Investigation of the bandwidth of resonators for frequency-coded chipless radio-frequency identification tags

Conventional radio-frequency identification (RFID) tags consist of a chip and a printed antenna. As the chip set is manufactured via the fabrication technology of application-specific integrated circuit, the cost of the chip is much higher than that of the antenna. Furthermore, soldering the chip to the antenna requires an additional procedure, so the main cost of an RFID tag comes from the chip. To eliminate this main cost, 'chipless RFID' removes the use of a tag chip. Chipless RFID tags encode data by using several electromagnetic resonators, which can be directly printed on paper or plastic films; thus, the cost of chipless tags is reduced to a level as low as barcodes. Nevertheless, in order to increase the capacity of chipless RFID, the resonators are required to depict a narrow bandwidth. Although several resonators have been proposed for this application, the design that leads to the narrowest bandwidth is still inconclusive. In this study, we investigate the bandwidth of twenty-four resonators that have been employed in chipless RFID tags. It is observed that the quarter-wavelength slot, the circular slot and the Hilbert-curve resonators provide a narrowband feature. These resonators have a potential to enable the chipless tag to enhance capacity.