Cross-polarization tolerance requirements of square microstrip patches

We normally feed a rectangular patch along the centerline of the H-plane (the non-resonant width). This is done to eliminate excitation of the patch in a mode along the other axis. The second mode radiates cross polarization. This article provides the tolerance requirements, given the cross polarization. The rectangular microstrip patch radiates its peak cross polarization in the diagonal planes, even when the antenna is fed perfectly. This cross polarization is low. When we locate the feed along the centerline of the H-plane of the patch, the impedance match to the TM₀₁ mode is poor, and the antenna does not radiate significant power at boresight to the patch. Often we design a square patch, so that we can radiate orthogonal-linear polarizations (or dual-circular polarizations) by using two feeds. If we fail to place the feed probe, aperture, or microstrip line along the H-plane axis, the cross-polarization level rises