Circularly Polarized Cross-Dipole Antenna with a Double Layer AMC Backing for UHF RFID Readers

This paper presents a circularly polarized ultrahigh frequency (UHF) antenna for radio frequency identification (RFID) readers. The proposed antenna is a cross-dipole configuration consisting of two dipole pairs printed on the top and bottom of the substrate. Each dipole pair is made up of arrow-shaped elements that are arranged diagonally and are linked by delay lines to produce circular polarization. An artificial magnetic conductor (AMC) is placed at a distance of /15 below the antenna to improve its performance, where λ is the free space wavelength at 900 MHz. The AMC-backed antenna has an impedance bandwidth of 850-950 MHz and an axial ratio bandwidth of 890-910 MHz. The peak gain of the antenna increases from -1.31 dB (without AMC) to 4.6 dB (with AMC) at 900 MHz.