A new approach for enhancement circular polarization output in square shaped microstrip patch antennas

Circular polarization (CP) is often desirable in WLAN applications for overcoming the multipath fading problem and enhancing the system performance. For practical applications in WLAN, CP bandwidth should exceed 75 MHz (3%). It is well known that the CP output of square shaped patch antennas is related to dual-mode operation of the square resonator. The problem is to obtain two orthogonal near-degenerate modes of the same magnitude in phase quadrature. We propose a new concept for achieving the CP output of square patch antennas. Previously, we have shown that the corner cut perturbation is not sufficient for providing CP and that traditional consideration of mode splitting is not always applicable to square resonator structures (Semouchkina, E. et al., IEEE AP-S Digest Proc. Int. Symp., vol.3, p.772-5, 2003). Additional data led us to conclude that the perturbation of diagonal symmetry does not shift the two orthogonal modes in opposite directions with respect to the center frequency. Instead, it causes both modes to split into two modes which differ in phase. The paper demonstrates how this understanding could be used to design a square patch antenna with an enhanced CP bandwidth.