Anisotropic Surface Impedance Research Articles

Electromagnetic Analysis of Anisotropic Impedance Spherical Metasurfaces

We investigate the interaction of electromagnetic (EM) waves with a spherical conformal metasurface, described by a anisotropic surface impedance. To this end, a full-wave method is developed, based on the expansion of the fields in terms of spherical vector wave functions. Appropriate treatment of the anisotropic impedance boundary condition leads to sets of linear equations that allow for the determination of the involved EM fields. The developed method can be used for design and anal- ysis purposes, as well as for the study of metasurface-based EM scattering control. We examine a related application of a cloaking setup and show that anisotropy breaks the spherical symmetry and leads to interesting new effects, such as a polarization-dependent switch function. The presented results suggest that metasurfaces with appropriately engineered impedance tensors may lead to desired scattering responses that otherwise cannot be obtained with their isotropic counterparts.

Beam wakes for canonical chambers with anisotropic surface impedance

Beam wakes for canonical chambers with anisotropic surface impedance

Modal Expansion Analysis, Inverse Design, and Experimental Verification of a Broadband High-Aperture Efficiency Circular Short Backfire Antenna Loaded With Anisotropic Impedance Surfaces

In this paper, a broadband highly-efficient circular short backfire antenna (SBFA) loaded with cylindrical anisotropic impedance surfaces (AISs) is proposed. An efficient modal expansion method (MEM) is developed to facilitate the analysis of the AIS-loaded SBFA with a finite-sized flange, which considerably reduces the computational resources as compared to commercial full-wave solvers. By combining the MEM with a multi-objective genetic algorithm, the geometrical dimensions of the antenna and the dispersive properties of the AIS are optimized through an inverse design strategy. As a proof-of-concept example, an <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> -band AIS-loaded SBFA with a broad bandwidth and high aperture efficiency is demonstrated, showing a good agreement between measured results and simulation predictions. An aperture efficiency of greater than 83% is experimentally achieved over a broad <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> < -10 dB bandwidth of 35.3%, with a peak aperture efficiency of about 100%. Moreover, the underlying operating principle of the antenna is revealed by investigation of the main propagating modes in the SBFA. The extraordinary performance demonstrates that the proposed AIS-loaded SBFA represents a promising candidate for various wireless communication systems.

A Broadband Low-Profile Dual-Circularly Polarized Reflect-Array Based on a Single-Layer Microstrip Patch for Ka-Band Application

A broadband low-profile dual-circularly polarized (dual-CP) reflect-array (RA) cells covering both <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> -band and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ka</i> -band is presented in this paper. As an anisotropic impedance surface, the functional layer is designed based on multi-resonant structure and printed on substrate with a thickness of 0.762 mm. The circuit is composed of elliptical ring, arc-shaped and rectangular metal microstrip lines. The air layer between the functional layer and the metal plate can effectively extend the cells bandwidth (BW), and the overall profile is 1.8mm. The dynamic phase and Berry phase can be adjusted by changing the size of the arc-shaped microstrip lines and the rotation of the cells. The diffraction results show that the phase of left-handed (LH) CP and right-handed (RH) CP waves can be controlled independently and reflected by high purity with the cross-polarization BW of 64% (19.72 GHz ~ 38.44 GHz). A dual-CP RA with 1013 cells is designed to realize reflect beams at θ = -20° for LHCP wave and θ = 15° for RHCP wave at 30 GHz. Furthermore, the 3-dB gain BW can cover the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Ka</i> -band and is expected to be used in wireless and satellite communication systems.

Modal Analysis, Inverse-Design, and Experimental Validation of Bandwidth-Controllable Suspended Patch Antennas Loaded With Cylindrical Anisotropic Impedance Surfaces

In this article, bandwidth-controllable suspended patch antennas (SPAs) loaded by anisotropic impedance surfaces (AISs) are proposed. A highly efficient yet accurate semianalytical modal expansion method (MEM) is developed for calculating the input impedance and radiation properties of the proposed antenna, which greatly reduces the simulation time and consumed memory compared to a commercial full-wave solver. The MEM is further utilized by coupling it with a genetic algorithm for effectively performing inverse-design, i.e., optimizing AIS-loaded SPAs with different predefined frequency responses. Three proof-of-concept antenna examples are designed, fabricated, and characterized, including an ultrawideband (UWB) antenna, a dual-wideband antenna, and a band-notched UWB antenna. The operating principle is illustrated by investigating the resonant modes of the AIS-loaded SPAs. The measured results of the three antennas exhibit good agreement with theoretical predictions, demonstrating that all three antennas have vertically polarized conical patterns in the E-plane and omnidirectional patterns in the H-plane with a cross polarization of smaller than −15 dB in their respective operational frequency band(s). The good performance demonstrates that the proposed AIS-loaded SPAs are promising candidates for broadband and multiband communications.

