Circularly Polarized Loop Tag Antenna for Long Reading Range RFID Applications

A novel circularly polarized (CP) loop antenna design for a UHF band (902-928 MHz) radio frequency identification (RFID) tag is proposed to overcome the polarization mismatch problem between reader antenna and tag antenna. The structure of the proposed tag antenna is a simple square-loop type loaded with an open gap, two feeding strips, and a matching stub. The open gap and two feeding strips were used to achieve good CP radiation. Desirable impedance matching between the tag antenna and RFID chip was attained by tuning the matching stub. Besides achieving satisfactory 3-dB axial-ratio and 10-dB impedance bandwidths of approximately 5.5%, good antenna gain and reading range of more than 2.8 dBic and 16.3 m, respectively, were also demonstrated.