[TowerTalk] Help with beam rotation

[Dick Flanagan W6OLD]

>From: Kurt Andress <K7NV@contesting.com>
>
>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.
>>
>[...]
>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.
>[...]

I always get in trouble when I disagree with professional hardware-type
engineers, but I usually end up learning something in the process, so
here goes!  :)

It would seem to my intuitive side that wind loading has nothing to do
with mast/rotor slippage.  Wind loading on the top of the tower will
tend to push the tower over in a strong wind, but the loading, per se,
would seem to have no bearing on the rotational forces applied to the
mast/rotor.

Envision a 20' diameter sphere in place of the antenna.  The
not-insignificant wind loading will try to push the tower over, but the
rotational forces on the mast/rotor should theoretically be nil.

I have always intuited that what an antenna seeks in the wind is
balance, not minimum loading (though the two will often coincide).  Put
a rudder on the end of your boom and I would think it would tend to
swing your TH7 into the wind, in spite of that being the orientation
presenting the greatest wind loading.

If I am worried about mast/rotor slippage, I aim my antenna boom into
the wind in an attempt to minimize the rotational lever the boom
presents to the wind.  Yes, the elements will try to rotate the
antenna, too, but because they are typically distributed along the
boom, the rotational forces somehow seem less concentrated.

So, there is my lay software engineer's grand, weasel-worded
interpretation.  Be gentle with me.  :)

73, Dick

--
Dick Flanagan W6OLD CFII Minden, Nevada DM09db (South of Reno)
Visit http://www.qsl.net/cvrc/



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