I don't think that anyone has mentioned the additional wind loading of the
105 foot mast. 105 feet times 4"/12" pipe diameter times about 0.6 to
convert to equivalent flat plate amounts to an additional 21 square feet of
I realize that this is not the same as putting an additional 21 square feet
at the top of the tower, but this would put a substantial additional load on
Gerald Boutin, VE6AVR
Date: Fri, 20 Jan 2006 09:43:15 -0600
From: "Stan Stockton" <firstname.lastname@example.org>
Subject: Re: [TowerTalk] Rotator at bottom of tower
To: "Wolfert, William R." <WWolfert@columbuspolice.org>
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Thanks to you and to all the others who responded to my request for
information. I am glad I asked for information and did not do something
stupid that was sure to not be a good situation.
It is clear to me that if I were to do this, I need a diameter of about 4
inches with about 12 inches of circumference in order to not have a little
twist translate into a lot of antenna movement. A quarter inch of twist in
both directions with a 1.90" mast would indicate a total movement of the
antenna of 30 degrees.
Given that the mast would have to be large, I would use schedule 40 aluminum
pipe about 4 inches in OD but will probably not do it unless I run into a
scrap deal on the pipe. Regarding the welding process, I am sure that a
plate, perhaps about 3/8" thick, 6 inches wide and 2 foot long at each joint
with a few heavy duty muffler clamps would hold it. You could even pin it
for extra insurance. Painting a perfectly straight line on the entire
length of the pipe would give visual confirmation as to whether it had ever
3.5 inch schedule 40 aluminum pipe would be 63 pounds per 20 foot length.
If I had a large enough hold in the top of the tower, I don't think I would
have a problem dropping them through the top.
Mounting the rotator independent of the tower would take the torque off the
tower and if a thrust bearing could also be independent of the tower there
would be no weight on the tower itself from the mast and antenna.
Sounds like a lot of work and perhaps I will not do it but still considering
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