Bidirectional Radiated Circularly Polarized Annular-Ring Slot Antenna for Portable RFID Reader

A novel single-layer, microstrip-to-slotline transition technique for coupling a circularly polarized annular-ring slot antenna in the UHF band is presented. In the proposed design, circular polarization is generated using a proper asymmetry within the annular-ring slot structure and feeding the annular-ring slot using a slotline feed at 135 degrees from the asymmetry. The direct slotline feed is used which is coupled to a 50-Omega microstrip line on the opposite side of the substrate. The asymmetry in the proposed design is a meandered-slot section and the proposed CP design is achieved by the two orthogonal linear modes of the annular-ring slot. Simulated and measured results indicate that the proposed structure can achieve good CP radiation performances, and an impedance bandwidth and a 3-dB axial-ratio bandwidth obtained for the design are about 17.6% and 3.72%, respectively.