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The quest for increasing the scanning range of a phased array is a challenging task for antenna engineers, and its solution could lead to significant advances in different applicative scenarios, ranging from 5G and beyond 5G communications to radar and satellite systems. For this purpose, the use of a deflecting meta-dome is one of the most promising solutions recently proposed that, however, still presents some inherent limitations, such as the significant reduction of the broadside gain of the array, due to the diverging effect of the dome, as well as the complexity of the implementation due to the need of a continuous phase profile. In this framework, the paper aims at proposing some technical solutions for maximizing the meta-dome performance and relaxing the implementation complexity. In particular, by properly discretizing and engineering the phase profile along the dome and by taking into account the different angles of incidence onto the meta-cells, we show how it is possible designing realistic meta-domes with reduced insertion loss at broadside, improved steering capabilities, and a reduced profile. In addition, a complete design workflow for a realistic meta-dome based on cascaded metasurfaces is presented., (in press)

Recently, the topological properties of composite vortices, i.e., the superposition between vortex modes of a different order, have proven effective in manipulating the radiation properties of patch antennas and making them suitable for new operative scenarios. In this paper, we exploit these properties to design a multi-antenna system exhibiting both pattern- and polarization-diversity. The low mutual coupling and the possibility to steer two orthogonal polarized fields in different directions, make this radiating element an optimal candidate for antenna diversity or MIMO systems and, thus, for increasing signal quality or channel capacity of a communication system.

The papers in this special section focus on recent advancements in this field and provide an overview of the potential applications of this technology in the next generation antenna devices, with emphasis on metamaterial-based enhancements enabling real-time control of their radiation characteristics.

Next-generation of wireless communications and beyond-5G systems will require extreme performance in terms of low latency, data rates, reliability, just to name a few, and the possibility to tailor the communication channel on demand would be a key enabling technology for the so-called smart environment systems. Therefore, conventional antennas with fixed functionalities cannot be considered an efficient solution, and a new paradigm change in antenna design is thus required. In this frame, here, we show some of the most interesting possibilities enabled by functionalized conformal metasurfaces coating wired antennas for enabling unprecedented electromagnetic behaviours. In particular, we report the design of metasurface coats loaded with electronic elements allowing scattering, frequency, and radiation pattern tunability, in view of the designs of intelligent antennas whose properties can be tailored on demand.

So far, the environment has been considered as a source of fading, clutter, blockage, etc., with detrimental consequences for the efficiency and robustness of communication systems. However, the intense research developed toward beyond-5G communications is leading to a paradigm change, in which the environment is exploited as a new degree of freedom and plays an active role in achieving unprecedented system performances. For implementing this challenging paradigm, it has been recently proposed the use of intelligent surfaces able to control almost at will the propagation of electromagnetic waves. In this framework, metasurfaces have emerged as a promising solution, thanks to their field manipulation capabilities achieved through low-cost, lightweight, and planar structures. The aim of this paper is to review some recent applications of metasurfaces and cast them in the scenario of next-generation wireless systems. In particular, we show their potentialities in overcoming some detrimental effects presented by the environment in wireless communications, and discuss their crucial role towards the practical implementation of a smart electromagnetic environment.

Recently, great efforts have been made in the research of new enabling technologies for beyond 5G communication systems. In this framework, Reconfigurable Intelligent Surfaces (RISs) are considered as one of the most promising solutions and, thus, they have been the subject of several investigations by both the electromagnetic and the communication communities. In this contribution, we further discuss, from an electromagnetic point of view, about the synthesis of a RIS. In particular, by exploiting the topological properties of vortex modes, we present a possible solution to simplify the synthesis and the reconfigurability of a RIS composed of concentric annular rings.

