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Re: Wind load of a mesh dish



I do think it depends on the size of the holes in the mesh.  At 2.4 GHz the 
holes can be pretty wide..
1/4 inch or so.  Yes the "turbulence" makes the dish solid-- almost-- the 
rule of thumb on Ku compatible mesh on TVRO dishes that above 40 MPH, the 
windload is 60% of what a solid would have on it.

At 100 MPH straight into the wind a 6' solid dish would have 700 lbs of 
force developed against it, a Ku mesh would have 420 lbs of force.  I 
imagine that a 1/4" homebrew mesh, ballpark would have 300 lbs or so force 
on it.

These are all kosher amount for dish actuators, but even the latter is too 
much for a G5400/5500/5600 in 100 MPH winds.  Meaning that this would likely 
work as most areas do not make 100 MPH winds and the dishes usually don't 
end up pointed into the wind-- but its like designing a roller coaster for 
75% of it's potential load because the riders are only over a portion of it 
for a short time.

Reference URL:  http://www.geo-orbit.org/sizepgs/grndpole.html#anchor27689

Fred W0FMS


>From: Doug Faunt N6TQS +1-510-655-8604 <faunt@panix.com>
>To: w9ae@earthlink.net
>CC: amsat-bb@AMSAT.Org
>Subject: Re: [amsat-bb] Wind load of a mesh dish
>Date: Mon, 19 May 2003 10:52:14 -0400 (EDT)
>
>    From: "Wayne Estes W9AE" <w9ae@earthlink.net>
>    Date: Mon, 19 May 2003 01:10:47 -0500
>
>    On May 12, Rick KG6IAL wrote:
>
>    I'll soon be migrating to a 6 ft.
>    dish which will include a triband patch feed that adds U-band support.  
>It's
>    beamwidth is still within the accuracy of a Yaesu 5400/5500 and because 
>it
>    consists of a lightweight aluminum dish frame, covered with 1/4" 
>hardware
>    cloth, it is very lightweight and offers almost no windload.
>
>    Wayne W9AE replies:
>
>    I just returned from a 2-week vacation, and I'm surprised to see that 
>nobody
>    responded to the last statement in Rick's message.  In high winds, mesh 
>has
>    almost the same wind load as a solid.  That's because the mesh creates 
>so
>    much turbulence that the turbulent air layer surrounding the mesh is
>    effectively a solid.
>
>I'd like to see a citation for this statement.  Last time I looked, I
>found references to formulas for wind loading that took into account
>the percentage of space to structure, and the hole size.  For lower
>percentages of structure (such as mesh), wind loading seemed to be
>significantly decreased, even at high speeds.  I'm traveling this
>afternoon, so can't relocate those references this week.
>
>Or, someone could actually experiment- take a 1' square of sheet, and a
>1' square of mesh, and hold them out a car window at 60MPH.
>
>    Wayne Estes W9AE
>    Mundelein, IL, USA
>
>
>73, doug
>----
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