Microstrip antennas having posts for circular polarization

To use circular microstrip antennas as circularly polarized radiators, perturbations such as slits are added to resolve degeneracy in the fundamental modes. In this paper, posts are inserted between the circular strip and the ground plane and the degeneracy is resolved. The theoretical investigations of this new circularly polarized microstrip antenna are discussed. In the derivation of the theoretical equations, the field is expanded in terms of Green's functions corresponding to the circular cavity region terminated by an admittance boundary. Numerical examples are given for the frequency characteristics of the axial ratio and the input impedance and for the post dimensions to resolve the degeneracy. The radiation patterns and gains are shown for this antenna. The theory has been confirmed by model experiments.