In this Letter, we discuss a novel, to the best of our knowledge, approach for designing passive nanoantennas with maximum forward and almost-zero backward scattering. The proposed approach is based on the use of high-index dielectric spheres supporting dipolar magnetic resonances, which are coated by ultra-thin surface impedance coatings. It is shown that, by properly engineering the radius of the coat and its surface reactance, it is possible to introduce an additional electric dipolar resonance and to make this overlap with the magnetic one sustained by the high-index dielectric sphere. A realistic design that is based on graphene and works in the low-THz range is also proposed and verified with full-wave simulations. Compared to earlier techniques based on the combination of multipoles or on the use of ellipsoidal particles, the proposed one is quite robust toward realistic ohmic losses and preserves the isotropic behavior of the nanoantenna.

Space-time-modulated metasurfaces can manipulate electromagnetic waves in space and frequency domain simultaneously. In this paper, an analytical design of space-time-modulated metasurfaces with modulation elements composed of two paths, In-phase (I) and Quadrature (Q), is proposed. The model is derived analytically, the space/frequency domain manipulations are achieved by designing the dimension and time sequence of I and Q paths. In the specular reflection direction, an objective frequency shift of the reflected first order harmonic can be obtained. While, in other directions, the opposite first order harmonic can be easily controlled by changing the dimension of I/Q paths and the objective first order harmonic remains unchanged. Furthermore, with a small dimension of I/Q paths, the first order harmonic can be used for beam scanning by pre-designing the start time of the modulation element. To realize the space-time-modulated metasurface with the required periodically time-varying responses, 2-bit unit-cells loaded with dynamically switchable pin diodes are employed as I/Q modulation. Both analytical and numerical results demonstrate that space and frequency domain manipulations of the reflected fields by the first order harmonics can be simultaneously obtained. The proposed designs have potential applications in wireless communications, radar camouflaging, and cloaking.

The use of fifth-generation (5G) new radio (NR) spectrum around 26 GHz is currently raising the quest on its compatibility with the well-established Earth exploration-satellite service, which may be blinded by the spurious radiation emitted above the horizon (AtH) by base station (BS) antennas. Indeed, AtH grating lobes are often present during cell scanning due to the large interelement spacing in BS array antennas for achieving higher gains with a reduced number of RF chains. In this letter, we propose an approach based on an electrically thin metasurface-based dome for the reduction of AtH grating lobes in 5G-NR BS antennas. The proposed scanning range shifting approach exploits the natural lower amplitude of the grating lobes when the antenna array scans in an angular region closer to the broadside direction. The grating lobe reduction is here demonstrated considering a 1x4 phased linear antenna array operating under dual-liner +/- 45 degrees-slant polarization. A simple design procedure for designing the metasurface dome is reported, together with the antenna performances, evaluated through a proper set of numerical experiments. It is shown that the grating lobe radiation toward the satellite region is significantly reduced, whereas the overall insertion loss is moderate.

In this contribution, we look at some recent applications of metasurfaces and we highlight their potential in the scenario of next-generation wireless systems. We show how metasurfaces allow overcoming some limiting effects of the environment on wireless systems, and we discuss their fundamental role in the practical development of smart electromagnetic environments.

In this contribution, we exploit virtual perfect matching to achieve impedance matching of purely reactive loads in lossless transmission lines, by using complex frequency excitation, i.e. a signal having a specific temporal shape. The proposed system is lossless, since the load is purely reactive, energy is not dissipated, but stored in the load as long as the complex frequency excitation condition is satisfied, giving rise to an interesting singularity in the transient regime, also known as virtual absorption. Then, the accumulated energy is released by the load once the applied illumination changes.

Time-varying metamaterials and metasurfaces have unlocked a novel way for the electromagnetic waves to interact with artificial materials. In this contribution, we present an overview of the recent components and devices based on time modulation of artificial media, surfaces, and illumination signals for achieving time-domain devices, magnet-less non-reciprocity, doppler cloaking, frequency conversion, and virtual energy accumulation into open cavities, respectively.

