Chipless RFID tags based on metamaterial concepts

Tags for chipless radiofrequency identification (chipless-RFID) based on chains of identical S-shaped split ring resonators (S-SRRs) are presented in this paper. The tag code is implemented by the presence or absence of resonant elements (S-SRRs) at predefined and equidistant positions. Tag reading is carried out by means of near-field coupling, by transversally displacing the chain of S-SRRs (tag) above a coplanar waveguide (CPW) transmission line. By injecting a carrier signal tuned to the frequency of the S-SRR, the amplitude of such signal is modulated by the presence/absence of S-SRRs in the code. Therefore, the tag ID code can be inferred from the envelope of the output signal. Through this sequential bit reading, the number of bits is only limited by the area occupied by the tag. Such tags are useful in applications where reading distance can be sacrificed in favor of data capacity (e.g. security and authentication of paper documents).