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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.