beamwidth vs diameter

[T3D Bob Wier <wier@xxxxxxxxxxxxxxx>]

This question has come up in a satellite tv list, but the optical principals
are the same, so I thought I'd ask here...

You may have seen the 18" direct broadcast TV dishes. The are standard
parabolic sections (offset a bit). The older "C-Band" dishes were normally
6' or larger. The differences are that the DBS satellites operate in the
12Ghz range, whereas the C-band were in the 4 Ghz range (if I remember
correctly). Now one of the reasons that the 18" dishes work is that
the satellites tend to be about twice the power of the older c-band
satellites, but maybe more importantly since they operate at 12Ghz,
an 18" dish has a resonable S/N ratio and gain at that frequency
whereas it would not at the lower c-band freq. One factor is that
for the same size parabola section, higher frequencies give better
gain and narrower beamwidth.

The question has come up - suppose you set up say four 18" dbs dishes
and install c-band electronics taking the combined signal from all
of them additively. Would that work?

My first impression was that the main limitation would be beamwidth
since it would be so broad at c-band freqs that you'd be getting
multiple satellites - the reduced gain could be made up by using
higher amplification factors  but the broader beamwidth produces
lower s/n so that  would be the main limitation.

But then, I remembered that multiple small optical (and I presume radio,
as in the VLA) units can be used to get effectively a very narrow beamwidth,
although the actual physical surface area is very small compared to
effective surface area (ie, diameter) of the virtual mirror/dish.

I can't seem to reconcile these two factors which seem to be
operating in opposite directions. 

Any thoughts?

THANKS

   ====== wier@xxxxxxxxxxxxxxx ======
  11:11 AM Tuesday, June 10, 1997
   keeper of the Photo-3d, Overland-Trails, LDS State
    Research Outline Guides and other stuff
     (currently in Ouray, Colorado)



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