Small RFID tag antenna for metallic surface

This paper presented an RFID tag antenna which can be mounted on metallic surfaces in UHF band. The proposed antenna has a proximity-coupled feeding structure composed of a microstrip line shorted to the ground plane. We can almost independently control the real and imaginary part of the antenna impedance using the proposed feeding structure. Therefore, we can directly match the impedance of the antenna to that of the tag chip. Moreover, the antenna has an impedance bandwidth of 13 MHz for S11 less than 3 dB, radiation efficiency of 56% at the center frequency of 911 MHz, and a read range from 5 m to 6 m on a metallic surface.