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Re: "Trash Can" or steel drum dish antennas



Scott

Don't be misled by the effects of microwave absorber.

Lossy material at a given temperature emits thermal noise - so if the 
shroud is coated (on the inside) with absorber then the feed will pick up 
just as much noise as if the shroud were not there.

Re sidelobe reduction - this is not going to help us much, unless the 
satellite is close to the horizon where the sidelobes might pick up some 
noise.

If you can tolerate the extra weight go for extra diameter on the dish, or 
extend it with a reflector.  This will certainly help.

Charlie G3WDG



At 23:05 03/12/01 -0700, you wrote:
>So far there are two votes for "high performance dishes".  Those votes
>are correct!
>BUT...the shrouding is accomplished by a "drum" or cylindrical surface
>that is covered in *microwave absorbing material*.
>At 2GHz this stuff is *heavy*.  Not impossible but a bit tricky.
>As to performance, it both 1. reduces spillover from the feed and 2.
>edge diffraction from the rim of the reflector.  Resultant antennas meet
>FCC Part 101 "category A" requirements for radiation pattern.  This is
>something like -50dB sidelobes and a front-to-back of more than 65dB
>(not sure of the precise numbers, but they are ridiculously small!).
>So...for satellite work they would be the cat's meow; Tsys nearly just
>that of cold sky and the receive preamp.  Just a bit bulky (and not sure
>where one would get the microwave absorbing material; Cuming Corp. used
>to/still does(?) make a product called EccoSorb which is what you'd
>want).

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