A comprehensive history and review on how the fascinating topic of non-radiating currents has been investigated is presented in this paper. From inverse scattering problems to radiationless theorems, a unified research on non-radiating currents is highlighted for the very first time in this work. Applications of non-radiating currents span from improving images in inverse scattering problems to make feasible antenna design, with a possibility to switch between radiating and nonradiating states.

Metamaterials (MTMs) and metasurfaces (MTSs) are engineered materials and surfaces constituted by a distribution of electrically small particles that collectively exhibit emerging properties that enable superior control over the electromagnetic (EM) field. This article reviews various EMs research areas relevant to the antennas and propagation community impacted by MTMs and MTSs, including extreme material responses, space and surface wave (SW) control, EM manipulation in space and time, nonlocal responses, hyperbolic phenomena, near-zero permittivity, nonreciprocal and nonlinear effects, and topological MTSs. Modeling, design, characterization, and applications of these devices are discussed, as well as their impact on antenna and EM engineering.

This paper reports the next step in the development of a Ka-band SatCom terminal that uses rotatable Metascreen apertures to realize wide-angle electronic beam steering with strongly suppressed grating lobes and broadened operational bandwidth. Spherical near-field measurements in an anechoic facility confirm robust scan performance over ~2 GHz in the receive band and ~2.5 GHz in the transmit band. The beam scans to ±65° from broadside in both bands while maintaining radiation efficiencies above 80%. Even at extreme scan angles (≥ 60°), the radiation patterns remain clean, with no observable grating lobes—representing a marked advance over earlier implementations.

This works describes a novel, low-profile, reconfigurable two-port antenna suitable for Wi-Fi in the framework of FTTR scenarios. A total of six beams can be generated by means of a Metasurface located on top of the main radiator, and including PIN diodes as reconfigurable elements., in order to ensure proper coverage across half hemisphere. The two antenna ports and their uncorrelated beams are able to manage independent data streams for improving MIMO system capabilities. Prototype testing showed that the desired coverage was achieved with a minimal number of reconfigurable elements, which makes this technology suitable for high-performance, low-cost, low-profile devices.

Wireless communication systems have rapidly evolved, becoming a driving force behind the global digital transformation. This has fostered connectivity and enabled the seamless exchange of information. Electromagnetic Information Theory (EMIT) represents a theoretical framework that explores the fundamental principles governing the transmission and reception of information through electromagnetic waves. Indeed, in wireless communication systems, the electromagnetic field acts as a medium for information transfer. EMIT has therefore emerged as a transformative paradigm, promising to redefine the landscape of next-generation wireless communication. At its core, the concept of Degrees of Freedom (DoF) of the electromagnetic field refers to the inherent capacity of the field to assume multiple independent configurations, allowing for the simultaneous transmission of diverse information streams. This capacity for simultaneous transmission is crucial in the context of next-generation wireless communication, where the demand for higher data rates, lower latency, and increased reliability is ever-growing. The DoF of the field can be harnessed to create multiple independent channels, each capable of carrying distinct information payloads. By understanding and manipulating these DoF, researchers and engineers can optimize the utilization of the electromagnetic spectrum, overcoming the limitations posed by traditional communication systems.

With his extensive experience of over 45 years in the field, Ross Stone was a towering figure and a guiding light in the Antennas and Propagation and Radio Science community. Throughout his career, he made significant contributions to industry, consulting, and research in antennas, propagation, and related technologies. Renowned for his exceptional skills and altruistic nature in supporting and fostering new initiatives, Ross became a key figure in major international societies and networks. His involvement with the IEEE Antennas and Propagation Society (AP-S), the International Union of Radio Science (URSI), and the European Association on Antennas and Propagation (EuRAAP) was profoundly influential. His active participation and leadership within these organizations was driven by his commitment to advancing the field and fostering and serving global scientific and technical communities. Through his roles, Ross not only contributed to the technical advancement of antennas and propagation but also played a pivotal role in bridging gaps between different scientific communities and enhancing international collaboration.

