Effects of inter-feed coupling on the axial ratio performance of circular polarized microstrip antennas

Microstrip antennas are widely used in various applications from L-Band to EHF. Many of these applications, such as mobile satcom, require circular polarized (CP) antennas with wideband performance. The simplest designs for CP microstrip antennas involve a single point feed and a slight perturbation in the antenna structure. This results in a CP antenna with a narrow bandwidth and an average axial ratio (AR) performance. Wider bandwidth can be achieved by using a thicker and lower permittivity substrate with air being the best. Better AR can be obtained by using two point feeds where they are located spatially 90 degrees apart and are excited in phase quadrature with equal amplitude. For these thicker substrate antennas, however, there may be significant amount of coupling between the two feeds which will degrade the AR performance of the CP antennas. This paper will present the investigation of the effects of inter-feed coupling on the AR performance and will examine methods for minimizing the coupling for the optimum CP operation.