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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] .

In this letter, we propose a magnet-less nonreciprocal isolating system based on time-varying metasurfaces. Two parallel time-varying metasurfaces, one for frequency up-conversion and one for down-conversion by the same amount, are used for realizing a region of space where incident waves from opposite directions experience an opposite Doppler frequency shift. As a result, any device within this region becomes sensitive to the illumination direction, exhibiting a different scattering response from the opposite directions, and thus breaking reciprocity. Very importantly, thanks to the opposite frequency shift of the metasurfaces, the frequency of the transmitted electromagnetic field is the same as for the incident one. Here, we demonstrate this general approach by using a Bragg grating as the device between the time-varying metasurfaces. The combined structure of the metasurfaces and the grating exhibits different transmission and reflection properties for opposite illumination direction, thereby realizing an isolator. More broadly, this letter presents a strategy for converting any conventional electromagnetic device to a nonreciprocal one by placing it between two time-varying metasurfaces. This approach opens the door to several new nonreciprocal components based on thin and lightweight metasurfaces, which are simpler to realize compared to their volumetric counterparts.

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.

Time-varying metamaterials are artificial materials whose electromagnetic properties change over time. Similar to a spatial medium discontinuity, a sudden change in time of the metamaterial refractive index induces the generation of reflected and refracted light waves. The relationship between the incident and emerging fields at one temporal interface has been subject of investigation in earlier studies. Here, we extend the study to a temporal slab, i.e., a uniform homogeneous medium that is present in the whole space for a limited time. The scattering coefficients have been derived as a function of the refractive indices and application time, demonstrating that the response of the temporal slab can be controlled through the application time, which acts similarly to the electrical thickness of conventional spatial slabs. The results reported in this Letter pave the way to creating novel devices based on temporal discontinuities, such as temporal matching networks, Bragg grating, and dielectric mirrors, which exhibit zero space occupancy by exploiting the time dimension, instead of the spatial dimension.

In this contribution, we review some of our recent efforts in the investigation of unusual properties of plasmonic and dielectric metasurfaces. We first outline the fundamentals of a surface impedance homogenization approach, which has resulted to be suitable for effectively describing the macroscopic response of metasurfaces made by spheroidal atoms for both normal and oblique incidences. Then, we will describe some of our findings about the unconventional applications of these structures at microwave and optical frequencies.

In this contribution, the fundamental bounds that apply to low-scattering antennas are discussed and the use of non-linear cloaking metasurfaces is proposed to overcome them. Different power-dependent mantle cloak designs, which are able to dynamically transforming their effective geometry depending on the power level of the impinging field, are introduced. Their effectiveness is assessed in several antenna and array scenarios. This innovative approach may find applications in all those radiating systems in which a considerable difference between transmitted and received power levels is present.

In this contribution, we propose the design in time-domain of two key components in electromagnetic theory, the Fabry-Perot cavity and the matching slab. Starting from the well-known reflection and transmission properties of a temporal medium discontinuity, we extend here the study to the case of the general case of multiple discontinuities, evaluating analytically the general reflection and transmission coefficients for a temporal metamaterial slab, i.e. a uniform homogeneous medium that is present in space for a limited time. It is found that the scattering response from this temporal device may be tuned to act as a temporal Fabry-Perot cavity and matching network by selecting the application time that acts like the electrical thickness for spatial slabs. Several novel devices based on temporal discontinuities can be derived by the presented concept of the temporal slab, such as temporal dielectric mirrors, broadband temporal matching networks, and temporal anomalous refraction.

In lossless transmission lines terminated on purely reactive loads reflections cannot be avoided, due to the significative impedance mismatch between the real characteristic impedance of the transmission line and the imaginary impedance of the load. Several techniques have been proposed to achieve matching, and all of them are based on the introduction of a resistive lumped element at the end of the line to obtain the power dissipation required to achieve zero reflection. In this contribution, we show a way to achieve the perfect matching condition for purely reactive loads by exploiting the properties of complex frequency excitation, i.e. a signal having a peculiar temporal shape. The proposed solution does not involve dissipation, since the load is purely reactive and energy is 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 stored energy leaks out the load as soon as the applied excitation varies or stops.

