Circularly Polarized RFID Tag Antenna Design for Underground Localization System

A circularly polarized antenna is proposed in this paper for a Radio frequency identification tag to be used in the underground localization system. The tag supports wideband performance around 915 MHz and the fully covered metal ground makes the tag less affected by the medium it will be attached to. An extended metal plate is also discussed to improve the tag read range and to make it easier to attach on the uneven wall in underground tunnels. With the extended metal plate, the tag −10 dB reflection-coefficient bandwidth is 37 MHz (890–926 MHz) and the 3 dB AR bandwidth is 6.4 MHz (911.4-917.8 MHz). This design provides good circular polarization characteristics and a read range of 2.2 m.