Reduction of back radiation and cross-coupling in dual polarized aperture coupled patch antennas

In mobile phone systems the most commonly used way to combat multipath fading on the uplink is to use space diversity at the base station. The disadvantage with space diversity is that the antenna systems become large as the antennas have to be separated by several wavelengths to receive independent signals. Polarization diversity for base station antennas has become of interest due to the need to reduce the number of antennas and the size of antenna systems in downtown areas. In a polarization diversity system two antennas with orthogonal polarizations are used to provide independently fading signals. The two orthogonal polarizations are normally horizontal/vertical or /spl plusmn/45/spl deg/ slanted. It was shown in Wahlberg et al. that a /spl plusmn/45/spl deg/ slanted polarization system has better performance than a horizontal/vertical polarization system. As the antennas do not have to be separated by space, the two antennas are normally combined in one. However, these polarization diversity systems put high requirements on the antenna performance such as high isolation between the channels and high cross-polar discrimination. In this paper we discuss the problems with resonances and undesired coupling between the polarizations of a dual polarized slot coupled patch antenna and some solutions to these problems. Results for a 4 element array for the GSM band (870-960 MHz) are presented.