Anisotropic impedance surfaces activated by incident waveform

Abstract Anisotropic impedance surfaces have been used to control surface wave propagation, which has benefited applications across a variety of fields including radio-frequency (RF) and optical devices, sensing, electromagnetic compatibility, wireless power transfer, and communications. However, the responses of these surfaces are fixed once they are fabricated. Although tunable impedance surfaces have been introduced by utilizing power-dependent nonlinear components, such a tuning mechanism is generally limited to specific applications. Here we propose an additional mechanism to achieve tunable anisotropic impedance surfaces by embedding transient circuits that are controllable via the type of incident waveform. By switching between the open and short states of the circuits, it is possible to separately control the unit-cell impedances in two orthogonal directions, thereby changing from an isotropic impedance surface to an anisotropic impedance surface. Our simulation results show that a short pulse strongly propagates for both x and y directions at 3 GHz. However, when the waveform changes to a continuous wave, the transmittance for x direction is reduced to 26%, although still the transmittance for y direction achieves 77%. Therefore, the proposed metasurfaces are capable of guiding a surface wave in a specific direction based on the incident waveform even with the same power level and at the same frequency. Our study paves new avenues regarding the use of surface wave control in applications ranging from wireless communications to sensing and cloaking devices.

Sensitivity analysis of rectangular waveguides with different anisotropic surface impedance boundaries using modal extended theory

This article presents a sensitivity analysis of rectangular waveguides loaded with anisotropic surface impedance boundaries on their vertical walls, by using modal extended theory (MET). Using this method, several dispersion diagrams are computed for various anisotropic impedance boundary conditions of the surface walls of the waveguides in order to assess how the slight changes on the surface impedance can affect on the bandwidth and the fundamental cutoff frequency of the waveguide. The results obtained with MET were validated by comparison with the commercial software ANSYS HFSS. The results show that small anisotropic variations of the surface wall impedances lead to variation on the fundamental cutoff frequency and the bandwidth of the waveguide. Most significantly, and compared to an isotropic metasurface case, the variation of the transverse impedance of the metasurface is the one that have more significant effect over the waveguide parameters. This is an useful information for the metasurface design in order to take into account the fabrication tolerance of these structures and how they can have an influence over the final constructed device.

Low-Profile Dual-Polarized Isoflux Antennas for Space Applications

Design, realization, and measurements of an X-band antenna for data downlink (DDL) from low Earth orbit (LEO) satellites are presented. This antenna has an isoflux radiation pattern and operates in dual circular polarization with cross-polar discrimination (XPD) higher than 18 dB on the operational bandwidth. Two solutions are first proposed: one realized by a metasurface with circular strips printed on a dielectric slab and another realized by concentric metallic corrugations. Starting from a basic scattering mechanism of a self-complementary ring-type metasurface excited by a perfectly polarized source, both these antennas are designed by optimizing an equivalent anisotropic impedance surface excited by a short hat feed. The corrugated antenna is realized and measured, confirming that the performances are compliant with the requirements for next-generation DDL antennas.

Anisotropic Impedance Surface-Enabled Low-Profile Broadband Dual- Circularly Polarized Multibeam Reflectarrays for Ka-Band Applications

We present the modeling, design, and experiments of a class of low-profile and broadband dual-circularly polarized (DCP) reflectarrays (RAs) with independent beamforming for CP waves with opposite handedness. The proposed DCP RAs are comprised by an array of unit cells containing two cascaded anisotropic impedance surfaces backed by a ground plane. The DCP phase compensation offered by the RA cells is enabled by simultaneously utilizing cell rotation-induced Berry phase and dimension variation-induced dynamic phase. Numerical investigations of the diffraction properties of a linear gradient array of the DCP cells reveal that the designed cells can offer independent and high-efficiency deflection of CP waves with opposite handedness over a bandwidth of about 40%. Two Ka -band DCP RAs with a profile of only $0.16\lambda _{0} $ are demonstrated. The single-beam DCP RA achieves a measured peak gain of higher than 30 dBic with a joint 1 dB gain and axial ratio < 0.5 dB bandwidth of around 30%, while the multibeam DCP RA exhibits a measured peak gain of about 26 dBic with a joint 1 dB gain and axial ratio < 1.5 dB bandwidth of more than 25%. The demonstrated broadband DCP RAs are promising candidates for various wireless and satellite communication systems.