In this contribution, we propose a quadratic-gradient metasurface dome operating in the near-field region of a dual-polarized phased array able enhancing its grating-lobe-free scanning range without affecting its other characteristics. The dome consists of an array of Huygens unit-cells exhibiting an extremely low insertion loss and a proper phase shift in transmission. The main characteristic of the proposed dome is the quadratic-gradient phase profile, which minimizes the beam divergence in broadside direction, whereas linearly increases the scanning region in off-broadside pointing angles. A proper set of numerical simulations have been performed showing the capability of the proposed dome to enhance the grating lobe free scanning region from -58° to -83°.

The term "electromagnetic cloaking" refers to a wide class of approaches whose aim is reducing the total scattering cross section of an object within a desired range of frequencies. In the last years, the use of electromagnetic cloaking for antennas have been widely investigated and several unprecedented effects have been proven. Just to cite a few examples, it has been shown that cloaking allows designing low-scattering receiving antennas [1] and overcoming the blockage and mutual coupling effects occurring between transmitting antennas in overcrowded telecommunication platforms [2] .

The aim of this Communication is to investigate and demonstrate the possibility to overcome the fundamental limitations of mantle cloaking for antennas by exploiting non-linear metasurfaces. First, we recap and give additional physical insights about the fundamental bounds that apply to the electric properties of an antenna that is made invisible at its own resonance frequency. Then, an innovative strategy is proposed to circumvent these limits through the introduction of non-linear elements that are able to dynamically transform the geometry of the cloaking metasurface depending on the power level of the impinging field. Different non-linear designs are discussed, and their effectiveness is assessed through the relevant benchmark example of a half-wavelength dipole antenna able to efficiently transmit high-power signals while being invisible to low power ones. In addition, the capability of such non-linear mantle cloaks to enable the shaping of the radiation pattern of an antenna array depending on the power level of the received/transmitted signal is demonstrated. These innovative cloaking devices may find applications in different radiating systems enabling unprecedented functionalities.

In this contribution, we present a metasurface-based dome to be placed above a bidimensional antenna array able to synthesize a radiation beam pointing towards the specific direction in elevation and azimuth defined by the antenna that is radiating. The metasurface is designed for acting similarly to the beamforming network, allowing controlling the beam direction by switching power to the desired beam port. The metasurface consists of a dense array of Huygens unit-cells, each of which exhibits an extremely low insertion loss and the desired phase shift. The profile of the transmission coefficient across the surface is engineered to transform the spherical phase fronts radiated by the antennas into a plane wave propagating towards a specific direction. Here, the operativity of the proposed metasurface is verified by illuminating it with a set of antennas arranged into a 6×6 array, enabling its capability to radiate towards 36 possible different directions. The metasurface dome reduces the complexity of the antenna system, avoiding the need for a beamforming network, and enables the activation of more than one beam, or even all of them, simultaneously.

In this contribution, we describe the design of a self-filtering horn antenna based on the innovative features of multipolar all-dielectric metasurfaces. We first analyze the scattering response of a dielectric core-shell meta-atom with a strong permittivity contrast and we show how such a geometry allows a fine control of the spectral positions of the dipolar and quadrupolar Mie resonances. Then, we describe how it is possible engineering the electric and magnetic surface currents flowing on the metasurface for achieving a band-pass filtering response. Finally, we show how such high-Q filters are suitable for the integration in an horn antenna and, thus, for the implementation of an ultra-selective filtenna. A realistic design based on commercial materials is proposed and checked with full-wave simulations.

In this contribution, we report a new approach for the design of frequency reconfigurable antennas equipped with cognitive capabilities. Specifically, we show how the complex-valued input impedance of a wire antenna can be tuned by using an appropriate conformal metasurface. We prove that, by properly engineering the metasurface response, it is possible to modify the operative frequency of the antenna without compromising its radiation pattern and impedance matching with the source. As a relevant example, the case of a reconfigurable dipole antenna coated by a multi-layered metasurface loaded with varactor diodes is discussed. By tuning the junction capacitance of the varactors, the antenna operative frequency can be continuously shifted within a quite broad range of frequencies (2/3 octave bandwidth), enabling wideband reconfigurability capabilities.

