Planar Broadband Circularly Polarized Antenna With Square Slot for UHF RFID Reader

Novel circular polarization (CP) design of planar broadband antenna with square slot for UHF RFID system is proposed and experimentally studied. By insetting the arc-shaped strip into the square slot, the proposed CP design can easily be achieved with the impedance bandwidth (RL >= 10 dB) of about 142 MHz (15.3% @ 931 MHz) and the 3 dB axial-ratio (AR) bandwidth of about 166 MHz (17.7% @ 940 MHz) for UHF RFID applications. The measured peak gain and radiation efficiency are about 6.8 dBic and 98% across the operating band, respectively, with nearly bidirectional pattern in the XZ- and YZ-plane.