[BC] chain link fence effect on satellite dish?

[Cowboy]

On Saturday 21 March 2009 03:25 pm, r.j.carpenter wrote:
>  
>  I don't believe anyone has mentioned that the dish is probably not 
>  uniformly illuminated by the feed.  They don't want to illuminate the 
>  earth behind the dish and get 300-deg K noise from it instead of sky 
>  noise temp. I think this means that the outer portions of the dish 
>  contribute less to the gain that their area would suggest. Obscuring s 
>  bit of the outer area of the dish shouldn't decrease the gain a lot, but 
>  might increase the noise temperature.
>  
>  Or am I as stupid as usual?

 No.
 Typically, only about 80% of the dish surface is illuminated, the outer
 20% used for mostly "guard band" type function.

 However, the surface area at the outer edge can provide a LOT more
 signal gain than the rest of the dish, simply due to real estate occupied.
 Surface imperfections become much more critical at the edges also.

-- 
Cowboy