The term filtenna [1] refers to a class of microwave components that, being able to perform both filtering and radiation operations, have been widely investigated for reducing the cost and space occupancy of a microwave system. In most cases, filtennas are designed by inserting a filtering element with a frequency-dependent behavior in the structure of a standard antenna. Given their wide operation bandwidth, which often exceeds the required one, waveguides and horn antennas have been extensively used for this purpose [1] - [3] . Moreover, by properly choosing the topology of the filtering element and by loading it with non-linear elements, the polarization state and power level of the incoming wave can be used as further discriminating characteristics between signals [3] - [4] .

Electromagnetic scattering typically occurs when a change in the material properties is perceived by the propagating wave, that inevitably splits into a reflected and refracted wave to maintain the continuity of the field components at the interface between the two media. However, such a scattering phenomenon occurs also when the entire media suddenly switches its properties to other values at a certain instant of time, realizing the so-called temporal interface. After a temporal interface, a couple of waves, one reflected and one transmitted, starts to propagate in the new media with the same wavelength but at a different frequency. Exploiting the analogies and differences between spatial and temporal interfaces, in this contribution we present the temporal counterparts of conventional electromagnetic devices based on dielectric slabs and a cascade of them, i.e., the multilayered structures. We discuss about the analysis and design strategies for synthetizing the desired scattering response in both transmission and reflection and present the possible families of devices based on multi-switched temporal metamaterials that can be conceived.

In this contribution, we present and discuss the concepts of virtual perfect matching of reactive loaded circuits and virtual perfect absorption in metasurface-bounded cavities. The anomalous scattering behaviours of the two cases are here analysed through a corresponding transmission line models, showing how it is possible to enable both phenomenon in realistic scenarios and deriving the operative limits for a given reactive load or surface impedance of the metasurface bounding the cavity, respectively. The presented study aims to highlight limits and connections of both virtual effects and derive the proper enabling complex frequency excitation.

A new approach for achieving frequency reconfigurability in wire antennas is proposed. In particular, we show that the antenna input impedance and its reflection coefficient can be tuned by properly engineering the geometrical and electromagnetic characteristics of a coating metasurface wrapped around. This mechanism is then exploited to design a reconfigurable dipole antenna coated by multi-layered metasurfaces loaded with varactor diodes. By tuning the capacitance of the varactors, the operative frequency band of the antenna can be dynamically and continuously shifted within a broad range of frequencies enabling wideband reconfigurability capabilities, while still preserving a stable omnidirectional radiation pattern.

Antenna arrays with large interelement spacing are marginally used in practical applications despite their highly directive main lobe. Their disuse is primarily attributed to the onset of grating lobes even for low scanning angles. In this contribution, we propose the design of a gradient metasurface dome able to reduce the grating lobe amplitude of a dual-polarized phased array. The dome consists of an array of Huygens unit-cells exhibiting an extremely low insertion loss and a properly designed phase of the transmission coefficient. The proposed solution is based on the scanning range shifting approach, which exploits the lower grating lobe levels exhibited by the array when radiating towards directions close to the broadside. A proper set of numerical simulations have been performed considering a 1×4 phased linear antenna array covered by a properly designed metasurface dome, showing its capability to significantly reduce the grating lobe amplitude without significantly reducing the performances in the pointing direction.

The research about all-dielectric metasurfaces is particularly popular in the last years because they have proven to enable the same effects of metallic metasurfaces but with reduced electromagnetic losses [1] - [5] . The peculiar electromagnetic behavior of these structures, consisting of a 2D array of sub-wavelength particles made by a high-permittivity material, stems from the excitation of orthogonal electric and magnetic dipole moments within each individual particle. The collective behavior of these dipoles can be tailored by acting on the shape of the individual particle as well as on the separation distance between neighboring elements [4] . While the possibilities enabled by the tangential electric and magnetic dipoles have been widely investigated, the study of the spatial dispersion of all-dielectric metasurfaces is a relatively new topic, which may enable new and intriguing effects [3] .

