A Broadband RFID Tag Antenna for Metal Surface with Characteristic Mode Analysis

In this letter, a broadband metal-tolerant tag antenna for ultra-high frequency (UHF) radio frequency identification (RFID) systems is presented. In the antenna design, three coupling patches are loaded on the top and both sides of the feeding patch separated by long coupling gaps. Two characteristic modes of the tag antenna are identified by employing characteristic mode analysis (CMA) to achieve broadband performance. Furthermore, an equivalent circuit model (ECM) for the antenna is established. The measured results show that its operating bandwidth is 51 MHz (888-939 MHz). When mounted on a 200 × 200 mm2 metal plate, the tag antenna achieves a maximum reading distance of approximately 10.4 m at 915 MHz, and maintains a reading distance of at least 7.1 m throughout the entire U.S. UHF band (902-928 MHz). Also, the antenna mounted on a 300 × 300 mm2 metal plate is measured and demonstrated.