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[TowerTalk] Noisy tubular tower

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Subject: [TowerTalk] Noisy tubular tower
From: (Kurt Andress)
Date: Tue, 23 Jun 1998 21:43:26 -0700
Rick Stasiak, VE3MM,  asks about eliminating the noises from a MA770
tubular tower. Others have commented that this type of tower is noise
There are some things that can be done to minimize the inherent situation.

Sounds irritating!
Your tower has only two modes of movement possible under a wind load.

The first is independent of the antenna on top ( I.E. would do this without
an antenna). This is bending along the vertical axis.
This loads up mating faces of the telescoping sections. If they have gone
dry, you will get sounds from the metal on metal surfaces. These will be
amplified by the hollow tubular structure. Regular application of a
waterproof grease will help this one!

The second mode is torsion. This is solely caused by the antenna and its
connection to the tower. Greasing will also help here.
I do not have the physical configuration of the C4SXL so I can't provide
specific help here.
I can, however, offer some information on basic antenna behavior, under
wind loading, that may be useful.

There are two reasons for a yagi to impart torsional loads into the tower. 

1) The boom/mast connection almost always results in an offset between the
centerlines of the boom and the mast. When the antenna is pointed into the
wind, the loads on the elements act along a line that is the centerline of
the boom. Since, this is offset from the centerline of the mast (or tower)
it results in a torsional moment that is applied to the tower. This torque
is impossible to eliminate unless one can redesign the boom to mast
connection so that the boom is split about the mast, so that the mast and
boom centerlines are aligned. 
The simple solution is not to point the antenna into the wind, which takes
us to the second source of antenna torque.

2) If the antenna had no coax or balun on it, we could say that the best
placement for the mast connection is the exact physical center of the boom.
This would cause the windloads on the front and back halves of the boom to
cancel each other out , resulting in zero torque.
If we run the coax on the top or bottom of the boom it will cause
additional windload on that end of the boom, resulting in a torque
imbalance. The best place to attach the coax is along the side of the boom.
This places it inside the projected profile of the boom and results in no
appreciable gain in windload.
If the antenna has a balun exposed to the wind, it will add to the
inbalance. In this case, it would be best to offset the mast connection to
counteract the coax and/or balun loads or add a boom torque compensator
(usually to the front end of the boom) to offset the imbalance.

Tricky matching stubs that are mounted on the boom also add to this

Note: Placement of the mast connection to balance the antenna for its
fore/aft weight distribution is almost always wrong! Weight balance and
wind induced torque balance are two completely independent things. The way
to correct the weight imbalance is to put a counterweight inside the boom,
at the light end, to get it balanced.

Hope this helps! Software is available to evaluate these things, or ask
your antenna manufacturer how to balance his antenna.

73, Kurt

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