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[TowerTalk] Help with beam rotation

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Subject: [TowerTalk] Help with beam rotation
From: (Kurt Andress)
Date: Mon, 15 Mar 1999 08:06:24 -0800
Dick Flanagan W6OLD wrote:

> 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!  :)

Dick is my neighbor, about 1 mile NE of me. Good thing he likes phone and I
like CW!

> 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.

Yes, any antenna loads that are developed by wind pressure on the antennae
create tower loads that want to make a tower lean over.

Every antenna that does not have its boom centerline passing thru the mast
centerline, creates torque loads on the system. The offset between the
connection of the boom to the mast, always generates torsional loads, when
the antenna is other than boom broadside to the wind.
We add to that the fact that some antenna builders mount the mast  at the
antenna CG, instead of the boom center. This causes imbalance when the boom
is exposed to the wind.

> 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.

Sounds like the mechanical equivalent to our isotropic radiator.
Theoretically useful but, hard to achieve in the real world. An antenna can
be designed to be completely torque neutral. The problem is that most are

> 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

The wind develops loads normal to the axes of the pieces of tubing.
Wind on elements results in loads acting along the axis of the boom. If
boom is offset from mast, we get torque.
Wind on boom develops loads parallel with elements. If mast is located at
the boom center no torque is generated. If mast is not at boom center, we
get torque.

My TH7 has the mast located very near the boom center. The problem is the
rotor connection to the pole is not up to the task. The torque caused by
the element loads acting along the boom, which is offset from the mast,
causes rotation until the boom is broadside. In that position the there is
no torque so it stays there.

That's my story and I'm staying with it.

73, Kurt

K7NV "That's K7 "Nevada" (ex - NI6W)

YagiStress - The Ultimate Software for Yagi Mechanical Design

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