Authors of this paper have recently formulated a maximum bound of directivity (super-directivity) of self-resonant antennas constrained by a maximum value of quality factor $Q$ (defined in such a way to be the inverse of the relative bandwidth) [L. Passalacqua, et al., “Q-Bounded Maximum Directivity of Self-Resonant Antennas,” in IEEE TAP, vol. 71, no. 12, pp. 9549–9558, Dec. 2023]. The above paper does not address the extent to which small losses can affect the performance of super-directive antennas. Indeed, it is widely acknowledged that super-directivity does not always translate into “super-gain” even for small losses, especially in cases involving high $Q$ factors. The relationship between super-gain and super-directivity has been a highly debated issue in antenna theory, prompting extensive research and numerous publications. The purpose of this paper is to assess the general impact of losses on the maximum super-directivity and, more specifically, to identify the ranges of $Q$ and losses for which the Q-bounded super-directivity corresponds to a Q-bounded super-gain.

In this work the authors present and discuss a technique to evaluate the near field radiated by an antenna when the far field is known, either from simulations or as a result of a measurement campaign, so as to determine and assess the possible hazards that an operator is exposed to when inspecting, maintaining or repairing equipment installed on cell phone towers.

In this paper, a high gain antenna based on low profile metasurfaces is designed and numerically analyzed. The operation principle is the same of series fed Continuous Transverse Stub (CTS) structures, but in comparison to prior designs, radiation efficiency and compactness are improved by feeding the radiating aperture with a planar lens working in reflection. The lens can be realized using a pin-type metasurface, resulting in a fully metallic construction. The radiating aperture is realized by etching slots in the upper wall of a parallel plate waveguide loaded with corrugations; this solution provides an almost frequency- and scan-independent active impedance, resulting in wideband performance for all the pointing directions. Furthermore, because of the axial symmetry of the feeding structure, the antenna has the capability of generating multiple beams from the same aperture while maintaining excellent port decoupling.

In this paper, it is shown that circularly polarized leaky-wave (LW) antennas based on a one-dimensionally modulated anisotropic impedance exhibit a complete suppression of the open-stopband when the beam is scanned through broadside. This is in contrast with what happens in an isotropic periodically modulated impedance surface. This general behaviour is illustrated here by using a homogenized penetrable impedance model to compute the LW dispersion. As a consequence, LW antennas based on modulated anisotropic metasurfaces (MTSs) are able to scan from backfire to endfire without any frequency region of high attenuation.

This paper reports the application of higher symmetries to the design of a Reflecting Luneburg Lens (RLL) with broadband response. RLLs consist of two circular parallel plate waveguides (PPWs) vertically stacked. The bottom PPW is filled with an azimuthally symmetric graded index (GRIN) medium. Owing to this GRIN medium, the wave launched by a primary feed in the bottom PPW is collimated in the top one, so that a plane wave with a different propagation direction is generated for any azimuthal position of the source. In this work, the GRIN medium in the bottom PPW is implemented by higher symmetry unit-cells consisting of metallic inclusions. This type of unit-cell achieves the required effective refractive index profile with reduced frequency dispersion, thus increasing the operational bandwidth of the lens. The proposed architecture constitutes a low-profile beam-forming solution that also provides complete azimuthal scanning in a wide frequency range. Moreover, it remains completely metallic, thus benefitting from structural robustness and reduced losses.

The term ``metasurface'' (MTS) denotes an artificial surface constituted by a distribution of electrically small elements that collectively exhibit equivalent homogeneous boundary conditions (BCs) to an interacting electromagnetic field. MTSs are becoming increasingly popular due to the technological simplification that they offer with respect to volumetric metamaterials. In this article, we review the basic theory behind microwave MTSs seen as reconfigurable intelligent surfaces (RISs), oriented to the future visionary challenge of a smart radio environment. To this end, two different typologies of MTS are reviewed: surface-wave-based MTSs and nonspecular reflective MTSs. Both types can be effectively characterized using simplified problems that locally match the homogenized, modulated BCs. A different use of these problems allows for an accurate design of radiated and scattered fields. An accurate ray representation is also suggested, which allows for an effective description of the scattered field also in the Fresnel region and for the insertion of the MTS description in ray-tracing tools for network planning. Several examples of practical implementation are shown, and the challenges in applying electronic reconfigurability are discussed.

In this work, the design of a locally lossless and passive anisotropic metasurface performing an anomalous refraction and a linear-to-circular polarization conversion is presented, using a numerically efficient surface field optimization. The metasurface consists of the cascade of three patterned metallic layers, modeled through homogenized impedance sheets. A certain number of evanescent Floquet modes are introduced through an optimization procedure aiming at minimizing the real part of the three sheet impedances. Numerical results show an almost perfect anomalous refraction without reflection and an efficient linear-to-circular polarization conversion.

