Planar, single-feed, circularly polarized microstrip antenna with enhanced bandwidth

The most common circularly polarized (CP) microstrip antennas are made by using a splitting network that feeds the two adjacent sides of a square patch with signals of equal magnitude and 90/spl deg/ phase difference. This splitting network becomes cumbersome especially in large arrays. Single-feed CP microstrip antennas have been investigated by a number of authors eliminating the need for a complex splitting network. Unfortunately these antennas have very narrow CP bandwidth. In the proposed antenna, a double layer microstrip is used. The two orthogonal modes occupy different cavities and the 90/spl deg/ phase difference between the modes is achieved by coupling holes between the two resonant cavities. The coupling hole concept was first introduced by Bethe (1944), and further refined by Collins (1966). In an earlier paper by Lee et al. (1996) each cavity had two side walls in order to suppress any unwanted modes. Unfortunately, side walls make fabrication difficult, especially in array structures. In this paper the side walls are eliminated and the CP bandwidth is substantially increased. In addition, the CP axial ratio does not degrade near the horizon.