Circularly polarized RFID tag for metal surface mount

A passive UHF band RFID tag antenna design is presented here. To allow circular polarization (CP) radiation, compact size, and metal surface mount characteristics, the radiating square patch is loaded with two pairs of unequal slits, while a quarter-wavelength microstrip line connected to a tag-chip and terminated by a short-circuited stub is used to excite the radiating patch. The 10 dB impedance bandwidth of the tag antenna is measured to be 31 MHz (896-927 MHz), while its 3 dB CP bandwidth is 6 MHz (911-917 MHz). Further experiment has demonstrated that the proposed tag antenna can provide better reading distance when mounted on a metallic surface as compared with the free-space condition.