Planar Broadband Annular-Ring Antenna with Circular Polarization for RFID System

Novel circular polarization (CP) design of planar annual-ring antenna with broadband operation for UHF RFID system is proposed and experimentally studied. By introducing the Wilkinson divider into the feeding network and dual Lshaped strips, the proposed CP design can easily be achieved with the impedance bandwidth (RL >= 10 dB) of about 246 MHz (25.0% @ 985 MHz) and the 3 dB axial-ratio (AR) bandwidth of about 180 MHz (19.5 % @ 925 MHz) for UHF RFID applications. The measured peak gain and radiation efficiency are about 7.7 dBic and 60% across the operating band, respectively, with unidirectional pattern in the XZ- and YZ-plane.