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Re: F/D cal of offset dish



IIRC, it was to ensure the LNB pointed up at the
cold(er) sky, vs picking up 'Earth Noise' from warmer
surroundings. The exact angle was probably a
compromise (what aint?) between available LNB gain and
NF, dish size, and downlink ERP.
I remember a whole department at DirecTV being devoted
to receiver/LNB/antenna design when I worked there.
73, Jim  KQ6EA

--- Cliff Buttschardt <cbuttsch@kcbx.net> wrote:
> I was about the suggest this program when Charlie
> beat me to the punch!
> Most of these dishes have a similar characteristic: 
> They seem to offset
> the beam by twenty two degrees.....why I don't know
> but this note is sent
> as a ham rather than an engineer for the moment! 
> The F/D is then adjusted
> to the point where the feed is JUST out of the field
> of the dish.  This is
> a great compromise indeed.  You can best visualize
> this as a PART of a full
> dish where the lower edge is slightly above the line
> between the feed and
> the center of a full circular reflector.  For most
> purposes this will get
> you to the 90 percent point and with a bit more
> effort all the way!
>      Cliff K7RR
> 
> 
> Jim Jerzycke wrote:
> > 
> > W1GHZ has a neat little DOS program you can
> download
> > from his website that will do this for you.
> > 73, Jim  KQ6EA
> > --- Charles Heisler -K3VDB- <ceheisler@suscom.net>
> > wrote:
> > >       Can anyone tell me how to calculate the
> F/D Ratio
> > > of a offset Dish
> > > so that I can build a 10G feed for it. The
> original
> > > feed was at a focal
> > > length of ~44.4cm.
> > > My dish is 66cm X 60.6cm and is 6.5cm deep. I
> don't
> > > think the cal. for a
> > > round dish is what I want to use.
> > >
> > >       TNX for any help
> > >       Charlie..K3VDB
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