Circular polarisation from dual-layer square-ring microstrip antennas

Stacked microstrip square-ring antennas are investigated for circular polarisation (CP). Enhancement of axial ratio bandwidth is obtained using stacked square rings and applying corner perturbations to both parasitic and driven rings. Both negative and positive corner perturbations are used and wide axial ratio bandwidths are demonstrated. A novel approach is presented, where a combination of positive and negative perturbations is adopted in the driven and parasitic rings, respectively, which resembles a sequential rotation and offers broad axial ratio bandwidth, along with a wide axial ratio beamwidth. The effects of finite ground plane on the CP performance of this antenna are studied, and methods are presented to overcome the problems associated with the degradation of the CP performance because of the finite ground plane. The effect of positioning the antenna asymmetrically on the ground plane is also studied, which shows that the ground plane symmetry is important for the CP antenna. The measured 3 dB axial ratio bandwidth of approximately 5.15 is achieved for dual-layer square-ring antennas. The measured right-hand CP (RHCP) gain is in excess of 7 dBic for a wide frequency range. The simulation results are in good agreement with the measured data.