Experimental study of circularly polarised microstrip antennas loaded with superstrate

The loading effects of the superstrate with various thickness on the circular polarisation (CP) performances of a cross-slot-coupled microstrip antenna are first investigated. Then, for improving the shifted centre frequency, the bad impedance matching, and the increased axial ratio, which are due to the superstrate effects, the key dimensions of the loaded antenna are adjusted to regain or even enhance the original unloaded CP performances. From the obtained experimental results, it is found that the CP bandwidth of the loaded antenna can be increased when the thickness of the superstrate is properly selected.