The characteristics of circular-polarization (CP) square-ring microstrip antennas loaded with superstrate are studied in this paper. Firstly, several CP antenna prototypes with various substrate thicknesses and inner-slot sizes are designed, and they are excited by a coupling-strip feed in order to overcome the high-impedance problem of the square-ring microstrip antenna. Then, superstrates with different thicknesses and permittivities are attached to the antenna prototypes in order to evaluate their effects on the CP performances. The experimental results show that the effects are more obvious for the square-ring microstrip antenna with a larger inner-slot size.
Microwave and Optical Technology Letters, 47(6): 567-570