A review of technological solutions for RFID sensing and their current or envisioned applications is presented. The fundamentals of the wireless sensing technology are summarized in the first part of the work, and the benefits of adopting RFID sensors for replacing standard sensor-equipped Wi-Fi nodes are discussed. Emphasis is put on the absence of batteries and the lower cost of RFID sensors with respect to other sensor solutions available on the market. RFID sensors are critically compared by separating them into chipped and chipless configurations. Both categories are further analyzed with reference to their working mechanism (electronic, electromagnetic, and acoustic). RFID sensing through chip-equipped tags is now a mature technological solution, which is continuously increasing its presence on the market and in several applicative scenarios. On the other hand, chipless RFID sensing represents a relatively new concept, which could become a disruptive solution in the market, but further research in this field is necessary for customizing its employment in specific scenarios. The benefits and limitations of several tag configurations are shown and discussed. A summary of the most suitable applicative scenarios for RFID sensors are finally illustrated. Finally, a look at some sensing solutions available on the market are described and compared.

A novel chipless RFID humidity sensor based on a finite artificial impedance surface (AIS) is presented. The unit cell of the AIS is composed of three concentric loops thus obtaining three deep and high-Q nulls in the electromagnetic response of the tag. The wireless sensor is fabricated using the low-cost inkjet printing technology on a thin sheet of commercial coated paper. The patterned surface is placed on a metal-backed cardboard layer. The relative humidity information is encoded in the frequency shift of the resonance peaks. Varying the relative humidity level from 50% to 90%, the frequency shift has proved to be up to 270 MHz. The position of the resonance peaks has been correlated to the relative humidity level of the environment on the basis of a high number of measurements performed in a climatic chamber, specifically designed for RF measurements of the sensor. A very low error probability of the proposed sensor is demonstrated when the device is used with a 10% relative humidity level discrimination.

An overview of transmission/reflection-based methods for the electromagnetic characterisation of materials is presented. The paper initially describes the most popular approaches for the characterisation of bulk materials in terms of dielectric permittivity and magnetic permeability. Subsequently, the limitations and the methods aimed at removing the ambiguities deriving from the application of the classical Nicolson–Ross–Weir direct inversion are discussed. The second part of the paper is focused on the characterisation of partially conductive thin sheets in terms of surface impedance via waveguide setups. All the presented measurement techniques are applicable to conventional transmission reflection devices such as coaxial cables or waveguides.

An equivalent circuit model for Frequency Selective Surfaces (FSS) comprising anisotropic elements is presented. The periodic surface is initially simulated with an arbitrary azimuthal incidence angle and its surface impedance matrix is derived. The impedance matrix is subsequently rotated by an angle φrot on the crystal axes χ1, χ2 thus nullifying its extra diagonal terms. The rotation angle φrot is derived according to the spectral theorem by using the terms of the matrix initially extracted. The diagonal terms of the rotated matrix, that is, the impedances Zχ1 and Zχ2, are finally matched with simple LC networks. The circuit model representation of the anisotropic element can be used to analyse anisotropic FSSs rotated by a generic azimuth angle. The methodology provides a compact description of generic FSS elements with only five parameters: the lumped parameters of the LC network Lχ1, Cχ1, Lχ2, Cχ2 and the rotation angle φrot. The circuit model can take into account the presence of dielectric substrates close to the FSS or a variation of the FSS periodicity without additional computational efforts. The equivalent circuit model is finally applied to the design of two transmitting polarization converts based on anisotropic metasurfaces.

A novel normalization-free reading scheme to recover data encoded in chipless RFID tags is presented. The proposed approach exploits the circular polarization to isolate the field scattered by the tag from that of surrounding objects. The desired polarization mismatch is achieved by exploiting the properties of high-impedance surfaces that can suitably manipulate the reflected field. A thorough analysis of the interaction between the circularly polarized interrogation and the field scattered by the chipless RFID tag as well as the hosting platform is provided. The proposed method does not require any a-priori calibration if a reading antenna with a suitable axial ratio is available. More importantly, the CP-based reading approach guarantees the correct tag detection regardless of the tag orientation. The case of the tag mounted on metallic platform is also analyzed and it is demonstrated that the circular polarization reading procedure is robust under the hypothesis of a proper axial ratio of the probing antennas. The minimum requirements of axial ratio are derived by using a theoretical model. Measurements confirmed the speculations obtained through the theoretical analysis both in the case of low-scattering items as well as for metallic platforms.

An ultra-wideband linear polarization converter based on a reflecting metasurface is presented. The polarizer is composed by a periodic arrangement of miniaturized metallic elements printed on a grounded dielectric substrate. In order to achieve broadband polarization converting properties, the metasurface is optimized by employing a genetic algorithm (GA) which imposes the minimization of the amplitude of the co-polar reflection coefficient over a wide frequency band. The enhanced angular stability of the polarization converter is due to the miniaturized unit cell which is obtained by imposing the maximum periodicity of the metasurface in the GA optimization process. The pixelated polarization converter obtained by the GA exhibits a relative bandwidth of 102% working from 8.12 GHz to 25.16 GHz. The analysis of the surface current distribution of the metasurface led to a methodology for refining the optimized GA solution based on the sequential removal of pixels of the unit cell on which surface currents are not excited. The relative bandwidth of the refined polarizer is extended up to 117.8% with a unit cell periodicity of 0.46 mm, corresponding to λ/20 at the maximum operating frequency. The performance of the proposed ultra-wideband polarization metasurface has been confirmed through full-wave simulations and measurements.