In this article, we review some of the recent efforts and advancements in the implementation of reconfigurable metasurfaces and metamaterials tailored for antenna applications. Originally limited by fixed properties, these structures are increasingly integrated with different technologies to achieve real-time reconfigurability and versatility in response to evolving operational conditions and constraints. This work investigates the motivations and technical approaches driving this development, with a specific focus on reconfigurable metasurfaces designed for antenna systems operating within the microwave range (L-band to K-band). By analyzing limitations and experimentally validated approaches, the article offers a comprehensive exploration of the performance and limitations of common reconfigurability schemes in microwave metadevices.

In this letter, we propose a nonlinear reflective metasurface-based frequency mixing system able to perform second-harmonic generation (SHG) and third-harmonic generation (THG). The metasurface consists of a planar bidimensional alignment of unit-cells, each of which encompasses a receiving element at fundamental frequency, and two transmitting elements operating at double and triple the fundamental frequency, respectively. The TX/RX elements within each unit-cell are connected via a microwave network to a RF mixing circuit that performs the conversion, assisted by an RF amplifier that improves the overall conversion efficiency. Efficient switchable SHG and THG frequency conversions are demonstrated by both full-wave simulations and experiments. For the input fundamental frequency of 3 GHz, the conversion efficiencies for SHG and THG are 55.5% and 42.6%, respectively.

 In this paper, we propose a multifunctional space-time modulated metasurface (STM-MTS) able to perform simultaneously a direction of arrival (DoA) estimation and radar cross section (RCS) manipulation in a single system. The metasurface consists of an alignment of unit-cells, whose scattering response can be dynamically controlled to be transparent (full transmissive unit-cell) or opaque (full reflective unit-cell), according to the amplitude of the modulating signal applied to the loading PIN diodes. The DoA estimation is performed behind the metasurface, i.e., in the transmission region, where multi-order harmonics of the incident field whose complex amplitudes are proportional to the DoA is transmitted. The multiple signal classification (MUSIC) algorithm is employed to estimate DoA. Simultaneously, the mono-/bi-static RCSs manipulation is performed in front of the metasurface, i.e. in the reflection region, where the reflected field is properly tailored by controlling the modulation scheme at unit-cell level. An X-band STM-MTS is fabricated to experimentally verify the feasibility of the proposed design. A DoA estimation errors are less than 2° in the incidence angular range of [-50°, +50°] is demonstrated, whereas mono-static and bi-static RCS manipulations of about 17dB and 36dB are achieved at operative frequency.Â

The design of metasurfaces and metasurface-based devices (e.g. reflective intelligent surfaces - RIS) is inherently affected by unpredictable variations of the unit-cell response, due, for instance, to fabrication tolerances, physical discontinuities, presence of bumps, etc. Such effects may dramatically affect the accuracy of the conventional design models, such as the one based on the local surface impedance (LSI). In this work, we present a statistical model resulting in a probability density function that allows defining a random local surface impedance (rLSI), that takes properly into account potential random variations derived from the von Mises distribution. To compensate for these variations, we propose a transformation kernel that matches the expected rLSI value to the design target. By using an analytical formulation and a proper set of full-wave simulations, we demonstrate the effectiveness and robustness of the method against unpredictable variations caused by the mutual coupling among the unit-cells and fabrication tolerances/errors. The proposed method paves the way to foster advancements in realistic metasurface engineering.

This paper presents a new design of diverse frequency time-modulation for linear frequency modulated (LFM) radar passive false target spoofing. To realize the passive diverse frequency time-modulation, the modulated metasurface is designed accordingly, which is composed by a set of elements, each of which is independently time-modulated to realize specific false targets in the high-resolution range profile (HRRP) of the radar system. Thanks to the control of the modulation scheme in each element and the combined response of the overall surface, we demonstrate the capability to realize several false target patterns consisting of single and multiple targets symmetrically, or asymmetrically, distributed around the actual target. A HRRP general expression of the time modulated diverse frequency metasurface at different observation angles is given to show the relation between the generated false targets and frequency components. Since having more flexible capacity of reflected signal manipulation comparing to false target spoofing only through time modulation with consistent modulation frequency, the proposed method has enhanced performance on control of false targets at different observation angles. Both simulated and measured results demonstrate the validity of false targets manipulation, demonstrating its effectiveness as electromagnetic countermeasure for achieving radar camouflaging and deception.

Temporal metamaterials have been recently exploited as a novel platform for conceiving several electromagnetic and optical devices based on the anomalous scattering response arising at a single or multiple sudden changes of the material properties. However, they are difficult to implement in realistic scenarios by switching the permittivity of a material in time, and new strategies to achieve time interfaces in a feasible manner must be identified. In this paper, we investigate the possibility to realize a temporal metamaterial without acting on the material properties, but rather on the effective refractive index and wave impedance perceived by the wave during the propagation in an empty guiding structure by varying the boundaries in time. We demonstrate analytically and through numerical experiments that suddenly changing the physical distance between the metallic plates of a parallel-plate waveguide will induce an effective temporal interface. In addition to the standard backward and forward scattered fields at different frequencies due to the temporal interface, we also identify the presence of a static field necessary to satisfy the continuity of the electromagnetic field across the interface. The proposed concept can be extended to temporally controlled metasurfaces, opening an easier path to the design and realization of novel devices based on time-varying metamaterials at microwave and optical frequencies.

In the last years, temporal metamaterials have been exploited as a novel platform for conceiving several electromagnetic and optical devices based on the anomalous scattering response achieved at a single or multiple sudden change of their medium properties, which however is difficult to carry out in a realistic scenario In this letter we investigate on the possibility to emulate the scattering response of a temporal metamaterial without acting on the medium properties, but on the effective refractive index and wave impedance perceived by the wave during the propagation within a guiding structure rather than the actual material, by acting on its boundaries. The work presents in closed form the scattering coefficients achieved when the boundary properties are suddenly modified for inducing an effective temporal interface. In this framework, temporally controlled metasurfaces can be used to implement the proposed concept, giving an easier path also to the design and realization of novel devices at microwave and optical frequencies.

Virtual absorption concept has been recently introduced as a new phenomenon observed in electromagnetics and optics consisting of an undefined energy accumulation within a finite volume of material without dissipation. The anomalous behaviour is achieved by engaging the complex zero scattering eigenmodes of the virtual absorbing system by illuminating it with a proper complex frequency, whose value is strictly determined by the system characteristics. In this paper, we investigate on the position of the zero-pole scattering pairs in the complex frequency plane as a function of the input impedance of the metasurface-based lossless virtual absorber. We analytically derive the conditions under which a properly modulated monochromatic plane wave can be virtually absorbed by the system and stored within its volume. The analysis is developed by modelling the propagation of a normally impinging plane wave though its equivalent transmission line model terminated on an arbitrary reactive load, which in turn models the input impedance of the metasurface-based system under consideration. The study allows to determine a priori whether the metasurface-based system can support the virtual absorption or not by evaluating the time-constant from its equivalent circuit.