Virtual absorption concept has been recently introduced as a new phenomenon observed in electromagnetics and optics consisting of theoretically unlimited accumulation of energy 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 ω = ω r + jω i , 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 through its equivalent transmission line model terminated in 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.

Patch antennas are among the most popular radiating elements, yet their quasi-2-D structure reduces the degrees of freedom available to tailor their radiation pattern. To overcome this limitation, a possible solution consists in etching on a grounded substrate two concentric radiating elements and combining two modes (one for each element) with proper amplitude/phase relations. Although this technique leads, in principle, to an infinite number of possible configurations (i.e., each patch element can support an infinite number of modes), the theoretical and experimental verifications available in the literature are limited to the first two radiating modes (TM 11 and TM 21 ) of a circular patch antenna. Recently, we have shown that the design of this circular patch can be effectively performed by exploiting the topological properties of vortex fields and, in particular, by controlling the phase singularity exhibited by the higher order right-handed circularly polarized (RHCP) TM 21 mode of the circular patch. Since the number of RHCP higher order modes of a circular patch is infinite, we can in principle deal with an arbitrary number of phase singularity points, whose combined control leads to unprecedented possibilities to shape the radiation pattern of a circular patch. In this article, we present a complete design tool to determine the number and position of phase singularity points arising when combining the RHCP modes of a circular patch antenna and, eventually, manipulate them to synthesize the required radiation pattern. As a realistic application example, we show how the proposed tool can be used to effectively design a single antenna whose radiation pattern can be properly tailored to switch between two different states, i.e., a sector and a saddle shape, widely used in base stations for mobile and satellite communications, respectively.

We develop a simple and reliable analytical model that allows describing the electromagnetic response of all-dielectric metasurfaces consisting of a single-layer array of high-permittivity spherical particles. By combining Mie theory with a bi-dimensional homogenization approach, we derive closed-form expressions of the electric and magnetic surface impedances exhibited by the metasurface and, thus, its reflection and transmission coefficients. The effectiveness of the proposed approach is validated through a set of full-wave simulations. The availability of the analytical model here developed allows a more in-depth understanding of the complex scattering response of these electromagnetic structures and enables the design of innovative devices operating throughout the whole electromagnetic spectrum, including, for example, unconventional reflectors for antennas, broadband optical mirrors, and highly efficient nanoantenna reflectarrays.

Recently, circuit loaded metasurfaces exhibiting different responses depending on the waveform of the incoming waves have been proposed. Here, we extend this approach presenting two different antenna applications. As a first example, we report an open-ended waveguide antenna capped with a band-pass filtering iris loaded with a lumped element circuit. Thanks to its peculiar frequency-and time-domain selectivity response the antenna exhibits different radiating patterns depending on the waveform of the transmitted/received signal. The same circuit is also exploited to design a mantle cloak able to hide an antenna to a pulsed radar system while keeping its radiation characteristics unaffected for a continuous-wave signal.

In radar countermeasures, the terms jamming and deception are used to indicate the intentional emission of radio frequency signals whose aim is to interfere with the operation of a radar by saturating its receiver with noise or false information. Electronic radar jammers typically consist of an antenna system and a corresponding circuitry, that captures, elaborates, and re-radiates the proper interfering signals. In this contribution, we present a radar jammer implemented through a time-modulated metasurface. Such a metasurface is an electrically thin artificial structure, whose properties are dynamically changed over time to realize an elaboration of the illuminating signal. We demonstrate that such a metasurface is able to jam Doppler radars, implementing sweep jamming and velocity pull-off techniques.

