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Patch Back Element -- when is the geometry incidental?



Wow. It was tough thinking up a title for this. :-)

Patch guru's, please indulge me:


For a round patch design ala W0LMD
http://www.ultimatecharger.com/Dish_Feed_S.html, at what distance from
the edge of the patch element does the geometry of the back piece of
metal no longer influence the pattern, resonant freq, or match of the
feed?

I built one patch for my primestar dish with a cup using a
rectangular Hammond aluminum box and a K3TZ
element. The box is just short of 4" by 5".
http://webpages.charter.net/acito/FD2003/newpatch.jpg

This patch was 1 s unit down from a helix with no cup, which seems to
agree with theoretical expected results (the patch should over
illuminate the offset dish), but not anecdotal results. Many have actual
usage results that the patch is still slightly better on offset fed
dishes http://home.swbell.net/k5oe/K3TZ_patch/K3TZ_patch.htm - a few of
those folks suggested that my cup may be too close to the edges of the
patch.


so....    I'm going to build another patch using the lid of the same box
(no cup) and the W0LMD design. I also have a larger, center fed 1.2 M
dish I am going to try and compare with the primestar dish. Remembering
that the Hammond box and lid are rectangular, and there will be about
5/8" spacing from the edge of the disk to the edge of the lid on the
short dimension, and about 1" in the other direction, will this degrade
the performance of the patch?

I like the Hammond lid because it is sturdy and aluminum (won't rust),
but you can convince me otherwise.

Bill

W1PA

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