| A microstrip antenna operating at the fundamental TM 11 mode generates a broadside radiation pattern. As the higher order modes are excited, boresight-nulled patterns are formed and the beam peak direction moves toward the endfire, as the mode order increases. The phase centre of the microstrip antenna, when it is excited at a single mode, is located at the center of the patch. But, when two or more modes are excited, the phase centre is no longer located at the center of the patch and phase loci may exist instead of a single point.;In this thesis, the phase centre of multi-mode circular microstrip antennas is studied. To guarantee exact mode excitations, stacked patch configurations are considered and three cases are investigated, stacked TM11, and TM21 modes, TM11 and TM02 modes, and TM11, TM21 and TM02 modes. In each case, the investigation is carried out for the phase centre, main beam tilt angle, radiation patterns and copolar and crosspolarization in terms of different mode amplitude and phase excitations. All three cases are treated both analytically and numerically. The case of special interest is when the beam is at broadside, generated by multiple modes, and the phase centre moves away from the patch center. This is equivalent to virtual antennas and has applications for remote sensing, and wireless communications. Dependence of the phase centre location on the mode excitation is investigated. Effects of the mode excitations on the radiation patterns, especially the crosspolarzation are also studied. |
