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RE: [TowerTalk] buying new tower

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Subject: RE: [TowerTalk] buying new tower
From: "Tod Olson" <>
Date: Tue, 11 May 2004 11:59:26 -0600
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>With 2 bolts on every joint, 
>I just can't see how any noise could occur.

A very quick calculation which takes the weight of the Force 12 antenna at
the end of a 72 foot arm should indicate that there is a very large bending
moment when the top is near the ground. The force will transmit down the
tower and every joint is subjected to either compressive or extension force.
In Minnesota I had a Heights tower and set it up so the top 48 feet folded
toward the ground. (Hinge placed about 32 feet above ground). I had a four
element quad on a 30 foot boom with a total weight of less than 65 pounds.
The rotator and thrust bearings added weight as well but were not as far
from the hinge as the antenna. 

After a very few folding operations I noticed that the bolt holes were
becoming slightly elongated, particularly in the vicinity of the hinge, and
the sections moved perhaps as much a 1/16 inch up and down as the wind blew
the antenna. The cure was to enlarge the holes very slightly, insert SS rod
of the correct diameter to pin the sections, remove the hinge and no longer
generate the very high forces on the joints.

Admittedly, the wind forces on an antenna create similar compression
-extension forces, but probably not as high as the fold over process does. 

My Minnesota Heights tower (non-folding) emits sound when the wind blows
above 25 mph, but it is more small 'creaks' than 'clanks'. Still, once a
year or more often I make an inspection trip from bottom to top and back
again to see if there have been elongation changes. I would expect almost
every free standing aluminum tower has this 'sound' phenomena simply because
the design is a segmented cantilevered beam which is sufficiently flexible
that it moves noticeably in the wind. I am certain that the noise is
associated with slight changes to the physical dimensions as the wind blows
and produces relaxation oscillations. My guess is that stand alone steel
towers with the same height and antenna wind load do not exhibit this
phenomenon because of the difference in the properties of the metals. I do
not have a steel free standing tower to compare with my aluminum tower, but
perhaps others on this list may have one and can provide data for us.



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