On the circular polarization operation of annular-ring microstrip antennas

Circular polarization (CP) operation of an annular-ring microstrip antenna with a pair of inserted slits at its fundamental TM11 mode is proposed and experimentally investigated. Two examples of the proposed antenna fed using a microstrip line at the inner and outer patch boundaries are studied. Results show that the proposed antenna can have good CP performance and, moreover, the required antenna size at a fixed CP operation can be much less than that of a conventional circularly polarized circular microstrip antenna operated at the TM11 mode, Details of the proposed antenna for CP operation are described and experimental results of the CP performance are presented and discussed.