Optimization of the polarization of reflectarrays using characteristic modes

Microstrip reflectarray antennas have raised a lot of interest due to their attractive features, such as low profile, low cost, conformability, and beam scanning capabilities. Basically, a microstrip reflectarray consists of a planar array of patches whose characteristics are slightly modified to get the proper reflection phase to form a planar phase front in a specified direction. Microstrip reflectarrays constitute an alternative to parabolic reflectors. Several published solutions for adjusting the phase of the field reflected by the individual elements are outlined. However, some of these techniques can degrade the cross-polarization level, especially for the case of oblique incidence. We use the theory of characteristic modes (Harrington, R.F. and Mautz, J.R. 1971) to analyze and optimize the polarization performance of a square reflectarray element. We find that, in the case of oblique incidence, the polarization purity of a square patch is improved by dividing the patch into two rectangular strips in the direction of the desired polarization; the bandwidth performance of the fundamental mode is preserved.