Re: [amsat-bb] "Trash Can" or steel drum dish antennas

[Scott Townley <nx7u@xxxxxxxxxxxxxx>]

Good points.  I was wondering about that as I wrote it...
In the terrestrial microwave environment Tant is assumed to be nearly
290K (not cryogenic) so that absorber wouldn't make much difference in
the receive mode.


Charles Suckling wrote:
> 
> 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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