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Re: Patch Back Element -- when is the geometry incidental?



>From some modeling I've seen, too little reflector will cause the patch to 
become less like a hemispherical pattern and more like a "Bow tie" or a 
dipole.  The "rearward" radiation will be RCHP on a LHCP feed in this case.  
Ideally the diameter of a patch at 2.4 GHZ should be at least 4" or larger.. 
typically larger being better.

Spacing between the reflector and patch element should be as wide as 
possible without generating parasitic modes.  This value is around 0.25" at 
2.4 GHz.  The wider the spacing within these bounds, the wider the bandwidth 
of the patch.  Also, the feedpoint impedance (or resonance) is effected by 
the spacing.

Although I've not tried it yet, putting a choke ring around the patch might 
be difficult withot test equipment as the high current points of the patch 
are on the outside edge of the radiator.  So any capacitive coupling effects 
will be greater on the outside of the radiating element.

I was going to wait to announce this until I could do a webpage for this, 
but here goes.. hopefully it can help someone building a W0LMD patch:

Several independant reports and my experience seem to suggest that W0LMD's 
patch resonates too high in frequency (problem #1).  On the network analyzer 
it resonated at 2520 MHz, but the locus showing the dip because of the two 
resonant modes of the patch (remember at 2401 one mode should present -45 
degrees shift and the other +45 to make CP.. and this is also the same as 
one freq being resonant higher and one lower than the desired CP frequency) 
was at roughly 2401 MHz.  Also (problem #2) the dip in the locus was not as 
pronounced as it should have been (meaning the too modes were too close 
together, less than the aggregate 90 degrees shift).

I fixed problem #1 by adding a #6 brass tuning screw on the side opposite on 
the axial line that the feed N-connector is on, 1 inch from the center of 
the shorting bar (almost to the edge) in the center of the patch.  Turning 
that screw down so its about 1/16" from the ground plane got the patch to 
resonate at the 2401 MHz,  the dip in the locus didn't move much.. almost 
not at all.

#2 was fixed by using a brass screw for the original CP adjustment, and 
soldering a brass nut onto the CP adjustment screw.  This gave me the 
ability to actually over adjust the spread in frequency of the two modes.  
The original #6 screw almost had to be shorted to the ground plane for me to 
see any dip in the smith chart locus at all.  My suggestion here is if I did 
it again I'd use a #10 brass screw for the CP adjustment screw instead.

W0LMD states that the resonance point of the patch moves down a bit (I swear 
I saw 4% somewhere but I've not been able to find it again) when the feed is 
placed on a dish.  Since I brought my feed into work to be tested, I 
couldn't practically bring the dish with me.  But I did notice when the feed 
is resonant at 2520 MHZ it was MUCH more sensitive to "large metal objects" 
than it was when brought into resonance at 2401 MHz.  So far the "lab tuned" 
2401 MHz patch seems to work wonders on the 1.37M DH satellite dish it's 
attached to.

Also as a commentary.. get all brass hardware for the tuning screws and 
nuts.  Don't bother with tapping the #6 (or #10) tuning screws-- solder the 
nut on to the top of the patch.  Having stuff stick out above the surface of 
the patch matters very little and mechanically the patch will be MUCH 
better.  Use "Q-dope" to seal the threads after tuning.  You can make 
"Q-Dope" by dissolving styrofoam cups in some acetone-- get it too about 
"snot" consistancy, apply on the screws and let dry for 30 minutes.

Anyway, if you have a way of measuring the VSWR, the "dip" in the locus 
should look like a very small "W" in the SWR curves with, say 1.05:1 at 2390 
MHz, 1.2:1 at 2401 and 1.05:1 at 2410 MHz or so roughly to have the thing to 
be resonant and CP at 2401 MHz.

There is about a S-unit difference in my experience on a properly 
illuminated center-fed dish with this patch properly tuned and a NON-CP 
patch.


I sometime wonder if a patch could be the radiating basis for a "disc" yagi 
to get the gain of the feed up a bit.  But I'm not enough of an 
Electromagnetics wizard to determine if it would work.  Certainly the feed 
impediances would be affected, and probably the CP tuning as well.

Hmm. a quadrature fed loop yagi or disc yagi is a possibility for offset fed 
dishes as well, isn't it?

Fred W0FMS

>From: "Bill Acito W1PA" <w1pa@hotmail.com>
>To: <Amsat-bb@AMSAT.Org>
>Subject: [amsat-bb] Patch Back Element -- when is the geometry incidental?
>Date: Fri, 20 Jun 2003 11:46:41 -0400
>
>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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