Re: [amsat-bb] Helix Feed for a Dish

[K5OE@xxxxxxx]

Fred,

fspinner@hotmail.com writes:
> Has anyone figured out if the "cup reflector" cavity makes any difference 
at 
>  all when used as a dish feed?  Obviously a standalone helix is does (more 
>  gain) but as a dish feed, pattern is more important than gain.  But then 
>  again, a choke ring on a Chapparrel type feed decreases sidelobes, so 
there 
>  is a strong probability that there will be a difference.

A helix has several very large sidelobes, especially around 35 degrees and 50 
degrees (reference the ongoing discussion with NX7U).  For a small dish, 
especially one of the partial-parabolas like the BBQ style, the sidelobe 
radiation can greatly over-illuminates the dish.  This degrades the G/T, 
hence the S/N ratio.  The cup reduces the sidelobes and "splash" beyond the 
dish's edge.  That's the theory, anyway...  

It would be very educational if someone who currently has a "regular" helix 
feed with reflector could take noise measurements and then install a cup 
around the feed and observe/report the difference.  
73,
Jerry, K5OE
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