On slot-coupled microstrip antennas and their applications to CP operation-theory and experiment

A simple theory based on the cavity model is developed to analyze microstrip antennas excited by a slot in the ground plane. By using an equivalent magnetic current source at the feed, the electric field under the patch is obtained in terms of a set of cavity modes. In particular, the loci of the slot feed location for achieving the circular polarization and the input impedance are computed and found to be in excellent agreement with the experimentally measured results. Simple but surprisingly accurate formulas for slot-fed circularly polarized microstrip antennas are derived and compared with those for probe-fed counterparts