[TowerTalk] Help with beam rotation

[Kurt Andress]

Randy Tudor wrote:

> I have a TH7 at 40 feet in a Ham IV. I am on the top of a hill at 2000 feet
> ASL. I get a lot of high winds. The beam gets off the correct heading in the
> strong winds.
>
> My question - should I pin the mast to the rotator to stop the rotation? I
> am not sure if I would cause a problem to the rotator.
>

Randy,
Along with the other suggestions to improve grip between the rotor and mast,
consider where to point the TH7 in a breeze.
I have a TH7 on a temporary military surplus stacked tube tower, turned by a
T2X. I've studied its behavior over the last 7 months and it clearly has told me
what it likes.
My site is on the eastern slope of the Sierra Nevada range and gets regular 70 -
80 mph winds during the winter storms, about 1 storm every two to three days. We
get 90-110 mph winds at least once per season.
My temporary setup cannot resist the forces on the antenna, so the TH7 always
ends up going where it wants to go. Where it wants to go is always to a position
with the boom broadside to the wind direction.

This makes a bit of sense when we look at the antenna projected areas. The
elements present 11.5 SqFt of area and the boom presents 4.1 SqFt. There are
several other factors involved in the behavior, but the simple solution is to
orient the antenna so that it presents the lowest area to the wind.
It's a natural thing for systems to seek  the lowest energy configuration or
"the path of least resistance." It's nice that some manufacturers are starting
to provide the information we need to make these decisions.

The fellows with swing arm sidemounts should orient the TH7 with the swingarm
aligned with the wind and the boom broadside, however that needs to be done with
their setup. This applys to most antennas that have larger element areas than
boom areas. But, cannot cover all installations, as a simple blanket solution.

Having to point the antenna in any particular direction during a blow is a sign
of a weakness in the system.
Many of us have this problem, for a variety of reasons. It can be remedied by
using gear that can handle the loads during all expected conditions. The
difference is usually defined by the wallet of the station builder, and the
absence of ample information to make the right decisions.

--
73, Kurt, K7NV

YagiStress - Software for Yagi Mechanical Design
Visit http://www.freeyellow.com/members3/yagistress/



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