Dual-Band Impedance-Matching Networks Based on Split-Ring Resonators for Applications in RF Identification (RFID)

This paper is focused on the design of dual-band impedance-matching networks of interest in RF identification (RFID) systems. By cascading an impedance-matching network between the chip and antenna, the performance of the RFID tags can be improved. The main aim of this study is to demonstrate the possibility of designing such networks by means of split-ring resonators coupled to microstrip transmission lines. These resonators are especially useful in this design since their equivalent circuit substantially simplifies the parameter calculation of the matching network. Dual-band conjugate matching at two different frequencies, $f_{1} = 867~{\hbox{MHz}}$ and $f_{2} = 915~{\hbox{MHz}}$, corresponding to the assigned bands for UHF RFID in Europe and the U.S., respectively, is demonstrated. The main difficulty for the synthesis of these dual-band matching networks relies on the proximity of $f_{1}$ and $f_{2}$ . Although the chip impedance cannot be considered a design parameter, the network design can be alleviated by allowing certain flexibility in the antenna stage. The fabricated prototype, a dual-band impedance-matching network based on split-ring resonators and loaded with a slot antenna, was characterized by measuring its reflection coefficient. The results reveal that conjugate matching at the above-cited frequencies for the chip impedance under consideration is achieved.