R: [TowerTalk] Tall verticals
Wed, 10 May 2000 20:29:52 -0400
> I would also like to shunt feed the tower on 160. Is there any way to
> ascertain analytically whether the electrical length of the tower or the
> loading at 20 meters above ground will cause serious complications so far
> as the impedance to be matched, and the height of the shunt-
Sure, model the tower and antennas on a modelling program. It
isn't necessary to get the antennas modeled exactly, just get them
Then insert a source near the bottom of the tower, and look at the
elevation pattern. If the tower is electrically too long, it will show up
as a high angle radiator. Even in that case, you can still feed an
insulated four wire cage surrounding the tower as a vertical.
I quite successfully (even working VS6DO on 160 SSB several
times from Ohio) used a 140 ft Rohn 55 tower with a full size two
element forty, and a five-element twenty fifteen feet above it (and
yes, the 20 meter antenna makes a BIG difference in operation of
the tower on 160).
That tower was self-resonant just above the middle of the AM BCB.
You can feed absolutely anything, but the elevation pattern that
might hurt performance if the tower is self-resonant too far below
the operating frequency. The tower most certainly does not need to
be resonant to radiate efficiently, as long as the feed system
matches the impedances.
73, Tom W8JI
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