Reconfigurable Dual-Beam Lensing Utilizing an EBG-Based Anisotropic Impedance Surface

An anisotropic impedance surface based on electromagnetic bandgap (EBG) theory is proposed to produce reconfigurable dual-beam radiation through surface-wave (SW) lensing by controlling SW propagation across the surface-to-air interface. The reconfiguration of the lensing effect is triggered by switching between 4.0- and 4.5-GHz frequencies, utilizing the anisotropy of the impedance surface created from two mutually exclusive EBGs along the two orthogonal spaces. The bandgaps are formed by a periodic arrangement of two different split-ring resonators (SRRs) with bandgaps at around 4.0 and 4.5 GHz along the orthogonal y- and x-axes, respectively, producing a strong axially symmetric anisotropy. As a result, a surface-bound wave, i.e., both transverse electric (TE) and transverse magnetic (TM), faces asymmetric propagation reluctance toward these two directions one of which is very high compared to the other at frequencies around the individual bandgaps of the unit cells. The proposed impedance surface is designed and simulated using commercial electromagnetic simulation software and then fabricated and measured to validate the model.

Integrated Broadband Circularly Polarized Multibeam Antennas Using Berry-Phase Transmit-Arrays for $Ka$ -Band Applications

In this article, integrated millimeter-wave (mmW) broadband circularly polarized (CP) single-beam/multibeam antennas (MBAs) are reported. The antennas comprised a Berry-phase (BP) transmit-array (TA) containing three layers of slotted elliptical metallic resonator arrays and a cluster of substrate integrated waveguide (SIW) -fed CP source radiators with an anisotropic impedance surface (AIS) superstrate. By utilizing broadband mmW CP radiators, BP-based transmission phase compensation, and tailoring the dispersive transfer function of the TA cell, broadband CP high-gain beams can be generated with improved gain flatness and an antenna profile of only $2.6\lambda _{0}$ . Moreover, by employing multifocal strategies in the design of TAs, the scanning loss for the multifeed MBAs can be alleviated, expanding their range of coverage. Two Ka -band prototypes were fabricated and characterized. The single-feed single-beam antenna experimentally achieves a peak gain of 22.6 dBic and a 2 dB gain bandwidth of 20.7%, within which the $S_{11}$ is below −10 dB and axial ratio (AR) is smaller than 3 dB. The second prototype, which contains five source radiators and a bifocal TA, generates five pencil beams that cover a range between ±33°. It features a peak broadside gain of 22.2 dBic, a joint AR $S_{11}$ is below −10 dB and mutual coupling is smaller than −20 dB. Finally, the numerical simulation of a broadband CP 21-beam antenna with a quad-focal TA is shown, which provides a 2-D coverage of 32° cone. The demonstrated single-beam antenna/MBA can be promising candidates for various mmW applications, such as satellite and fifth-generation (5G) communications.

Wideband Transmit Arrays Based on Anisotropic Impedance Surfaces for Circularly Polarized Single-Feed Multibeam Generation in the Q-Band

In this article, a class of wideband transmit arrays (TAs), composed of cascaded anisotropic impedance surfaces (AISs), for circularly polarized (CP) multibeam generation from a single feed horn are reported. The dispersionless phase compensation is achieved by the Berry phase (BP) via imposing a spatially dependent rotation angle on the TA unit cells. A homogenized model for the BPTA unit cell is proposed and utilized for obtaining a wideband response by tailoring the dispersive properties of the AIS layers. Two modeling methods, an analytical vectorial field analysis and a full-wave strategy incorporating the homogenized model, were employed to efficiently evaluate the performance of the BPTAs. In order to validate the proposed unit cell and the modeling methodologies, a Q-band single-beam BPTA is demonstrated, which achieves a peak gain of 30.2 dBi and a 1 dB bandwidth of 11.1% within which the axial ratio is smaller than 2 dB. Furthermore, by employing the intersection approach for pattern synthesis, several Q-band BPTAs supporting multiple concurrent symmetric/asymmetric CP pencil beams and circular-shaped flat-top beams are designed. A BPTA prototype for producing quad CP pencil beams with unequal gain values was fabricated and characterized, yielding good performance with an overall operational bandwidth of about 11%. The proposed BPTAs are promising candidates for point-to-multipoint communication and point-to-multiregional coverage in wideband millimeter-wave communications for wireless and satellite applications.