This papes describes the application of higher symmetries to enable broadband operation of Reflecting Luneburg lenses (RLL) at Ka-band. RLLs are a new type of beam-former, consisting of two vertically stacked parallel plate waveguides (PPWs) of circular shape, of which the bottom one is filled with a graded index (GRIN) medium with azimuthal symmetry. The rays launched by a source in the bottom PPW follow curvilinear paths such that they emerge collimated in the top PPW after encountering a reflecting boundary. Owing to the lens' symmetry, one can generate plane waves with arbitrary directions by simply changing the azimuthal position of the source in the bottom layer. In this paper, the GRIN medium is implemented by loading the bottom PPW with higher symmetric unit-cells consisting of metallic posts. This solution offers a double benefit: it allows one to synthesize the somehow high refractive indexes in the RLL profile and it also mitigates frequency dispersion, thus increasing the lens operational bandwidth. The proposed architecture constitutes a metal-only, low-profile beam-former that can provide full azimuthal scanning in the whole Ka-band.

This paper investigates the conditions for a perfect anomalous reflection through a modulated metasurface consisting of a metallic cladding printed over a grounded slab. Differently to what has been previously published, the problem is rigorously addressed by modeling the metallic cladding through an equivalent penetrable impedance and accounting for the grounded slab through the problem's Green's function. It is shown that without polarization transformation, the exact solution exists only for the special case of retroreflection, and, in that case, it can be done simultaneously for the two orthogonal polarizations, with an arbitrary phase shift among the two. On the other hand, changing the polarization of the reflected wave allows one to find an exact solution for arbitrary combinations of incidence and reflection angles. The exact solution is found by imposing that the induced currents radiating with the Green's function of the background problem simultaneously create the desired reflected beam and cancel the specular reflection from the grounded slab. This approach leads to the derivation of a closed-form expression for the homogenized penetrable impedance profile providing perfect anomalous reflection, i.e., ensuring the vanishing of all the coefficients of the waves associated with unwanted diffraction orders, including the specular reflected wave and the evanescent waves. This result is of great practical interest, since the derived penetrable impedance profile can be readily implemented through a simple distribution of metallic patches. The feasibility of this approach is verified through full wave simulations of both the ideal impedance and the patch-based structure, which confirm the effectiveness of the proposed solution.

The transmission and reflection characteristics of a bent square transverse electromagnetic waveguide constituted by two opposite perfect electric conductor and perfect magnetic conductor walls are analyzed. This waveguide exhibits a parity, timereversal, duality (PTD) symmetry with respect to both the diagonal axes. It is found that this property is maintained even when the structure is bent in a plane orthogonal to one of the two PTD symmetry axes (PTDbend). As a consequence, the transverse electromagnetic mode propagation is protected against backscattering by this class of discontinuities. The preservation of the PTD symmetry in presence of a geometric flexibility of the bend is also analyzed, thus introducing a new class of bendable waveguides that are largely immune to backscattering by bend discontinuities.

Two analytical methods are presented to retrieve the equivalent refractive index in a parallel plate waveguide loaded by non-homogeneous metasurfaces. The first one is based on the Abel transform and the second one on a Regularized Ray-Congruence Equation. These methods enable the design of new flat metalenses.

This paper presents two different design methodologies to cope with the somehow unexplored design of dual-band modulated metasurface (MTS) antennas with circular shape. In the first approach, one overlays two different modulations. Each modulation is appropriately chosen to provide a broadside beam for one of the bands and a very weak radiation for the other frequencies. The second approach builds on the active region modulated MTS concept, recently applied to broadband designs. In this case, the periodicity of the modulation at the central region of the aperture is selected to radiate at the high frequency band. In turn, a larger periodicity in the outer annular region provides the broadside beam at the low frequency band. Both approaches are compared and their advantages and drawbacks, discussed.

This paper presents a metasurface (MTS) beam-former based on Reflecting Luneburg Lenses (RLLs) operating in the sub-THz range. RLLs consist of two circular parallel plate waveguides (PPWs) vertically stacked. The bottom wall of the lower PPW is loaded with a MTS with an axially symmetric modulation. The wave launched by a primary feed in the bottom PPW is collimated in the top one, so that a plane wave is identically generated for any azimuthal position of the source. The proposed solution uses a bed of nails, well-suited to fabrication by Si micromachining, to implement the RLL refractive index profile. Simulation results yield a 30% -3dB directivity bandwidth around the center frequency (280 GHz). This device can be used as a beam-former for multi-beam antennas for Earth observation or for front- and back-hauling in beyond 5G wireless.