Re: [amsat-bb] covering

[Bruce Bostwick <lihan161051@xxxxxxxxxxxxx>]

>>> Hello my friends:
>>> Was wondering which would be better. Covering  my 1.2 meter dish 
>>> frame with
>>> fiberglass screening  or aluminum screening (hardware cloth), which 
>>> would be
>>> better?
>>>
>>> Don...w2xb
>>
>> Fiberglass is nonconductive and won't reflect at all.  It might 
>> affect the signal slightly due to near-field capacitive loading but 
>> not in the way you'd want.  Go with the aluminum screening -- it's 
>> conductive and should reflect RF quite well ..
>
> Plastic is non conductive and it reflects, is fiberglass different?  
> I'm looking for a strong light weight 3' dish.   My 3' oblong 
> primestar like dish is too heavy for portable.
>
> KB7ADL

Fiberglass itself is the most nonconductive stuff there is -- glass 
fiber.  The window screening is typically fiberglass with a plastic 
overcoat.  It's not even the best stuff for covering windows, and it's 
useless for covering a dish.  The fiberglass patio dishes wouldn't work 
either except that they are coated with a very specially formulated 
reflective layer that I believe is conductive paint.  I do know that 
painting a fiberglass dish (or in some cases even a metal one) with the 
wrong stuff can virtually ruin its reflectivity .. the student TV 
station at our university here in town wanted me to paint their logo on 
the patio dish they were using for a TVRO until I found out about that 
from some colleagues.

The surface of a dish is subject to skin effect, basically, and 
anything in that narrow space from a few microns inside to a few 
microns outside the surface will affect the dish reflectivity 
dramatically.  I know of a few dishes once owned and operated by 
certain three-letter agencies that even had strategically cut and 
positioned tuning patches applied to specific areas of the dish surface 
to correct small surface irregularities and increase efficiency.  
Overall, any surface with holes smaller than a certain fraction of a 
wavelength will appear solid, but that small scale skin effect will 
affect the overall "brightness" of the surface at various frequencies ..

Heard from a flight instructor:
"The only dumb question is the one you DID NOT ask, resulting in my 
going out and having to identify your bits and pieces in the midst of 
torn and twisted metal."

----
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