Analytical Study and Experimental Verification of the Refraction Angle as a Function of Frequency Due to Surface Waves Incident Onto a Tensor Impedance Sheet

In this paper, using the theory of tensor impedance boundary conditions, a surface structure with new applications is presented. The proposed structure includes a tensor impedance sheet (TIS) alongside a scalar impedance sheet (SIS) which is designed by arrays of cells containing subwavelength patterned metallic cladding over a grounded dielectric substrate. In this structure, a method is presented to study the electromagnetic surface wave (SW) passing through a boundary between an isotropic and anisotropic impedance surface. If an SW with a specified frequency is excited on a SIS and orthogonally propagates onto the boundary of a TIS, due to the refraction of the SW, the propagation direction of the SW on the TIS will change. Therefore, the SW will enter the TIS with a specified angle, which is called the refraction angle. In this paper, the frequency variations of the refraction angle are studied in the proposed structure. Finally using the presented method, a sample structure is designed and simulated in 15.1-15.75 GHz frequency. In addition, the structure is fabricated and the results of the proposed model are verified with simulation and measurement. Moreover, an equation with an analytical solution is presented in order to model tensor impedance boundary conditions using printed circuits. This can make TIS design quick and accurate.

Method for Extracting the Effective Tensor Surface Impedance Function From Nonuniform, Anisotropic, Conductive Patterns

A patterning technique known as the point shifting method has enabled the generation of smoothly varying and highly anisotropic impedance surfaces with a wide range of patch sizes and shapes. Previously, the surface impedances of different shapes of unit cells were assumed by the impedance of similar size rectangle cells. In this paper, we study an approach to calculate the surface impedances for anisotropic polygon unit cells more accurately, based on the area moment of inertia equations. We define an alternative unit cell called the equivalent rectangle, which has the same tensor impedance properties of a general polygon unit cell in the surface impedance pattern. The size of the equivalent rectangle cell is calculated by using the moment of inertia equations between the polygon and the rectangle. The extracted surface impedance from the equivalent rectangle is compared to the surface impedance of polygon in the unit cell simulation, validating our method. We also verify the method by comparing the results between PEC patterns and impedance boundary sheets to which the extracted impedances are applied. Simulations of the patterns are verified by measurements as well.

Design, fabrication, and measurement of an anisotropic holographic metasurface for generating vortex beams carrying orbital angular momentum.

In this Letter, to the best of our knowledge, a novel anisotropic surface impedance holographic metasurface antenna for generating orbital angular momentum (OAM) is developed through design, fabrication, and measurement at radio frequencies. The classical leaky-wave theory and a microwave holography method are combined to realize vortex waves carrying different OAM modes flexibly. The holographic metasurface composed of subwavelength quasi-periodic anisotropic metallic patches on a grounded dielectric substrate operates by exciting the interference patterns. The interferences are generated between a reference wave excited by the monopole antenna and a field with the desirably shaped wavefront-carrying helical phase. Numerical simulation has shown good agreement with the experimental results, which lays a solid foundation for holographic metasurface antennas having the potential for OAM generation at radio frequencies.

Simulation Analysis of Electromagnetic Surface Wave Suppression by Soft Surfaces, Including Effects of Resistive and Active Elements

The soft surface is defined by its anisotropic surface impedance, and it has the property of suppressing surface wave propagation for any polarization of the electric field. It can be used as a general design element for reducing scattering or controlling coupling between nearby antennas. In this letter, the suppression capabilities and bandwidth limitations of soft surfaces are analyzed using simulations to draw the following conclusions: First, a soft surface is as good as an ideal perfect magnetic conductor for suppressing vertically polarized surface currents; second, adding loss to the soft surface actually increases coupling between nearby antennas; third, the bandwidth of soft surfaces can be extended using active loading; and fourth, a soft surface boundary condition can suppress coupling below even the expected free-space wave coupling. Several simple examples are illustrated using simulations, including the effect of lossy substrates, designs for active loading, and the effects of soft surfaces on the contributions to coupling between nearby planar antennas.

