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Re: receiving system design tradeoffs



At 11:06 PM 2/5/2003 -0600, Jon Ogden wrote:
>on 2/5/03 7:54 PM, Doug Faunt N6TQS +1-510-655-8604 at faunt@panix.com
>wrote:
>
>> I believe that a 3' aluminum dish that was perforated properly would
>> give just as much gain as a solid dish of that size, but would be much
>> lighter in both weight and windload.  Now, if we could easily locate
>> such.  
>
>It's a question I've thought of and with living in a warm climate, you are
>OK.  But for those of us who live where we have snow and ice, a perforated
>dish isn't going to offer much help during the winter...
>
>One interesting thing though about BBQ grill type dishes I've heard is that
>with snow and ice on them their windloading is actually GREATER than a solid
>dish.  I had the systems engineering manager for a cellular telephone
>company tell me that once.  Can anyone back up his claim?
>
>73,
>
>Jon
>NA9D
>-------------------------------------
>Jon Ogden
>NA9D (ex: KE9NA)

Jon,

I'll try to guess.  In wind area I can't see that it would be true, but
mass (weight) would definitely increase so wind-buffeting of the heavier
object might cause higher forces on the mount.

As far as perforated or mesh dishes having lower wind loading that is
largely a myth.  It only applies at low wind speeds.  Once winds reach
about 20mph, the mesh dish presents the same loading as a solid surface.
So why were all those C-band TV dishes made of mesh?  Mostly for lowered
manufacturing costs, cheaper shipping weight, and aesthetics.  A mesh dish
blends into the background better!

I suspect a solid dish would be superior in snow/ice country since less ice
build-up would occur.  Still the weight of snow filling a ten-foot TV-dish
will wreck havoc on it.  In snow country, stowing the dish pointing on the
horizon may be the better position.  Bird-bath (pointing at zenith) is only
advisable during high wind to minimize surface area.

These observations are mostly from personal experience, rather than
engineering texts.

Ed - AL7EB

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