I know that there are a number of fellows on this reflector who are skillful
at computer modeling of antennas -- I wish I was one of them.
I have, however, made a stab at modeling an inverted L antenna which I think
was done properly. As a part of looking at that model I decided I would hang
the inverted L from a tower and simply separate the vertical part of the L
from the tower by a couple feet or so. Rather than model the tower itself I
decided to approximate the tower by using a two inch diameter tube about 57
feet high. [This is a mathematical model so I did not have to worry about
how to get the tube up and then hold it up in place].
In the process of entering the wires and the tubing parameters into the
model I neglected to bring the bottom of the tube to ground. That meant I
had a 57 foot long piece of metal in the vicinity of the vertical wire of
the L. I began to wonder if there would be some effect if I grounded the
metal -- this would be the situation if we ran a wire up alongside a tower
and then ran out a horizontal top loading wire making the 'classic' inverted
L.
The wire only model calculated result was as I had expected; at 1.8 MHz the
source impedance was about 18 ohms and the average gain was about 0.45 dB
when I used a simple #14 wire 57.5 feet high and with a top wire length of
78 feet. There was a reduction in signal strength in the direction of the
top wire which has been reported before. [I used the real MINNEC ground of
the EZNEC model].
When I added a 2" diameter aluminum tube spaced about 6 inches from the wire
on the side opposite the top wire there seemed to be essentially no change
in the calculated source impedance and field strength. It made no difference
if I grounded the tube or left it ungrounded. I made a second run with the
wire spaced 1/8" from the tube and again there seemed to be no proximity
effect. These results were not the ones I expected. I expected to see that
moving the wire closer to the tube caused changes in the source impedance
and the field strength.
I have to believe I am not the first person to run such a model experiment.
Has anyone else run the same experiment with different results? That is,
they found that placing the vertical wire in proximity to a grounded [or
ungrounded] tower in a model produced a change in source impedance or field
relative to a model where the wire was substantially separated from the
tower.
Does anyone have any suggestions on what I might have done to make the model
'incorrect'.
If this result is correct does it not imply that one can use an inverted L
[with even a very short top hat wire] that is positioned close to a tower as
a vertical and feed it against ground. That would suggest that 150 foot high
towers could hold full sized 160 meter verticals and that shunt feeding
towers would be unnecessary.
I am sure that I am missing something or people would already be doing it.
Clearly, I need some help creating a correct model.
Tod, K0TO
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