&lt;italic&gt;X&lt;/italic&gt;-Band Circuit-Analog Absorbers Using Unidirectional Carbon Fiber

This letter explores single-ply pre-impregnated (pre-preg) unidirectional carbon-fiber-reinforced polymer (CFRP) laminas for use in X -band (8.2–12.4 GHz) absorbers. Specifically, the unidirectional CFRP sheets were used as lossy frequency selective surfaces in single- and two-layer circuit-analog absorbers (CAAs). The aim of this letter was to assess the feasibility of using only the fiber orientation and areal weight of the material as a tuning aid. A free-space materials characterization system was used to characterize the anisotropic complex permittivity and anisotropic surface impedance of two different areal weight unidirectional CFRP samples. The absorbers were prototyped in a mechanically reconfigurable fixture with the aid of a vacuum-bagging system. The reconfigurable approach allowed reflectivity measurement performance to be measured without the need to fabricate cured sandwich constructions. Reflectivity measurements were compared to transmission-line models as well as full-wave models developed using commercial simulation software. Regions of validity for the transmission line models were identified. The results show that single-ply pre-preg unidirectional CFRP laminas have the potential to be used in single- and two-layer CAA configurations for use in X -band absorbers.

Design and Experiments of Bandwidth-Controllable Broadband Monopole Antennas With Conformal Anisotropic Impedance Surface Coatings

In this paper, we report an ultrathin and lightweight coating approach for enhancing the impedance bandwidth of a conventional monopole antenna by using a single-layer dispersive anisotropic impedance surface (AIS). The design is facilitated by a rigorous and efficient modal-expansion analysis of a monopole coated by a homogeneous AIS, which is developed by approximating the AIS as a slab of anisotropic medium with a deep subwavelength thickness and introducing an auxiliary perfectly matched boundary above the AIS-coated monopole. This method is further utilized to efficiently optimize the dispersive properties of the AIS, which has a deep subwavelength radius, for tuning the impedance bandwidth of a monopole, thereby achieving a greatly broadened yet controllable bandwidth. Three AIS-coated antenna examples with different ratio bandwidths are illustrated, with their corresponding discrete realistic structures designed, fabricated, and characterized. The measured results agree strongly with simulation predictions, experimentally exhibiting a VSWR < 2 ratio bandwidth of 1.91:1, 2.79:1, and 3.81:1, respectively. The proposed antennas possess a stable omnidirectional pattern with a low cross-polarization level of less than −20 dB. The demonstrated AISs can serve as redeployable conformal coatings for obtaining broadband and ultrawideband antennas with a controllable bandwidth, which will be useful for various wireless systems, including the vehicle-to-X network.

Perfect reflection control for impenetrable surfaces using surface waves of orthogonal polarization

For impenetrable electromagnetic surfaces, a metasurface design approach for perfect control of the reflection phenomena using gradient anisotropic tensor surface impedance is presented. It utilizes a set of orthogonally polarized auxiliary surface waves to create pointwise reactive impedance characteristics by channeling power along the tangential direction of the surface in the near zone in a carefully designed manner. The propagating incident and reflected fields do not interfere with the surface waves due to the polarization orthogonality. Design examples of an anomalous reflector and a power splitter for an incident plane wave are presented and numerically verified. Realization possibilities using an array of rotated metallic resonators on a thin grounded dielectric substrate are discussed.

Reduced Electromagnetic Edge Scattering Using Inhomogeneous Anisotropic Impedance Surfaces

Electromagnetic scattering characteristics highly depend on the geometry and material property of the scatterers. In particular, electromagnetic wave scattering occurs at the discontinuity of the propagation path, such as the edge of a scatterer. In this paper, the edge scattering from a triangular object is largely reduced by patterning the triangular surface with inhomogeneous and anisotropic impedance surface. Because surface waves propagate toward the direction with the lower surface refractive index on an anisotropic surface, the direction of the wave propagation can be controlled by designing the distribution of the refractive index. Consequently, the proposed anisotropic impedance surfaces can make the current flows toward or away from the edge so that the edge scattering can be reduced or redirected to a different angle. For demonstration, anisotropic unit cells are analyzed and designed. Also, the anisotropic impedance surfaces are simulated, fabricated, and measured. Here we propose two types of impedance profiles, which are capable of changing the surface current direction either toward or away from the scattering edge. The experimental measurement demonstrates a 7–10 dB edge scattering reduction.