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Re: Multi band dish feeds



Great idea Ed, but after thinking about it a while it seems that you still
cannot get all the feeds at the same focal point.  Yes, you can "fan" them
out, but then they are no closer to the center focal point than before.
If you "fanned" them arond the focal point, then they are all "in front"
of the focal point by their own depth..

So it seems since you can't get them all at the same focus anyway, so may
as well, bunch them the best you can and take advantage of the whole
dish surface, not just part of it.  let their resulting beams be
offset.  You'd be surprised at how far you can go off center and still get
a reasonable beam (similarly offset)...

Bob

 On Wed, 14 Mar 2001, Edward R. Cole wrote:

> Howard etal:
> 
> After sending you my reply, I got to thinking more about the idea of
> feeding a portion of my 8-foot dish in an offset mode.  
> 
> I think a really neat method of multi-feeding a dish would be using a
> normal center-fed dish that is twice the diameter you need.  If you need a
> 2-foot offset dish with feeds for four bands, then get a 4-foot dish and
> mount your four feeds so each feed looks at 1/4 of the dish surface [from
> the dish center to the edge].  Position each feed to illuminate a separate
> quarter of the big dish.  Each feed will essentially operate as a 2-foot
> offset dish.  Since they are pointing in different directions, interaction
> between feeds should be minimum [there is some aperature overlap].  Dish
> pointing will be fairly easy as the four beams should all point in the same
> direction away from the dish [providing you align the feeds correctly].
> Just mount the dish on a normal az-el system.  There are no feed pivoting
> systems or special angle adjustments for each band as in other multi-feed
> arrangements.
> 
> Switching bands would be easy, or if wanted, simultaneous operation could
> be done [but all in either TX or Rx].  Separate dishes for Rx and Tx is
> still a better idea.  Setting up each feedhorn at the focal point might be
> tricky, but solvable, I think.
> 
> I've got a 4-foot dish sitting in the barn raising mice.  Maybe I'll try
> this out.  If it doesn't work the feedhorns are still useable with a normal
> offset dish...hmm.
> 
> Ed
> 
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de WB4APR@amsat.org, Bob

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