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 L-shaped 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.