[TowerTalk] Angle of maximum wind force on Yagis

[EUGENE SMAR]

Hi, Kurt:

     If what you're asserting here is accurate (which I haven't satisfied
myself that it is yet), then what you're saying is that my calculations show
exactly the OPPOSITE of what I think they show.  This is a serious
discrepancy.

     I don't have the 19th Antenna Book (the 18th is my latest), but there
are a couple of local Hamfests coming up.  I'll pick up a copy and read
through the section you cited.

     In the meantime, how about this?  Take a hypothetical crossed dipole or
turnstile antenna, made with the same amount of aluminum on either element.
Say it's large enough to worry about wind loading.  Orient it so the two
dipoles are NS and EW.

     If the wind is from the east or west, you get some force, F, applied to
the mast.  The force is proportional to exposed area, L X W, of the element.
If the wind is from the north or south, you get the same force, F, applied
to the mast 90 degrees from the first example.  I'm ignoring the end-on wind
force.  Also, I'm not using shape factor as it cancels in my analysis,
below.

     Now let's say the wind is from the northeast.  Both dipole elements
experience the same force, whatever it is, right?  The force on one dipole
is proportional to effective area exposed to the wind.  In my equation, that
area is L X W X sin theta or, for 45 degree angle, L X W X 0.707.  Total
force at 45 degrees is twice this (two dipoles) or L X W X 1.41.  This force
is 1.41 times the force experienced by the single dipole broadside to the
wind (merely L X W).  I further assert that this is the MAXIMUM force that
the crossed-dipole array would experience.

     If there is an error in my equation (and I'm not convinced there is
yet), then it's in the cos theta term.  I'll let you'all know.


73 de
Gene Smar  AD3F
-----Original Message-----
From: Kurt Andress <K7NV@contesting.com>
To: EUGENE SMAR <SPELUNK.SUENO@prodigy.net>; towertalk@contesting.com
<towertalk@contesting.com>
Date: Monday, May 21, 2001 11:14 AM
Subject: Re: [TowerTalk] Angle of maximum wind force on Yagis


>Hi Gene,
>The direction of maximum exposed antenna area, hence maximum force
>developed by the antenna on the support structure, is either with the
>elements or the boom broadside to the wind, whichever direction produces
>the largest sum of projected antenna member areas is it. It's not somewhere
>in between, as was thought a decade ago, somewhere in between produces the
>lowest antenna loads on the structure.
>
>You can read about it in the K5IU articles cited in the post N8SM referred
>to, or the latest ARRL Antenna Book, 19th Edition, Pg 22-18. Or, you can
>get some software that does it.
>
>--
>73, Kurt, K7NV
>
>http://yagistress.freeyellow.com


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