Bidirectional radiated circularly polarised square-ring antenna for portable RFID reader

A novel single-layer, dual-fed technique for a bidirectional radiated circularly polarised square-ring antenna operated in the UHF band is presented. The two feed ports of the square-ring radiating element are placed in orthogonal directions and connected to a feeding network with a Wilkinson power divider in the same plane. Properly adjusting the size of the capacitive coupling groundplane results in good impedance matching and circularly polarised radiation, and a broad impedance bandwidth ( S 11 les -10 dB) of about 45.2% and a 3 dB axial-ratio bandwidth of about 8.7% were obtained.