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Re: Dish Painting???



At 02:19 PM 8/10/2002 -0700, Dave Guimont wrote:
>What am I missing here??
>
>The optical focus of the two off-sets and one circular dish I have is 
>somewhere on
>the feed.  I could put my feeds (patches and helices) in the oven, and they 
>would
>survive!!
>
>The small amount of heat that is conducted to my preamp (converter is 
>behind the dish)
>from the feed doesn't raise the temperature of the preamp anymore than the 
>radiant
>heat of the sun that I can detect??
>
>I just tested it today, at high noon, and our ambient is ~82° F.....
>
>The pre-amp appears to be about the same temperature as a similar size 
>aluminium(for Reg!)
>  box lying in the sun??

Dave,

I believe Jeff Davis initiated this topic with question about directions on
his G3RUH spun-aluminum "in-painted" dish.

If you are using a shiny metal dish then solar radiation is nearly all
reflected (see Bob Bruninga's treatsie on reflectivity/absorbtivity) and
concentrated at the focal point!  After all sunlight is a form of
electomagnetic radiation just as 2.4 GHz is, and if you point directly at a
huge powerfull light source (the sun), the light will be concentrated (you
have gain)!  This can reach several hundreds (or thousands depending on
dish size) of degrees if the dish is pointed at the sun (on purpose or
accidentally).  

When not pointed at the sun it is not (very) relevant.  {unless you live
near the tropics or in a desert; Then I think you will get a very nasty
burn if you pick up a metal object that has sat in the summer sun a few
hours...five years working experience in the Mojave Desert!}

Most commercial dishes are painted or surfaced with a flat or matte
covering to disperse sunlight!  That is most likely the case with your
dish, Dave.  Also bbq grill dishes do not reflect much light since they are
mostly "air"!  The G3RUH dish is unfinished spun aluminum...very shiny!
Thus it is prudent to paint it...per the discussion that has resulted on
the bb.

Ed, AL7EB

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