Any lossless transmission line terminated on an arbitrary reactive load suffers from reflections, due to the high impedance mismatch between the real characteristic impedance of the transmission line and the imaginary impedance of the load. Although several techniques have been proposed to achieve matching, a resistive lumped component must be always added at the end of the line for ensuring power dissipation and achieving the required zero reflection. Here, we present a way to achieve the perfect matching condition for purely reactive loads by exploiting the properties of complex frequency excitation. By exciting the circuit with a signal having a proper time profile, we demonstrate that the amplitude of the reflected wave can be brought to zero. Being the load purely reactive, energy is not dissipated, but stored in the load as long as the special excitation condition is satisfied, giving rise to an interesting singularity in the transient regime, referred to as virtual absorption. Then, the stored energy can be released at will by changing or stopping the applied complex excitation.

In this contribution, we discuss two different strategies aimed at tailoring the electric and magnetic response of an all-dielectric metasurface. Specifically, we investigate the possibilities enabled by two different families of core-shell meta-atoms: concentric all-dielectric spheres and homogenous spheres covered by a reactive layer. The analysis of the scattering properties of these two classes of particles reveals intriguing effects that can be used for the design of innovative metasurfaces exploiting the entire multipolar response of their meta-atoms. Several examples are considered and investigated both analytically and numerically.

We propose a novel class of ultrathin high Q passband filters designed by properly combining different multipolar resonances sustained by an all dielectric metasurface. A rigorous analytical model, based on surface impedance homogenization and accounting for the effects of both dipolar and the quadrupolar contributions to the overall scattering response, is derived and verified through numerical simulations. Then, it is described how it is possible to engineer the interactions between dipoles and quadrupoles in a metasurface made by core shell spherical elements to design ultrathin and broadband dielectric mirrors with a narrow transmission band. The proposed filters exhibit high Q factor resonances and can be implemented using realistic materials at either microwave or optical frequencies. Finally, we discuss how the proposed dielectric filters can be used to design self filtering aperture antennas exhibiting higher out of band selectivity compared with those implemented through metallic resonators.

We present the design of an innovative wire antenna able to automatically hide or reveal its presence depending on the waveform of the received/transmitted signal. This unconventional behavior is achieved through the use of a novel waveform-selective cloaking metasurface exploiting a meander-like unit cell loaded with a lumped-element circuit capable to engineer the scattering of the antenna depending on the waveform of the impinging signal. Due to the time-domain response of the lumped-element circuit, the antenna is able switching its scattering behavior when interacts with either a pulsed wave (PW) or a continuous wave (CW) signal. The proposed configuration paves the way to a new generation of cloaking devices for intelligent antenna systems, extending the concept of antenna as a device capable to sense the external environment and change its electromagnetic behavior accordingly.

In this contribution, we investigate several examples of antenna applications of waveform-selective devices, i.e., structures exhibiting different responses depending on the waveform of the incoming waves. In particular, as a first example, we describe an open-ended waveguide capped with an iris band-pass filter loaded with a lumped-element circuit. Thanks to the frequency- and time-domain selectivity properties of the loaded iris, the aperture antenna exhibits different radiating patterns depending on the waveform of the transmitted/received signal. Then, the same lumped-element circuit is used to design an antenna that is made invisible/visible to either a short pulse or a continuous signal. Exploiting a peculiar meandered cloaking metasurface loaded by the lumped-element circuit, the cloaking effects is automatically turned on/off depending on the time-domain characteristics of the incoming signal. This study aims paving the way for the design of antenna systems equipped with both frequency- and time-domain selectivity properties.

In this contribution, we propose a magnet-less non-reciprocal isolating system based on time-varying metasurfaces. The system consists of a conventional frequency selective device placed between two time-varying metasurfaces, one for frequency up-conversion and one for down-conversion by the same amount. The overall reciprocity of the system is broken by exploiting the different scattering response from the embedded device when illuminated from opposite directions, since the two time-varying metasurface imparts an opposite frequency conversion for the two directions of illumination, but the transmitted fields are always the same as for the incident one, since the metasurfaces vanish the frequency conversion of each other. This strategy can be successfully exploited for conceiving a new family of non-reciprocal isolating systems starting from any conventional electromagnetic device.

