Wideband RFID Tag Antenna for Metallic Surfaces Using Proximity-Coupled Feed

A low-cost, wideband patch antenna for radio frequency identification tags mountable on metallic surfaces is presented. A novel proximity-coupled feed structure, in which the direction of the microstrip feed line is parallel to that of the resonant length of the radiating patch and one end of the feed line is shorted to the ground, is proposed to perform simple and wideband impedance matching between the antenna and tag chip without any additional matching networks. The proposed design is verified by simulation and measurement. The measured 3-dB return-loss bandwidth is about 49 MHz, which totally covers the 26-MHz bandwidth requirement in North America. The measured read range is about 8.9 m when the tag antenna is mounted on a metal plate with a size of 200 × 200 mm2.