The use of non-linear electronic components is proposed in this contribution to further expand the potentialities of mantle cloaking for antennas. In particular, the possibility to design phased arrays featuring a radiation pattern that can be made self-sensitive to the level of received/transmitted power by the system will be demonstrated. Towards this end, a phased array exhibiting a directive pattern when operating for high-power signals while presenting an omnidirectional pattern for lowpower ones will be designed and numerically validated. This solution can be of particular interest for designing radar systems able to seamlessly/transparently switch between selectively scanning the environment when transmitting high power pulses, and omnidirectionally sensing the environment when handling low power scattered signals.

We investigate an anomalous scattering phenomenon exhibited by a lossless system based on metasurfaces. Electromagnetic energy is neither reflected nor transmitted but stored within the system to be available again at a different time. We analytically derive the proper excitation conditions and verify the response of the system through a proper set of full-wave simulations, demonstrating the key role of the metasurface in enabling such a zero-scattering condition. The practical feasibility and the opportunities offered by the proposed metasurface-based system may open the door to the design of virtual absorbers with dynamic properties in energy absorbing, storing, and releasing.

In this contribution, we present the latest developments from our group on metasurfaces for antenna applications. In particular, we present the properties and a possible implementation of non-linear and time-varying metasurfaces: the first one have been used for conceiving powerdependent and waveform-dependent mantle cloaks for antennas, allowing them to become invisible/visible to an electromagnetic wave depending on the power level or waveform of the incident wave, respectively; the second one allows to realize Doppler mantle cloaking, which can vanish the Doppler frequency shift due to the motion of the antenna system. Finally, we present the design of patch antennas with reconfigurable radiation characteristics exploiting the position of the phase singularities of vortex fields.

The storage of electromagnetic energy is a typical capability of closed cavities, whose impenetrable walls do not allow energy leakage in the form of electromagnetic radiation. Recently, the interest in open or partially-open cavities able to absorb and store the energy carried by an external illuminating field has stimulated research efforts in the exploration of special cavities exhibiting anomalous scattering properties. In this contribution, we investigate the scattering properties of a partially open cavity, bounded on one side by an infinite reflector and on the other side by an infinite metasurface. We show that for a specific illumination signal, the cavity may operate in its virtual absorption state, exhibiting no reflection nor transmission. Being the system lossless, the impinging energy is totally stored in the cavity between the metallic reflector and the metasurface. The proposed structure, which can be easily implemented, may enable the design of lossless systems with dynamic energy properties.

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.

In this paper, we discuss the electromagnetic homogenization of graphene-based nanoparticle arrays using the surface impedance approach. For this purpose, we first investigate the contribution of the different harmonics to the equivalent surface impedance of the array. We conclude that the size of the nanoparticles is small compared to the operating wavelength, and the higher-order modes have a negligible impact on the overall response of the array. Then, the accuracy of the homogenization method to describe the macroscopic response of graphene-coated spherical particle arrays is discussed. In particular, it is demonstrated that the surface-impedance modeling provides reliable results when the level of coupling between the nanoparticles is moderate and the model accuracy can be slightly improved by considering the Floquet expansion-based interaction coefficient. Finally, approximate formulas for the array surface impedance, as well as an equivalent circuit model of the metasurface, are extracted.

Conventionally, antennas are reciprocal devices, exhibiting the same properties in transmission and in reception. In order to break antenna reciprocity, in this letter, we exploit the properties of space-time varying metamaterials, whose dielectric properties are modulated in both space and time. We propose to surround the antenna by a space-time varying cloak that imparts a momentum bias to the electromagnetic field propagating through it. The propagating wave interacts with the space-time modulation only in one direction, breaking time-reversal symmetry and, thus, the reciprocity of the system. Here, we show that a properly designed space-time varying metamaterial cloak can be used to significantly change the realized gain exhibited by an antenna in its transmission and reception mode. Since the proposed concept is based on weak coupling between propagating modes, electrically long propagation distance in the cloak is required, increasing the overall dimension of the system. However, the cloak thickness can be controlled by modifying the modulation parameters. The proposed concept may enhance the performance of radio-frequency communication systems, reducing the in-band captured noise or interfering signals.

