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Re: CP Patch Antennas? {YUP}


There are several different configurations for a microstrip patch antenna.

The design that the "G3RUH" patch uses, circular, typically needs two 
feedpoints (if someone knows how to do it with one, let me know) to make it 

Rectangular patches can be made circular by making one side longer than the 
other by making the patch so that the one side is -j50 and the other is +j50 
Ohms. (90 degree phase shift)  Then the patch is fed on (through the 
dielectric) the appropriate diagonal to make the LHCP needed for a RHCP dish 
feed.  The opposite diagonal would produce RHCP on the patch (for a LHCP 
dish feed, the sense changes on reflection).

A (symmetrical) square patch can be made circular by "cropping" the corners 
at the appropriate point on one corner, and the corner opposite to that 
corner, the cropping being the correct dimensions at a 45 degree angle on 
each "crop".  What this does is provides electrically a 90 degree phase 
shift, and a secondary mode is generated on the axis of polarity that isn't 
the one being directly fed.  The directly fed axis is just like how you'd 
feed a linear polarization patch, in the center of the patch in one axis, up 
the center line at the point that is the 50 Ohm match, through the 
dielectric.  Woo Boy, I don't know if that description can do it or not....

Of course one can feed two sides of a square patch 90 degrees out of phase 
(two feedpoints) on the edges for CP as well.  This is commonly how patches 
are fed on PC boards or in arrays.

Anyway, one other interesting thing I've noticed is that if you want to make 
a patch be DC grounded, the way the "through the dielectric" feed works is 
that at either end of the patch (or the outside diameter of a circular 
patch) the high current part is at the edges.  The high current part is the 
radiating part of the antenna.  So a square patch can be grounded in the 
center with a small grounding post and not effect the patch much (the 
dimensions change a little due to the fact that the short is not of 
infinately small diameter).  This also explains why feeds like the 
multi-band ones at W0LMD's site
http://www.ultimatecharger.com/dish.html work with a higher frequency patch 
in front of the lower frequency one.  As long as the edges are exposed, most 
or all of the radiation isn't blocked.

As far as "scalar rings" go-- on a dish they block (actually reflect, if 
they blocked the G/T ratio would suffer) the sidelobes or overillumination 
that a feed would produce on a dish.  The "scalar ring" dimension is 
dependant on the f/d ratio of the dish the patch will be mated to, and 
ideally (and no one's done this yet, afaik) should be made adjustable on a 

I hope this explains patches and patches used as dish feeds.  It's currently 
about all I know about them.  And I'm sure there are others out there that 
know a LOT more about this than I do....

OH, BTW, I'm going to release my drawings of the two different types of 
patches I've come up with as soon as I finish network analyzing the 
"cropped" corner, DC grounded design I'm currently playing with.

73, Fred W0FMS

>From: Clair Cessna <ccessna@pe.net>
>To: amsat-bb@AMSAT.Org
>Subject: [amsat-bb] CP Patch Antennas?
>Date: Wed, 02 Jan 2002 10:23:46 -0800
>Hello all,
>I have been looking for a pattern to build a CP patch feed for a dish
>antenna for reception of A0-40.
>I found a couple of sites on the web which show what is claimed to be a
>CP patch with only one feed point.  I read in on another site that to
>get CP from a patch, the active element must be fed in two places
>connected by a quarter wave matching section.
>I would appreciate a clarification of this and also a source for a CP
>patch pattern for  mw beginners like me.
>Thanks,  Clair      K6LG@arrl.net
>Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
>To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org


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