Bill, K4XS wrote:
"I have a 200 foot tower which is pretty much in the clear and is guyed
with Phillystran at all points. It has a 5 element KLM 20 meter beam
which of course has all elements insulated from the boom. It also has a
bunch of other big Yagis at various heights from 27 to 165 feet on it.
I am aware that the 5 element beam on the top will further lengthen the
effective height from the 200 feet by (just estimating here) around 20-25
more feet. Will the other antennas which are mounted lower on the tower
have any loading effect? They are all element insulated from the boom
and are 10 and 20 meter yagis."
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Not only the highest beam, but all aluminum on the tower will have a
significant effect on the resonant frequency of the structure including
all booms and driven elements. Although the elements are insulated from
the boom, all driven elements can be considered to be grounded to the
boom (on 160m) by virtue of their feedlines which are tightly coupled to
the tower (hopefully, they go down a tower leg before leaving the
vicinity of the tower!).
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"I realize any numbers are estimates due to the complexity of the
antennas mounted on the tower."
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My estimate of the resonant frequency of the structure is that it is
below 900 kHz (electrically longer than 1/2 wave on 160m).
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"I would like to feed the tower as a vertical but am concerned about
several things.The effective height of the tower will be in excess of
1/4 wave, but I
believe even with the top loading probably is slightly less than 1/2
wavelength. I realize too, that generally the effective height of the
tower
the more important it is to extend the radials beyond 1/4 wavelength. I
am limited to around 110 to 130 feet on 180 degrees of the antenna base
and can go somewhat farther on the other 180 degrees. Putting down 60 or
more radials would be no problem ( except for the work)."
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Just put down the 60 or so 110- to 130-foot radials and don't worry about
it. That should be quite sufficient.
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"I am aware of the need to add capacitance and have a huge variable
capacitor around that looks like it came from a California KW."
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Because the structure is close to 1/2-wave electrically, the value of
capacitance will be quite low (on the order of 100 pF) and the RF voltage
across it will be quite high when running a kilowatt (6 kV or more).
It's adviseable to use a vacuum variable rated at 15 kV or better.
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"My soil situation is kinda different. Near the antenna it is a mix of
sandy loam, and clay, probably average at best in conductivity. However
the water table of fresh water is never more than three or four feet
below the surface and I have cypress swamps as well as drainage canals
within 150 feet of the tower.
How effective would this antenna be compared to a regular 1/4 wave
veritcial with the same ground system?"
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If the far-field ground is of "average" soil conductivity, a 1/4-wave
vertical will slightly outperform a 1/2-wave vertical. This is because
of more attenuation of the signal at the lower angles (a 1/2-wave
vertical radiates at a lower angle than a 1/4-wave vertical over the same
ground). If the far-field soil conductivity is "very good" or better, a
1/2-wave will slightly outperform a 1/4-wave vertical.
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"How do you think it would play compared to a suspended wire slightly
top loaded 1/4 wavelength long with an elevated ground plane of eight
wire at a base height of 50 or 60 feet?"
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Debatable. Are 8 elevated radials enough at only 50 or 60 feet high?
You'll never know how it compares until you try it.
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"What do you estimate the impedence of the shunt fed tower to be?"
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The object of shunt feeding is to find the 50-ohm tap height and spacing
of the shunt wire so that all you have to do is insert a series
capacitance to tune out the inductive reactance. So my answer is 50 ohms
resistive when proerly tuned and the SWR is 1:1 on the feedline.
73, de Earl, K6SE
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