Clay,
If you stick a 25G on a pier pin, you can pretty much ignore what typical
tower movement might be. The tree is going to be holding up the tower.
I sort of liked the suggestion to put up a ladder instead. So, what would be
a good ladder? How about 100 feet of 25G?
If you still want an answer to your initial question, it is relatively easy
to approximate when you know how much guy wire stretches and use the data
from the Rohn catalog and the law of cosines equation.
Law of Cosines Equation
(http://www.ajdesigner.com/phptriangle/law_of_cosines_equation_angle_a.php)
Guy Wire Stretch
6.77 inches of stretch per 100 feet of 3/16" EHS when under a 3000 pound
load
http://k7nv.com/notebook/topics/guycable.html)
--
Gerald Boutin, VE1DT
From: "W7CE" <w7ce@curtiss.net>
Subject: [TowerTalk] Horizontal tower movement at the top
To: <towertalk@contesting.com>
Message-ID: <270a01c79024$6357f680$3500000a@claylaptop>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=original
Does anyone know how much horizontal movement is considered acceptable at
the top of 100' tall Rohn 25G, 45G and 65G towers? I don't see any mention
of it in the Rohn catalog. I know that the tops move, I just don't know how
much is considered normal.
I'm considering a rather unusual tower installation at my QTH. Because of
large trees that occasionally lose branches during wind storms and property
line constraints, a guyed tower isn't practical. For years I've had my 3
element yagi on top of a 95' Doug Fir tree and it has worked well there. I
watched it during our record setting wind storms last winter and the top of
the tree was only moving about 6-7" (less than the diameter of my Ham IV
rotator). This is a big tree with a 42" diameter base and an 8" diameter
stump at the 95' level where the antenna is mounted. The problem with this
installation is getting up to the antenna to do maintenance, plus I'm
planning on replacing it with a new SteppIR as soon as I figure out how I'm
going to mount it. What I'm thinking about doing is installing 100' of
tower as close to the tree as practical (less than 3') and attaching it to
the sides of the tree with angle iron at 20' intervals. With this approach,
I don't think the tower could ever come down unless the tree itself did
also, which is unlikely based on our soil type and the excellent health of
the tree. I would then mount the antenna on the top of the tower. This
would give me a much easier way to climb up to the top. I can use 25G, 45G
or 65G for the installation, whichever would be best. 25G would be the
easiest to install because of it's weight and I suspect that it is the best
choice.
I'm looking for opinions on the viability of this solution. Will the
movement of the tree in the wind stress the tower too much or will it about
the same amount of movement that a guyed tower would normally experience in
a big wind? Opinions? Suggestions? Am I thinking a little too far outside
the box?
73,
Clay W7CE
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