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 compact, low-profile, two-port dual-band circularly polarized (CP) antenna for portable RFID readers is presented. The radiating element for the UHF-RFID band (902-928 MHz) has been realized through a circular array of four inverted-F meandered monopoles, where the array elements are excited with a 90 ^° phase offset (sequential rotation feeding technique) through a microstrip feeding network. In order to also provide the portable reader with a wireless web access, a miniaturized CP patch has been added for the WLAN IEEE 802.11b,g radio link, by exploiting the available space between the four array elements, without increasing the overall antenna volume. The meandered monopoles and the patch are both etched on the same layer of an FR4 substrate, so getting a significant antenna compactness (60 mm×60 mm×7 mm). The patch loading effect on the monopoles can be compensated by adjusting the meandered monopole length. Reflection coefficient, port isolation, axial ratio and radiation patterns are evaluated by numerical simulations and compared with measurements on an antenna prototype.

The robustness of wearable UHF-band planar inverted-F antennas (PIFAs), with respect to body-antenna separation and human tissue dispersion, is addressed through numerical investigations. The main goal is gaining physical insights into the relationship between the grounded antenna performance and the distribution of the electric and magnetic energy densities in the antenna near-field region close to the ground plane border. A criterion for choosing a proper shape of the antenna ground plane is suggested, which can improve the antenna robustness with respect to the random variations of the body-antenna coupling scenario, but with a minimal impact on the antenna size.

The robustness of wearable antennas to the human body proximity can be improved by properly placing the radiating element over a ground plane, or by shaping the ground plane, on the basis of the antenna electric and magnetic energy density distributions, aiming to confine the electric energy density in specific regions far from the antenna border. The effectiveness of the above design criterion has been assessed through both numerical analysis and experimental measurements, by optimizing the position of the radiating element of a wearable grounded UHF RFID tag operating at the ETSI band (865-868 MHz), with respect to its ground plane. A flexible tag characterization and performance evaluation platform is used to measure the tag sensitivity as a function of the antenna-body distance.

A low-profile printed antenna is here proposed for vehicular communications, covering wireless standards in a wide frequency range (698-2690 MHz) that includes the LTE (Long Term Evolution) band. Multiple monopole antennas are printed on the same compact substrate easily embeddable under a sharkfin cover, and they are properly optimized to resonate in the considered frequency band. A tapered profile and a thin slot have been introduced to achieve a good impedance matching, especially at the higher frequencies. Numerical simulations and measurement results on a prototype are here reported in terms of reflection coefficient, gain and radiation pattern. The proposed wideband antenna is suitable for other wireless scenarios in addition to the automotive industry.