A popular absorbing structure, often referred to as Perfect Metamaterial Absorber, comprising metallic periodic pattern over a thin low-loss grounded substrate is studied by resorting to an efficient transmission line model. This approach allows the derivation of simple and reliable closed formulas describing the absorption mechanism of the subwavelength structure. The analytic form of the real part of the input impedance is explicitly derived in order to explain why moderate losses of the substrate is sufficient to achieve matching with free space, that is, perfect absorption. The effect of the constituent parameters for tuning the working frequency and tailoring the absorption bandwidth is addressed. It is also shown that the choice of highly capacitive coupled elements allows obtaining the largest possible bandwidth whereas a highly frequency selective design is achieved with low capacitive elements like a cross array. Finally, the angular stability of the absorbing structure is investigated.

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 RF identification based on a multiresonant high-impedance surface is proposed. The structure is based on a finite metallic frequency-selective surface (FSS) comprising 2 × 2 (30 mm × 30 mm) or 3 × 3 (45 mm × 45 mm) unit cells. The FSS unit cell is formed by several concentric square loop resonators. The thin structure performs deep absorptions of the impinging signal at several resonant frequencies related to the loop resonators. If one of the printed loops in the unit cell is removed, the corresponding absorption peak disappears from the reflected signal giving the possibility of encoding a desired bit sequence. The proposed structure exhibits some intrinsic advantages, such as scalability (bit number increase) without any size increase, polarization independence, large read range, and the capability of operating when mounted on metallic objects. A transmission line model is employed to illustrate the operation principle of the structure, whereas measurements on realized prototypes are provided to assess the reliability and effectiveness of the proposed design.

A solution for reducing the radar cross section (RCS) of a microstrip antenna based on the use of frequency selective surfaces (FSSs) is described. The goal is accomplished by replacing the solid ground plane of the device with a hybrid structure comprising a suitable FSS. The behavior of the hybrid ground plane illuminated by a plane wave is analyzed by using a periodic method of moments (PMM), and it is modeled by resorting to a transmission-line equivalent circuit. Similarly, the propagation of the quasi-TEM mode along the modified feeding line of the array is represented by an equivalent circuit for surface waves. The two simplified analyses provide useful design criteria for the hybrid ground structure. The presented solution guarantees a decrease of the out-of-band radar signature of the target while preserving the desired in-band radiation characteristics of the low-profile array. A careful comparison to alternative configurations employing different ground planes has revealed the superior performance of the proposed design. Measurements on a realized prototype show a good agreement with simulations and prove the reliability of the design approach.

A comprehensive analysis aimed at reducing the radar cross section (RCS) of array antennas, preserving at the same time their radiating performance, is presented. A microstrip slot array is considered as a test case to illustrate the proposed strategy for radar cross section reduction (RCSR). It is shown that a remarkable reduction of the radar signature can be accomplished over a frequency band as wide as two octaves by employing an array of periodic resistive elements in front of the radiating apertures. The monostatic and bistatic RCS of the proposed structures are investigated both for normal and oblique incidence. Different arrangements and geometries of the periodic resistive pattern are thoroughly analyzed showing the benefits and the drawbacks in terms of antenna gain and level of the scattered fields. Furthermore, the use of metallic parasitic elements for enhancing the antenna gain is considered, and the scattering phenomena caused by their presence are addressed, taking into account the appearance of grating lobes. The antenna designs are also analyzed by resorting to a bidimensional color plot presenting the variation of the reradiated field both in frequency and spatial domain. The guidelines illustrated by the proposed examples can be easily applied to other antenna architectures.

In this letter, the bandwidth of high-impedance surfaces (HISs) is discussed by an equivalent circuit approach. Even if these surfaces have been employed for almost 10 years, it is sometimes unclear how to choose the shape of the frequency selective surface (FSS) on the top of the grounded slab in order to achieve the largest possible bandwidth. Here, we will show that the conventional approach describing the HIS as a parallel connection between the inductance given by the grounded dielectric substrate and the capacitance of the FSS may induce inaccurate results in the determination of the operating bandwidth of the structure. Indeed, in order to derive a more complete model and to provide a more accurate estimate of the operating bandwidth, it is also necessary to introduce the series inductance of the FSS. We will present the explicit expression for defining the bandwidth of a HIS, and we will show that the reduction of the FSS inductance results in the best choice for achieving wide operating bandwidth in correspondence with a given frequency.