Recent years have witnessed an increasing interest in topological states of condensed matter systems, whose concepts have been also extended to wave phenomena. Especially at optical frequencies, several studies have reported applications of structured light exploiting topological transitions and exceptional points or lines, over which a field property of choice is undefined. Interesting properties of light beams with phase singularities (such as the creation, annihilation or motion of these topological points) have been observed in composite vortices consisting of superimposed light beams with different topological charges. Here, we discuss how these concepts may have a relevant impact on antenna technology at microwave frequencies. We obtain the superposition of vortex fields with different topological charges by simultaneously exciting different modes of a patch antenna. This can be useful to give a physical interpretation for the behavior of some structures, already proposed at microwave frequencies, which use superposition of different radiating modes to manipulate the radiation pattern of patch antennas. Moreover, this approach may open new strategies to design at will the directivity properties of a patch antenna with inherently robust responses, and it may find applications in the design of smart antenna systems, requiring pattern reconfigurability.

Boosted by the progresses in nano-optics, highly sensitive and label-free optical transducers for sensing applications have been proposed over the last years. Among them, plasmonic nanoparticle based transducers have had a great success because their resonant behavior is extremely sensitive to the changes of the local surrounding environment, being interesting candidates for real-time measurements and diagnoses. Here, we propose the employment of the core-shell super-spherical nanoparticle to design new sensing devices. We demonstrate that, thanks to this particular geometry, a sensor working in the visible spectrum may exhibit an higher sensitivity with respect to all the conventional solutions currently adopted in the field. The analysis is conducted by developing an analytical model for the polarizability of the core-shell super-spherical particle and verified through a proper set of numerical experiments. Finally, the sensing performances of the core-shell super-spherical nanoparticle are discussed and compared with the state-of-the-art optical sensors working in the same frequency range.

In this paper, we show how mantle cloaking can be effectively used in satellite scenarios to reduce the deteriorating effects of the deployable equipment on the link budget between a miniaturized satellite and its ground station. The proposed idea is applied to a nanosatellite scenario and is validated through a simple analytical analysis of its communication link with the ground. The effectiveness of the designed cloaking metasurfaces is evaluated through a proper combination of numerical strategies taking into account the electrically large domain of the problem and the electrically small features of the metasurfaces. This innovative solution can be used to enhance the performances of existing satellite communication systems or to design new extremely compact nanosatellite platforms equipped with multifunctional antennas and sensors.

Some recent works have demonstrated that phase singularities of vortex fields can find different applications at microwave frequencies. In particular, it has been shown that topological properties of phase singularity points can be used to design patch antennas with reconfigurable radiation characteristics. The aim of this contribution is to generalize the aforementioned approach and, thus, to present a general design tool for antenna pattern shaping based on topological properties of vortex modes.

In this contribution, we present a time-modulated full-reflective metasurface able to realize a frequency shift of a normally impinging electromagnetic plane wave. The frequency conversion is achieved by a dynamic control of the reflection phase, which emulates the phase advance (phase delay) of the field when reflected by a scatterer moving towards (away from) the source. The metasurface consists of three layers: the first two layers are used for realizing an array of mirrored mushrooms, i.e. two metallic patches printed on the two sides of a dielectric substrate and connected by a via; the third layer is a metallic ground plane that enforces the zero transmission. The patches are loaded by a set of varactors controlled by a low-frequency time-varying voltage signal. The metasurface and biasing network are properly designed for ensuring the operation for both polarization of the illuminating plane wave. We report here our results, showing the ability of the metasurface to reflect a frequency-shifted signal according to the modulation imparted by the varactors. The proposed metasurface can be used for realizing electrically thin Doppler cloak, which can restore the source illumination frequency of a moving object, as if it were not in motion.

