Re: [amsat-bb] Cupping a patch feed

["Edward R. Cole" <al7eb@xxxxxxxxxxxxx>]

At 12:52 PM 10/23/02 -0400, Bill Acito wrote:
***snip***
>A patch over-illuminates (+/- 130 deg), but better axial ratio (less
>QSB) and no side lobes.
>
>Not knowing all the physics and the math of the pattern, if I cup the
>patch enough so it only "sees" the offset dish (effectively cutting off
>part of the "volume" of the patch's pattern), will I effectively reduce
>the gain of the patch?...
>
>In other words, some practical experiences describe a patch on an offset
>dish having the same relative signal strength for signals vs a helix,
>less QSB, but more background noise. If I cup the patch, can I reduce
>the noise component, and keep the same QSO signal strength, when I have
>effectively cut off the outer ring of the pattern?
>
>Bill
>W1PA

Bill,

Hopefully the patch experts will reply.  What you prpose sounds reasonable
from an optical standpoint.  The questions that occur to me is whether ther
is near-fiedl interaction at the edge of the patch with the "cup" wall?
How deep will the cup need to be to effectively truncate the pattern?

Why not experiment and see what results?  Checking the feed pattern at 2400
MHz is not too hard.  You need a cw signal source (handy for checking your
converter frequency, too).  Just rotate the feed to determine the pointing
angle that the signal drops to -10 dB or -15 dB.  For an offset feed dish
the target beamwidth is 70-degrees (35 either side of center).  Don't worry
too much about being too technically correct...hams discover stuff this way
all the time!

If you do try this, be sure to share what you find out!  Be a great
one-page how-to article for the Amsat Journal!

73, ED - AL7EB
PS:  I have a question about patch feeds (actually two of them):
1)  How broadband are they...I have the need for a circular pol dish feed
on 1268-1296-1420 MHz!  (f/d=0.4)
2)  Are there designs for two feedpoint patches (one RHCP and the other LHCP)?

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