Dual-band RFID tags based on folded dipole antennas loaded with spiral resonators

The purpose of this paper is to demonstrate the possibility of implementing folded dipole antennas operative at two closely spaced frequencies of interest, for instance, in ultra-high-frequency radio frequency identification (UHF-RFID) applications. The dual-band functionality is achieved by coupling a spiral resonator to the antenna, which operates as an unbalanced transmission line. This modifies the impedance of the antenna and dual-band functionality can be achieved by virtue of the conjugate matching between the antenna and the chip at the frequencies of interest. Two prototype devices have been designed and fabricated: a mono-band folded dipole UHF-RFID tag, and a dual-band UHF-RFID tag based on a spiral resonator coupled to a folded dipole antenna. The measured read ranges at the operating frequencies are in the vicinity of 6 m. The read ranges of the designed tags have been also measured by attaching them to different objects (a CD-ROM box and a plastic ID card). A certain frequency shift is obtained, but this shift can be corrected by merely modifying the distance between the resonator and the antenna.