Effect of a Finite Ground Plane on the Axial Ratio of Circularly-Polarized Microstrip Arrays

The degradation of the axial ratio due to surface waves in a circularly-polarized microstrip array was investigated. A prototype, consisting of a 2x2 microstrip array with feeding network, operating at 4 and 6 GHz simultaneously, was developed to investigate this effect experimentally. The measurements were compared with results from a detailed numerical model and from a simple approximate model. It is shown that diffraction at the edges of the grounded dielectric slab causes a high level of cross polarization within the beam width of the antenna. Based on these insights, an Electromagnetic Band Gap (EBG) structure is proposed to reduce the effect of surface waves on the polarization properties. Measurements on this structure have shown that the axial ratio can be reduced down to 1.4 dB within the 3 dB beam width of the antenna.