Topological concepts have found several applications in different branches of physics. As a notable example at microwave frequencies, in this contribution, we focus on the topological properties of phase singularity points of vortex modes, which can be generated by using higher order modes of patch antennas. In particular, we report our recent results on the possibilities offered by structured fields, i.e. the collinear superposition of vortex modes with different topological charges, for designing radiating systems with novel or enhanced functionalities.

In the last two decades, the concept of metasurfaces has evolved, leading, nowadays, to unprecedented possibilities. The first generation of metasurfaces was based on regular homogeneous periodic structures. This 2D version of volumetric metamaterials had a dramatic impact in many application fields, ranging from radio up to optical frequencies. Following the evolution path of the metasurface concept, the second generation of metasurfaces was based on inhomogeneous structures characterized by quasi-periodical or gradient-like arrangements. The spatial modulation of the metasurface properties allowed enhancing the capabilities, thanks to the higher number of degrees of freedom available. Finally, the third generation of metasurfaces consists of thin patterned structures whose properties can be controlled in both space and time. This possibility, enabled by different control mechanisms (e.g. electronic, optical, thermal, chemical, liquid crystal based, etc.) and boosted by the recent advances in integration and micro/nano-fabrication technologies, allows conceiving tunable, reconfigurable, and programmable structures that can be used in a variety of applications. In this contribution, after a review of the three generations of metasurfaces and a detailed focus on the current third generation, the potential impact of these structures as a key enabling technology for beyond 5G communication systems will be discussed.

In this work, we review our recent research on advanced cloaking metasurfaces used to cover monopole antennas. Cloaking devices based on passive metasurfaces have demonstrated a dramatic enhancement of the antenna functionalities, thanks to the additional the degrees of freedom available in the design. Integrating electronic circuits and enabling time modulation in cloaking metasurfaces can further lead to more advanced functionalities, making the covered antennas ever more intelligent. In this framework, our vision is to extensively use reconfigurable metasurfaces to make the antennas cognitive and adaptive, enabling sensing capabilities of the surrounding electromagnetic environment, and dynamic changing of their properties (e.g. operation frequency, visibility/invisibility, polarization, radiation pattern shape, power level, etc.) accordingly.

In this contribution, we expand the possibility offered by structured fields at microwave frequencies by exploiting phase singularity points of vortex modes for designing a single radiating structure exhibiting orthogonal radiation patterns pointing in different directions. This kind of structure, being able to radiate two orthogonal polarizations with low-coupling levels, can be exploited in compact multi-antenna systems for antenna diversity or MIMO applications.

The aim of this contribution is to describe an innovative wire antenna able to automatically hide or reveal its presence depending on the waveform of the received/transmitted signal. This unconventional behavior is achieved through the use of a cloaking metasurface (MTS) made of a meander-like unit cell loaded with a lumped-element circuit. Due to the engineered time-domain response of the lumped circuit, the antenna is able switching its behavior when interacts with either a pulsed signal (PW) or a continuous signal (CW). The proposed configuration paves the way to a new generation of self-adaptive cloaking devices for antenna applications.

In this contribution, we present a time-modulated full-reflective metasurface able to realize a frequency shift of the illuminating electromagnetic wave. The metasurface consists of an array of metallic patches printed on a grounded electrically thin dielectric substrate. The patches are loaded by a set of varactors controlled by a low-frequency time-varying voltage signal. The varactors change in real-time the response of the metasurface realizing the required reflection response for achieving the frequency shift. We report here our preliminary results, showing the ability of the metasurface to reflect a frequency-shifted signal according to the modulation imparted by the varactors. The proposed metasurface is a bi-dimensional version of the recently proposed Doppler cloak, a spatiotemporally modulated metamaterial cover to be placed in front or wrapped around a moving object to restore locally the source illumination frequency, as if it were not in motion.