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

In this letter, a broadband metal-tolerant tag antenna for ultrahigh 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 are 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 MHz to 939 MHz). When mounted on a 200 mm × 200 mm 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 US UHF band (902 MHz to 928 MHz). Also, the antenna mounted on a 300 mm × 300 mm metal plate is measured and demonstrated.