Re: [amsat-bb] Cupping a patch feed

["Bill Acito" <w1pa@xxxxxxxxxxx>]

http://www.qsl.net/k3tz/k3tz_test2.html

Interesting.

btw, I just did some back-of-the-envelope calculations:

Assuming Square patch for 2.4 Ghz (58mm on a side)
4" circular reflector
18" DSS dish (f/d .69 or .7)

For the right edge of the patch to just "see" optically the right edge
of the dish, the cup needs to be 4.15 cm high, and for the corner, 19mm
high (between 3/4 and 1.5 inches...  hmmm, sounds familiar). How a
line-of-sight view from the edge or corner of the patch equates to the
pattern of the patch seeing the dish, well, that's beyond me.

Maybe I'll build one and see...  :-)

Bill
W1PA



----- Original Message -----
From: "Edward R. Cole" <al7eb@ptialaska.net>
To: "Bill Acito" <w1pa@hotmail.com>; <amsat-bb@AMSAT.Org>
Sent: Wednesday, October 23, 2002 1:45 PM
Subject: Re: [amsat-bb] Cupping a patch feed


> 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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