Polarization of Patch antenna - Intuitive explanation

Question

I was looking into the polarization of patch antennas (for GNSS use). I found this website (http://kempbros.github.io/antennas/Patch_Antenna_Generator/) which gives the geometry of the antenna starting from a couple of parameters (frequency, dielectric constant, etc..). From what I can see the polarization of the antenna (LHCP or RHCP) is determined by little cuts of the corners of the patch.

I found fascinating that such a little change has such an impact on the characteristic of the antenna. Is there a simple intuitive explanation for this? (I do not have any antenna theory background)

Answer 1

A patch antenna can be understood and approximated as a cavity resonator whose top and bottom are PEC while the sides are PMC walls. Because of the nearly square geometry there are two (linearly polarized) modes that can be simultaneously excited any frequency. The two resonance peaks do not coincide because the patch is not a perfect square, and the corner notches are used to tune the small resonant frequency separation of one mode relative to the other. The patch operates at the nominal midpoint between the peaks and thus the relative phase of the two modes are also tuned with the notches just as it could be done with an LC resonator circuit. When the relative phase between the modes is $\approx \pi/2$ we have circular polarization. Whether that is RH or LH depends on the asymmetry of the notches relative to the feedpoint, i.e., which resonance is ahead and which is lagging in phase.