Chipless RFID transponder design (2017)

Radio Frequency Identification (RFID) is a wireless data transmission and reception technology for automatic identification, asset tracking and security surveillance. RFID technology has gained momentum in market penetration, with both the US Department of Defense and retail giant Wal-Mart introducing mandatory tagging of goods in their supply chain. However, for large-scale RFID implementation, the cost of the systems cannot compete with optical barcodes. The main hindrance is the cost of the microchip used in RFID. An RFID tag that can compete with the barcode will need to be fully printable on plastic or paper. To date, only a few fully printable chipless RFID technologies have been reported in the open literature. The reported chipless RFID tags have a limited number of bits. To compete with the optical barcode, the chipless tag needs to encode 64 to 128-bits. The present thesis has addressed the problem and conceptualized fully printable multi-bit chipless tags. In the present thesis, three novel designs of multi-bit chipless RFID tags have been presented. The tags operate over the ultra wideband (UWB) frequency domain with more than 100% bandwidth. The first prototype is a Pythagorean tree (PT) fractal resonator based chipless RFID tag. A UWB monopole is loaded with the PT fractal resonator to generate multiple frequency signatures in the frequency spectrum. To achieve 64-bits from the PT chipless tag, eight PT fractal resonators with different frequency signatures are connected in parallel with a 1-to-8-way power divider and the input port of the power divider is connected to a UWB monopole antenna. The Radar Cross Section (RCS) of the monopole antenna shows distinct nulls in the respective resonant frequencies of the PT fractal resonators. Therefore, the resonant nulls corresponds 1:1 to data bits for the identification tag. The second prototype is the fine slot loaded UWB monopole antenna. The antenna resonates at many narrow resonant bins and they are resolvable in the return loss vs. frequency plot of the antenna. A single such antenna is encoded up to 32-bits. Two such orthogonally polarized antennas are connected together via microstrip transmission lines that include a Real Frequency Technique (RFT) designed broadband matching section. Thus a 64-bit chipless tag is obtained. Another set of two orthogonally polarized antennas on the opposite polarization of the former antenna yields the ability to encode 128-bits of data. Thus a 128-bit chipless RFID tag concept is proved. The tags were tested with input return loss, transmission amplitude and phases and finally the RCS. Distinct nulls are present in the RCS vs. frequency plots. The third and final prototype is a phase encoded square patch antenna reflector loaded with open circuited reactive stub elements in two orthogonal planes of the patch antenna. The relative phase difference generated between the two degenerate modes at the same frequency due to the variation of the stub length is used as the encoded data. In this final design six 2x2-sub-arrays of the patch antennas are designed to encode many data bits in six different frequency points. The RCS measurement shows distinct phase shifts in the RCS vs. frequency plot due to the variation of the stub length of the patch. Moreover, due to the array geometry instead of the single element design, the shift in phase and increase in RCS amplitude has increased four-fold. This ensures that low power transmission is able to detect the tag. All the designed prototypes have the potential to replace and/or co-exist with optical barcodes for low cost item tagging.