High Polarization Purity for Aperture Coupled Microstrip Antennas

A design method to suppress the cross-polarization of an aperture-coupled microstrip antenna is presented. The design consists of three substrate layers with embedded vias in the middle layer surrounding the coupling slots to constrain the electric field. A cross-polarization suppression of more than 20 dB in the orthogonal E- and H-planes is achieved at 20 GHz. Furthermore, the design nearly doubles the impedance fractional bandwidth without adversely affecting the antenna's performance in terms of gain and backlobe level. The proposed design is thus a suitable candidate for antenna arrays with high cross-polarization isolation requirements.