Multibit Chipless RFID Tags Based on the Transition Among Closed- and Open-Loop Resonators

In this article, a novel and simple design procedure for multibit chipless radio frequency identification (RFID) tags is introduced, which is based on the characteristics of dual-mode loop resonator (DMLR) and open-loop resonator (OLR). By removing a small patch element from the symmetry axis of the DMLR, a simple geometrical transition to an OLR to obtain different frequency codes can be achieved. Since the DMLR and OLR provide dual- and single-resonant frequencies, three different frequency codes can be observed using one resonator. Thus, 3 $<^>{N}$ codes can be performed by coupling N resonators to input/output (I/O) ports. Even-odd mode analysis over the coupling mechanism of a single resonator is provided to characterize the frequency responses of the proposed resonators. To prove the proposed approach, two multiresonator circuits having four resonators were fabricated and tested in a good agreement. In the test environment, two horn antennas and a vector network analyzer have been utilized as transmitter and reader. Next, in order to achieve size reduction, meandered resonators are utilized instead of conventional DMLR and OLR. Based on these resonators, two RFID tags, including wideband monopole antennas integrated into the seven-resonator circuits on the same board, were also fabricated and tested.