Compact and Circular Polarization UHF/UWB RFID Reader Antenna

A circular polarized, dual-band radio frequency identification (RFID) reader antenna is designed to simultaneously operate at ultra-high frequency (UHF) and ultra-wide band (UWB) bands for positioning systems. The phase shifters are loaded on the Wilkinson power divider, so that the current phase difference between each two ports of the four inverted F antenna is 90 degrees, which can achieve circular polarization on the UHF band. The circularly polarized electromagnetic dipole antenna of UWB band is composed of a pair of patches, which are two rotary symmetric shapes of the same shape as electric dipoles, and two vertical patches with the metal ground as equivalent magnetic dipoles. With a rotary symmetric structure, the design can achieve a wide 3 dB axial ratio bandwidth. The overall size of the antenna is 200 mmx200 mmx20 mm. The simulation results show that the impedance bandwidth of the UHF antenna ranges from 0.68 to 1.4 GHz and 3 dB axial-ratio bandwidth is 44.1% (0.76-1.19 GHz). The impedance bandwidth of the ultra-wide band antenna is 59.5% (3.32 - 6.13 GHz) and it covers a wide axial ratio bandwidth (AR <= 3 dB) of 71.3% (3.16 - 6.66 GHz) with a peak antenna gain of 9.6 dBi. The mutual coupling between the two ports is less than -35 dB over the two bands.