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.

The advantages of adopting circular polarization in multiple-input-multiple-output (MIMO) systems are illustrated for both line-of-sight (LOS) and multipath propagation. More in detail, an analysis of the MIMO performance attainable by employing orthogonal circularly polarized (CP) radiators with respect to orthogonal linearly polarized (LP) ones, has been addressed. At first, an accurate analysis is presented aimed at the evaluation of the channel matrix by comprehensively including also the effects of the antenna in LOS condition. In particular, the channel matrix has been calculated as a function of the antenna parameters and orientation, demonstrating that CP radiators are capable of obtaining better average values of the matrix eigenvalues with respect to LP ones. The analysis is therefore completed by evaluating the characteristics of a CP MIMO system operating in indoor environment representing this latter a more challenging condition where multipath propagation occurs. In this latter case, some meaningful numerical experiments have been performed by using a reliable ray-tracing solver, followed by a measurements campaign conducted in a real environment for validation purposes. Measurements, which are in good agreement with simulations, confirm the benefits of adopting circular polarization in MIMO systems with respect to LP.

A review of technological solutions and advances in the framework of a Vertical Heterogeneous Network (VHetNet) integrating satellite, airborne and terrestrial networks is presented. The disruptive features and challenges offered by a fruitful cooperation among these segments within a ubiquitous and seamless wireless connectivity are described. The available technologies and the key research directions for achieving global wireless coverage by considering all these layers are thoroughly discussed. Emphasis is placed on the available antenna systems in satellite, airborne and ground layers by highlighting strengths and weakness and by providing some interesting trends in research. A summary of the most suitable applicative scenarios for future 6G wireless communications are finally illustrated.

A novel low-cost null-steering antenna operating at 2.45 GHz is designed by exploiting an asymmetric excitation of two characteristic modes. This compact antenna allows a null shift of the radiation pattern by changing the relative phase between the two capacitive exciters (CCEs) located above the conductive plane. As a proof of concept, a prototype of the radiator has been realized by resorting to a discrete phase shifter in order to scan the null. The measured radiation patterns prove that the proposed design strategy is reliable and applicable to other kinds of structures. The manufactured antenna is capable of steering the null within a beam of 64° centered at broadside direction (θ = 0°), with depth nulls greater than 18 dB.

This work addresses the use of the Characteristic Modes Analysis (CMA) for tailoring a Metasurface (MTS) in view of exploiting it in compact, low-profile, Circularly Polarized (CP) antennas. The investigated non-uniform MTS is exploited as a superstrate in the design of a novel compact and low-profile antenna for nanosatellite applications. The MTS consists of a 3 × 4 array of unequal patches arranged in a rectangular lattice. The antenna parameters are carefully tailored by using the CMA to achieve a significant performance enhancement with respect to a uniform MTS in terms of both Axial Ratio (AR) bandwidth and aperture efficiency. The reliability of the CMA approach is verified by assessing the overall performance of the whole radiating structure comprising the stripline feeding excitation. The proposed MTS antenna is compact (0.068 λ0) provides a remarkable aperture efficiency going from 86 % up to 96 % and an AR coverage in the upper hemisphere greater than 84 % within the desired S-band (2.025 – 2.29 GHz).

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.