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.

I. Introduction

The tag antenna is one of the key components in the RFID system, and it is commonly designed in the commercial domain as a dipole type with linear polarization (LP) in the UHF-band [1]. Thus, to mitigate multipath effects and polarization mismatch loss, the RFID reader antenna is usually designed with CP polarization. The most difficult part when designing an RFID tag antenna is to consider the input impedance of the tag chip. This is due to the fact that the input impedance of the tag chip is low in resistance value and high in capacitive reactance. Hence, to achieve maximum power transmission, the input impedance of the RFID tag antenna must be conjugate matched to the tag chip with similarly low resistance but high in inductive reactance. Notably, it is not simple to identify a feeding position that can satisfy both the required resistance and reactance values from the patch antenna, while allowing a CP radiation to excite.