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Re: [TowerTalk] Dish on a tower

To: "J. Gordon Beattie, Jr., W2TTT" <>,"'Tom Ed Moore'" <>, <>
Subject: Re: [TowerTalk] Dish on a tower
From: Jim Lux <>
Date: Sun, 20 May 2007 21:03:16 -0700
List-post: <>
At 07:07 PM 5/20/2007, J. Gordon Beattie, Jr., W2TTT wrote:
>Tom et al,
>What are the size and spacing of the holes of a typical TVRO dish?
>I have a six foot, 900 MHz grid dish that I overlayed with 1/4 inch
>galvanized hardware cloth and would like to know the wind loading at various
>wind speeds.

The drag of a perforated screen or grill is tricky. There's all sorts 
of interactions between the wires or holes, and the effective 
porosity will change with wind speed.  Usually, you look it up in a 
table which is based on empirical measurements, or do a worst case 
assumption that it's solid.

If you're in an icing/snow location, then you might as well assume 
it's solid anyway.

>This may have already been addressed? A TVRO mesh dish becomes a solid
>object at winds around 35 to 40 MPH. This will vary with pressure and
>humidity(usually an inverse relationship).

I don't know that humidity will affect it very much. Temperature and 
pressure, since they affect density, will affect it a bit (i.e. hot 
day will have less drag because the density is lower).  You're not 
getting close to sonic speeds, so the temperature effect on speed of 
sound is negligble (i.e. you can assuming incompressible flow) 


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