Abstract-An electrically small planar passive UHF-RFID tag based on an edge-coupled split ring resonator (EC-SRR) antenna is presented in this work. In order to explore the potentiality and limitations of the SRR antenna and to aid the tag design, an analytical study of the SRR radiation properties at its fundamental resonance is presented for the first time. Radiation resistance, efficiency, polarization, bandwidth and impedance matching with the RFID ASIC are treated in the study. Based on such analysis, the tag design process is presented, and a tag prototype of size 30 mm × 30 mm (λ 0 /11 × λ 0 /11) is designed to operate in the North-American UHF-RFID band (902−928 MHz) and manufactured. The measured read range is in good agreement with the simulation, and reaches 9.3 m at 911 MHz. The tag also features a mitigation of the blind spots, providing a minimum measured read range of 4.2 m.Index Terms -Split ring resonators (SRRs), electrically small antennas (ESAs), radio frequency identification (RFID).

Abstract-The radiation properties of split ring resonators (SRRs) at their second resonance frequency are studied for the first time in this work. In particular, the electric and magnetic dipole moments of the edge-coupled SRR (EC-SRR) are calculated analytically under the assumption of strong coupling between the internal and external rings. Based on these results, the radiation resistance and the radiation efficiency are obtained theoretically. Electromagnetic simulations of the structure reveal that there is very good agreement with the theoretical predictions, pointing out the validity of the proposed analysis. As a proof of concept, an SRR antenna prototype is designed and fabricated. Experimental data are in good agreement with the theoretical and simulated results, and demonstrate the validity of the SRR working at its second resonance frequency as a radiating element.Index Terms-Split ring resonators (SRRs), planar antennas, radiation efficiency, metamaterials.

A novel planar Yagi-Uda antenna is presented in this letter. The proposed antenna uses electrically small resonators as radiating elements that behave as short electric dipoles. Its radiation pattern, gain, front-to-back ratio (FBR) and efficiency are maintained close to that of a Yagi-Uda antenna of stacked half-wavelength dipoles. However, its physical dimensions are considerably reduced. The choice of the resonant particle and its radiation properties, along with the antenna structure, are discussed. Simulated results show a good impedance matching (29 dB) at the working frequency of 5.5 GHz for a 15-elements antenna, as well as a high gain (11.5 dBi). These characteristics are experimentally validated and compared with that of others Yagi-Uda designs from the literature.Index Terms-Electrically small resonators, end-fire array, front-to-back ratio, gain, planar Yagi-Uda antenna.

Abstract:The use of complementary split-ring resonators (CSRRs) as radiating elements in low-profile on-metal UHF-RFID tags is explored in this work. Firstly, the radiation properties of the edge-coupled (EC-CSRR) and the non-bianisotropic (NB-CSRR) versions of the CSRR are studied. The tag design strategy is then discussed in detail. On that basis, a compact (λ0/7 x λ0/7) low-profile (1.27 mm) tag prototype based on the NB-CSRR antenna is designed and manufactured to operate in the North-American UHF-RFID band. The experimental results validate the theoretical and simulated behaviour, and exhibit a maximum read range of 6.8 m.

The definition of a precise illumination region is essential in many applications where the electromagnetic field should be confined in some specific volume. By using conventional structures, it is difficult to achieve an adequate confinement distance (or volume) with negligible levels of radiation leakage beyond it. Although metamaterial structures and metasurfaces are well-known to provide high controllability of their electromagnetic properties, this feature has not yet been applied to solve this problem. We present a method of electromagnetic field confinement based on the generation of evanescent waves by means of metamaterial structures. With this method, the confinement volume can be controlled, namely, it is possible to define a large area with an intense field without radiation leakage. A prototype working in the microwave region has been implemented, and very good agreement between the measurements and the theoretical prediction of field distribution has been obtained.