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Re: rectangular "dish" illumination?



>From: "Greg D." <ko6th_greg@hotmail.com>
>Hi folks,
>
>So what is the rule of thumb when illuminating a not-round "dish"?
>My screen-covered grill measures about 30x36".  Do I pick a feed
>with a 10db width of 30 or 36 or somwhere in between?  It will be
>a compromise, but what is best?  Do I gain more signal or noise as
>the pattern spills over the width, and illumiates the length?
>
>Thanks,
>
>Greg  KO6TH

Well Greg,

Ideally you would have a feed with a rectangular pattern with the ratio of
30x36 that would provide a constant illumination out to the edge of your
dish and then fall instantly to no [or zero] illumination.  Now you know
such a feed does not exist.  All feeds have pattern which usually is
maximum toward the center of the dish and gradually drops off in intensity
toward the edge.  The design of feeds is the subject of most dish engineering.

If you were only transmitting with your dish, then gain would be the
predominant goal.  Getting the maximum signal going in one direction.  In
that case you would want the fullest use of the dish area, and maybe
trade-off wasting some signal off the narrow side of the dish in order to
illuminate the long dimension.  But realise that any radiation from your
feed that does not hit the surface of the dish is wasted, gone, and has no
contribution to your forward signal strength [or gain].

When you design a dish for receiving, the problem is somewhat similar, but
the problem of radiation coming from directions other than from the dish
surface causes deterioration of your receiver system noise figure [any
extra noise is added to the total receiver input].  Your goal is to
maximize [amplify] the signal and minimize the noise.  Therefore it is
better to not use part of the dish and lose some of the signal that it
collects, rather than collect noise beyond the narrow dimension edge.  Why?
 Well the sky temperature runs about 10K at 2400 MHz and a "good" receiver
adds about 50 to 70K.  The ground is 290K.  If you see it at -10 dB, you
are adding 29K to the total.  

Tsys = Tsky + Trx + Tgnd.
  
If Tsky = 10K, and Trx = 50K then Tsys = 60K if you don't see any ground
noise.
  
If you see Tgnd < 29K, then Tsys is < 89K.  This is slightly more than a
noise figure of 1 dB, and that is pretty darn good for an amateur system.
  
If Tgnd = 100K, then Tsys = 160K and NF grows to 1.9 dB.

So do you get the picture?  A short answer would be to design the feed to
illuminate the narrow 30 inch dimension and ignore the wider dimension.
Now it turns out that a dipole feed is pretty well matched to a rectangular
shaped dish because it has wider beam width in one direction than the
other.  A dish using a dipole feed is probably designed for maximum gain
rather than minimum sidelobes or spillover, though.

Ed

Reread my reply to the topic "Question regarding gain of Dish Antennas with
different feeds" I sent yesterday.


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