Radio-frequency identification (RFID) is a wireless data capturing technique that utilizes radio frequency (RF) waves for automatic identification of objects. RFID relies on RF waves for data transmission between the data carrying device, called the RFID tag, and the interrogator.

A fully passive printable chipless RFID system is presented. The chipless tag uses the amplitude and phase of the spectral signature of a multiresonator circuit and provides 1:1 correspondence of data bits. The tag comprises of a microstrip spiral multiresonator and cross-polarized transmitting and receiving microstrip ultra-wideband disc loaded monopole antennas. The reader antenna is a log periodic dipole antenna with average 5.5-dBi gain. Firstly, a 6-bit chipless tag is designed to encode 000000 and 010101 IDs. Finally, a 35-bit chipless tag based on the same principle is presented. The tag has potentials for low-cost item tagging such as banknotes and secured documents.

A low-power consumption, small-size smart antenna, named electronically steerable parasitic array radiator (ESPAR), has been designed. Beamforming is achieved by tuning the load reactances at parasitic elements surrounding the active central element. A fast beamforming algorithm based on simultaneous perturbation stochastic approximation with a maximum cross correlation coefficient criterion is proposed. The simulation and experimental results validate the algorithm. In an environment where the signal-to-interference-ratio is 0 dB, the algorithm converges within 50 iterations and achieves an output signal-to-interference-plus-noise-ratio of 10 dB. With the fast beamforming ability and its low-power consumption attribute, the ESPAR antenna makes the mass deployment of smart antenna technologies practical.

A novel low-cost chipless radio frequency identification tag sensor is presented. The tag sensor provides identification data as well as monitors relative humidity (RH) of tagged objects. The tag sensor is made of passive microwave circuit that uses humidity sensitive polymer material for RH sensing. The aim of this paper is to investigate RF sensing properties of moisture absorbing polymer polyvinyl alcohol at microwave frequency. In addition, frequency shifting technique is used to encode data bits for high data capacity. The overall size of the proposed tag sensor is 15 mm × 6.8 mm and has 6 bit data capacity for ID generation and single bit for humidity sensing. Results presented here show ~ 607 MHz frequency deviation for 50% RH increase. The tag sensor has potential to be printed on flexible laminates such as plastic and paper for ultra-low cost item level tagging and ubiquitous sensing.

A novel compact design of an ultralow-cost fully printable slot-loaded dual-polarized chipless radio frequency identication tag is presented with four near- and far-field reading techniques. The tag consists of four rectangular metallic patches loaded with multiple slot resonators. Slots with the same polarization for adjacent frequencies are placed alternately into two patches to reduce the mutual coupling between the slots. Then two similar sets are placed in horizontal and vertical polarizations to double the number of bits within the same frequency bandwidth. The tag can be detected using dual-polarized waveguide(s) or dual-polarized antennas. This single-sided compact chipless tag has higher data capacity and lower cost compared with the existing printable chipless tags and can be used in personal ID or credit cards and banknotes and can be directly printed on paper or plastic packets for item-level tagging.

A novel 3 b chipless RFID transponder is presented. The transponder comprises three microstrip patch antennas, which are loaded with open circuited (O/C) high impedance stubs. The antennas are resonant at nearby frequencies, and when excited with their respective resonant frequency signals, they re-radiate backscattered signals with distinct phase characteristics. This phase information is encoded as hexadecimal bits for the proposed chipless RFID tag. Both the CST Microwave Studio full-wave solver and the anechoic chamber measurements show a 30deg phase step for the 3 b chipless RFID transponder. It it easily transferred to printed technology, and it may find applications in the mass identification market.