Chipless RFID tags and a tag system are provided, wherein the stepped impedance higher-order mode resonator includes two sets of composite transmission lines each set having an equal line length and including a plurality of transmission lines each having a plurality of specific levels to which the transmission line characteristic impedance is assigned, the two sets of composite transmission lines each connected in series, the sets being connected at the center to form an electrically symmetrical configuration and wherein the chipless RFID tags and the tag system allow each of the codes to be assigned to each of the structures of the stepped impedance higher-order mode resonator, and allow each of those codes to be identified by detecting each of the combinations of the higher-order mode resonance frequencies that may be produced from each the structures of the resonator that corresponds to each of the codes that have been assigned.