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Re: "Trash Can" dish antennas

In a message dated 12/3/01 2:39:08 PM Pacific Standard Time, 
Jim.Walls@sce.com writes:

> Ray - WB3ABN asked (in part):
>  > Most larger Navy and USCG ships have "trash can" antennas mounted on the
>  flying bridge.  The best way for me to describe them would be to take a
>  metal trash can, cut half of the top off, and mount the parabola in the
>  bottom with the feed in the open end.  This in effect is like wrapping a
>  "fence" around the outside edge of a normal circular dish.  Does this serve
>  to eliminate background noise from over illumination of the dish, so the
>  feed only sees signals from the front of the focal point?
>  That's one way to describe them.  Your last sentence is exactly right.  In
>  the terrestrial communications business, it's refered to as a high
>  performance dish.  The "fence" as you put it, reduces radiation (and
>  reception) in undesired directions.  You will see lots of them (only bigger
>  in most cases) at major communications sites.  Most of them have a covering
>  over the open end.  It is made of a canvas like material and is commonly
>  called a "tom-tom".

Okay, so it's a proven concept to reduce over illumination.  Is there 
substantial benefit to modify any of the typical S-band or TVRO dishes (I 
have a 10 ft Winegard round with mesh covering waiting for a use...) with an 
"illumination fence" or "shield" like on a Tom-Tom?  Would it improve the 
offset dishes, or the BBQ grills, at least enough to be worthwhile?  Or is 
the benefit more or less significant at various frequencies - Bob B says the 
military versions operate at low-UHF range.

Laura mentions a flat bottom version of this antenna called a "backfire", but 
I can't find much info on these in my normal references.  How does a backfire 
compare and would it prove to have sufficient gain and be less costly to 
construct that a COTS (sorry, military acronym "commercial off-the-shelf") 
BBQ or 1.2m offset dish?

Ray - WB3ABN
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