Broadband Circularly Polarized Loop Antenna Based on High-pass and Low-pass Filters for Handheld RFID Reader Applications

Recent researches on portable circularly polarized (CP) Radio Frequency Identification (RFID) reader antennas have 3-dB axial ratio bandwidth (ARBW) lower than 80 MHz. In order to cover all the Ultrahigh Frequency (UHF) RFID bands simultaneously, this paper present a novel broadband handheld RFID CP reader antenna. The proposed antenna consists of a square-loop radiating element with 2 feed ports and a broadband feeding network in the same plane, measuring 95 mm x 95 mm x 1.6 mm. The feeding network, which realizes broadband ARBW, is composed of a Wilkinson power divider, a high-pass filter (HPF) and a low-pass filter (LPF), connected to the radiating element, producing broadband right-hand circular polarization (RHCP) in z direction and left-hand circular polarization (LHCP) in -z direction. The measured broad impedance bandwidth (S-11 < -10 dB) is from 710 MHz to 1150 MHz. The measured 3-dB ARBW of the proposed antenna is from 800MHz to 940 MHz. The peak gain in the maximum radiation direction is 1.3 dBic at 910 MHz.