-dimensional estimation.
73, Guy.
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