Recently, we have presented several configurations of microstrip antenna, which exhibit peculiar radiation characteristics due to the topological properties of the vortex modes radiated by the antenna itself. In particular, the performances of these structures are all related to the proper superposition of two different circular polarized modes of patch antennas, radiated by two concentric radiating elements. In this contribution, we show that the proposed configurations are particular cases of a more general design tool, which can be exploited for shaping the radiation pattern of patch antennas in many other different forms. In particular, being the pattern shaping capabilities related to the positions of phase singularity points in the overall radiated beam, we determine the equations that relate the singularity locations to the structural and feeding properties of the radiating structure. Moreover, as a practical example, we show the conditions that allow designing a single radiating structure with a multi-beam radiation pattern.

In this contribution, we discuss a convenient technique for designing rectangular waveguide filtering antennas with reconfigurable radiation properties. The basic structure we consider is an open-ended WR-137 rectangular waveguide working in the frequency range 5.85-8.20 GHz, with a stable realized gain around 7 dB. We will show that by placing two electrically small resonators at the end of the waveguide, the bandwidth of the overall structure can be reduced with important advantages in terms of noise/interference rejection. Moreover, by loading each resonator with a varactor diode, the phase shift introduced by the two elements can be controlled and, thus, exploited for steering the radiation pattern of the antenna.

Here, we discuss the design of a cloaking metasurface (MTS) capable to engineer the scattering of an antenna depending on the waveform of the impinging signal. The MTS consists of vertical metallic strips loaded with lumped-element circuits exhibiting a peculiar time-domain response. Depending on the waveform of the impinging signal, the designed MTS behaves either as an invisibility device (when the signal is a pulse) or a perfect electric conductor (for a continuous wave excitation). The proposed configuration paves the way to a new generation of self-tunable and reconfigurable cloaking devices for antenna systems.

In this contribution we present the opportunities offered by artificial non-reciprocity and frequency shifting induced by time-varying metamaterials and metasurfaces. Such artificial metastructures are characterized by constitutive parameters (permittivity, permeability and/or surface impedance) that are modulated in both space and time through an external control. At first, the physical insights of the unusual interaction arising between the electromagnetic field and time-modulated metamaterials are presented, with emphasis on the resulting non-reciprocal behavior and Doppler cloaking capabilities offered by the control of the frequency content of reflected and transmitted fields. We present some applications of Doppler cloak, such as motion invisibility and restoration of antenna matching in moving systems. Finally, we introduce the time-varying metasurfaces, which are able to (red-)blue-shift the frequency content of an illuminating electromagnetic wave by changing continuously the reflection phase, emulating their motion in a certain direction and velocity.

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.

The design method for the broadband compensation of the intrinsic dispersion of a Composite Right/Left-Handed (CRLH) transmission line is presented. The design is based on Cauer’s method that is typically used for the design of microwave networks that must exhibit a specific input impedance profile over an arbitrary frequency band. Here, we consider a conventional microstrip CRLH-TL and the method is applied. The compensating network has been designed for linearizing the dispersion relation, keeping the left-handed propagation regime unaltered. The network consists of a series and shunt connections of inductors and capacitors, some of which exhibit negative value, meaning that a Non-Foster elements are required. When the compensating network is connected to CRLH-TL, a significant reduction of dispersion of broad band signal propagating through the line is achieved.

In this contribution, we describe a simple and powerful analytical approach to homogenize all-dielectric metasurfaces. The proposed model is based on a combination of the Mie scattering theory of the individual scatterer with a bi-dimensional homogenization approach accounting for the interaction between the electric and magnetic dipoles excited by the external field. Through full-wave simulations, we show that the proposed approach is able to effectively characterize the electromagnetic response of an array of Mie resonators, even for small inter-element separation distances. Several applications of analytically-designed all-dielectric metasurfaces at optical and microwave frequencies are also discussed.

Electromagnetic invisibility or cloaking represents one of the most important breakthroughs enabled by metamaterials and metasurfaces. Among its possible applications, cloaking of antennas has been emerged as a topic of particular interest since it increases the degrees of freedom available for the design of complex telecommunication platforms. In this contribution, we will show that the potentialities of cloaking for antennas can be further increased if we make use of power-dependent metasurfaces, i.e., textured engineered surfaces loaded with non-linear elements. We will show that such structures enable shaping the radiation pattern of an antenna array depending on the input or received power by the driven element.