Dual-Band Circularly Polarized RFID Reader Antenna With Combined Dipole and Monopoles

This article presents a new approach to implement shared-aperture dual-band dual-sense circularly polarized (CP) antenna array for dual-band (0.92 and 2.45 GHz) radio frequency identification (RFID) reader applications. In this approach, each antenna element is obtained by combining one asymmetrical half-wavelength dipole and two monopoles. The asymmetrical half-wavelength dipole operates at low frequency (LF) (0.92 GHz) with right-hand CP (RHCP) radiation. For the left-hand CP (LHCP) radiation at high frequency (HF) (2.45 GHz), the asymmetrical quarter-wavelength monopole works with fundamental mode and the higher order mode plays a major role in three-quarter wavelength monopole. An optimized
$2\times2$
array is designed to further extend the 3-dB axial ratio (AR) bandwidth and enhance the realized gain by using a simple 1-to-4 sequential-phase feeding network. An artificial magnetic conductor (AMC) with
$9\times9$
unit cells is adopted to reduce the profile height of the antenna array. A prototype of the final optimized antenna array is fabricated and measured to validate this new approach. Experimental results show that good CP characteristics are obtained in 0.92-/2.45-GHz frequency bands, which agree with the simulation results well. Due to its excellent dual-band CP performance, this antenna can be well used in some multifrequency applications.