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A nearly square microstrip patch gives you circular polarization by activating two modes that have their resonant frequencies very close to the design frequency desired. There are plenty of ways to do this..
A very good article about this is at
I haven't gone through the mathematics of it all but this is what this article explains.. Another way is to have a truncation on the edges of the antenna to create the same effect being talked about in the above article.. Also, you can find good, although very mathematical, explanation to how this is achieved in the Microstrip Antenna design handbook by Bahl and Bhatia..
If I could add to vfurlan's comments.. it is not necessary to have the antenna fed by two probes.. You can do it with a single probe which is what the above article talks about..
I would like to add two more things:
1)the CP bandwidth will be very limited if the antenna is fed with only one probe.
2)the coupling between two feeding ports should be taken into design considerations if the antenna is fed with two probes
how do nearly square patch gives circular polarization?