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In this article, we review recent advances in static and reconfigurable Huygens’ metasurfaces (HMSs) for antenna beamforming and beam steering. First, the fundamental theory of HMSs is summarized along with basic approaches for their physical realization. Subsequently, precise antenna beamforming without feeding networks by utilizing static HMSs is described. This is followed by the description of using HMSs as radomes for extending the scan range of phased arrays. Finally, several strategies are described for realizing reconfigurable HMS by means of electronic control devices. [ABSTRACT FROM AUTHOR]

An accurate equivalent circuit model (ECM) for general multiwire multiorder printed circuit board (PCB) metagratings (MGs) is developed. The effective impedances of the grating wires are modeled as lumped loads in the ECM. The numerous propagating Floquet harmonics supported by the grating are modeled as circuit ports. Based on the desired field transformation for the MG, a set of required scattering parameters for its ECM is derived. The equivalent lumped loads are subsequently optimized to realize these parameters, which is analogous to shaping the diffraction pattern of the MG to match the desired functionality. Within this framework, it is possible to encode multiple functions into a single device by simultaneously optimizing all relevant entries of the ECM scattering matrix. Using the proposed technique, an angle-multiplexed multifunctional MG is designed and investigated. [ABSTRACT FROM AUTHOR]

We propose a new topology for printed circuit board metagratings (MGs) for manipulation of electromagnetic waves at microwave frequencies. By interleaving two single-band segmented gratings (SGs), a composite device capable of performing dual-band operation is realized. The constituent SGs are designed independently, meaning the two operating frequencies need not be harmonically related. The SGs are shielded from one another through inclusion of via fences which suppress near-field coupling. They are each engineered to be inactive within the other’s operating band, which helps retain their individually encoded functionalities. Using the proposed topology, a dual-band MG that performs perfect anomalous reflection of plane waves at 10 and 15.11 GHz is designed. The device is verified both numerically and experimentally. [ABSTRACT FROM AUTHOR]

According to the Equivalence Principle, any distribution of electromagnetic fields can be generated within a closed region by suitable electric and magnetic sources lying on its bounding surface. In electromagnetic theory, these sources have been used as fictitious analytical tools. However, this has changed in recent years with the development of "Huygens's metasurfaces," which are subwavelength thin sheets of artificial active and/or passive elements. With proper configuration, these elements can physically approximate the aforementioned sources, enabling precise manipulation of electromagnetic waves in extraordinary manners. Given their potential, this work aims to provide an overview of Huygens's metasurfaces with a focus on cloaking applications. After an exposition of general operation and design principles, both active and passive Huygens's cloaks are described. [ABSTRACT FROM AUTHOR]

A new topology for Huygens’ metasurfaces (HMSs) is proposed as a means for dramatically simplifying the design of metasurface-based devices. Traditionally, the constituent unit cells for such metasurfaces are designed and optimized with many assumptions such as local periodicity, normal incident rectilinearly propagating excitation, etc. For applications demanding precise wave transformation, it is necessary to further optimize all the cells collectively to compensate for the inaccuracies of these assumptions. For complex designs with many degrees of freedom, this can be extremely time-consuming. In this paper, we propose a new unit cell topology, which intrinsically enforces the aforementioned assumptions, enabling rapid construction of metasurfaces from a library of preoptimized cells without the need for any subsequent tuning. To illustrate the effectiveness of the proposed topology, several devices are designed and verified numerically. For practical demonstration, a wide-angle reflectionless refracting metasurface employing the proposed unit cells is fabricated and experimentally tested. [ABSTRACT FROM AUTHOR]

A systematic design method for high-order dual-band bandpass frequency selective surfaces (FSSs) with a low profile is derived from classical filter theory and presented here. To complement the design procedure, a multilayer double-slot resonator unit cell topology is proposed for realizing dual-band operations. For simplicity, the resonators are made to work for only a single polarization. To design the FSS, first, a classical dual-band bandpass filter circuit is designed by performing successive frequency transformations on a lowpass prototype. The filter is then transformed into a form resembling the equivalent circuit of the proposed multilayer FSS structure. Finally, the transformed filter is mapped to a set of FSS geometrical parameters. The method presents very few inherent limitations to realizing a diverse range of filter responses. The resulting designs lend themselves to fabrication since very few layers of metallization are required. Two FSSs with third-order passbands at 4 and 7 GHz but different passband characteristics are designed and verified numerically. One of the designs is fabricated and experimentally verified. The overall thickness of the designs is $0.08\lambda _{l}$ where $\lambda _{l}$ is the free-space wavelength at 4 GHz. The unit cell size is approximately $\lambda _{l}/8$. [ABSTRACT FROM AUTHOR]

A systematic technique for designing and optimizing multilayer frequency selective surfaces (FSSs) with low overall profile is presented. Periodic scatterers in the shape of loaded dipoles (dogbones) are used on each layer to create a single-stopband response. Multiple such layers are cascaded together to create the desired multistopband response. An equivalent circuit model for a multilayer FSS that explicitly and intuitively accounts for electromagnetic coupling interactions between the layers is proposed and investigated. This model is used in a novel design method, which precompensates for the effect of coupling during circuit-based design stage rather than postcompensating through iterative full-wave (FW) optimization after the design stage, as in most traditional approaches. As a consequence, this approach has the potential to greatly speed up the design process by enabling considerable simplifications during FW simulations. The proposed method is used to design several ultralow-profile triple-layer, triple-stopband surfaces intended for Wi-Fi applications. The interlayer spacing is as low as $\lambda /75$ at the highest operating band (5.2 GHz), making the overall thickness extremely small. The unit cell size for the designs is about $\lambda /5$ at 5.2 GHz. The designs are fabricated and tested to validate the proposed methodology. [ABSTRACT FROM PUBLISHER]