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Re: Wind load of a mesh dish




> 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.
>
> The rotor and support structure for a 6 foot mesh dish should be designed
to
> withstand the wind load of a 6-foot solid dish, because that's what the
mesh
> dish will effectively be in a 60 mph wind.  A mesh dish may be cheap,
light
> weight, and easy to form.  But it does NOT have significantly less wind
load
> than a solid.  If you have a good memory, you could always park your 6
foot
> mesh dish pointing straight up when you're not using it, to minimize the
> wind loading.  But a Yaesu rotor is almost certain to be damaged in a
storm
> if the dish isn't pointing straight up.
>
> Wayne Estes W9AE
> Mundelein, IL, USA


You are technically correct that at some given wind speed, the mesh begins
to act more like a solid material; however, the key here is the wind speed
at which that effect occurs.  This varies based upon the size of the mesh
wire and the size of the mesh "holes".  I ran across the formula for
calculating this some years ago and can't locate it right now, but at that
time I was calculating wind load for wire mesh dishes to be operated by my
Yaesu G5500 AZ/EL system.  As I recall, for dish sizes up to 7 feet in
diameter, using 1/4" hardware cloth, the rotators' safety limits (dish at
elevation 0 degrees rather than "bird bathed") began to be exceeded at wind
speeds in excess of 120 MPH.  When using 1/2" hardware cloth, that danger
point increased to 150 MPH.  My 4-footer, covered with 1/4" wire mesh,
survived several 80+ MPH storms this past winter without ever being "bird
bathed" (Don't try this trick with your solid dish, however.) Of course,
either mesh size can be used with no noticeable reduction in dish
performance, but I found the 1/4" mesh to be somewhat easier to work with.

73,

Rick
KG6IAL
www.teksharp.com
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