Equivalent Surface Impedance Research Articles

Atmospheric Sound Propagation over Rough Sea: Numerical Evaluation of Equivalent Acoustic Impedance of Varying Sea States

This work presents a numerical study on atmospheric sound propagation over rough water surfaces with the aim of improving predictions of sound propagation over long distances. A method for generating pseudorandom sea profiles consistent with sea wave spectra is presented. The proposed method is suited for capturing the logarithmic nature of the energy distribution of the waves. Sea profiles representing fully developed seas for sea states 2, 3, 4, and 5 are generated from the Elfouhaily et al. (ECKV) sea wave spectra. Excess attenuation caused by refraction and surface roughness is predicted with a parabolic equation (PE) solver. A novel method for estimating equivalent effective impedance based on PE predictions at different sea states is presented. Parametric expressions using acoustic frequency and significant wave height are developed for effective surface impedances. In this work, sea surface roughness is on a scale comparable with the acoustic wavelength. Under this condition, the acoustic scattering is primarily incoherent. This work shows the limitations of using an equivalent surface impedance in such incoherent scattering cases.

Broadband Chiral-Type Linear to Linear Reflecting Polarizer With Minimal Bandwidth Reduction at Higher Oblique Angles for Satellite Applications

This work presents a broadband linear–cross polarizer for K- and Ka-band applications. The design consists of a simple asymmetric width-based meander-line metasurface printed on a thin FR-4 grounded dielectric substrate. Normal incidence demonstrates 90% polarization conversion ratio (PCR) bandwidth (BW) of 20.64 GHz from 18.31 to 38.95 GHz with 72.1% fractional BW (FBW). Due to the destructive interference involved at greater oblique angles, reflective polarizers experience severe degradation of 90% PCR BW response. The proposed polarizer’s performance is obliquely stable up to 43° for transverse electric (TE) and 45° for transverse magnetic (TM) incidence with minimal 90% PCR BW reduction of only 13.95% (TE) and 15.25% (TM) compared with normal incidence. For the first time, transfer matrix method (TMM)-based equivalent surface impedance technique is modeled for oblique incidence analytically, closely resembling the full-wave analysis. The design is compact with a periodicity of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda \text{o}$ </tex-math></inline-formula> /5.98 and a thickness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda \text{o}$ </tex-math></inline-formula> /13.69. The surface current patterns at resonant frequencies illustrate the reason behind broadband behavior. Bistatic radar cross section (RCS) analysis of the proposed converter is studied with reference to PEC. To the best of the author’s knowledge, this structure demonstrates broadband response with wide angular stability with less than 90% PCR BW reduction at higher oblique angles reported so far.

Modeling Large-Size Round Steel Plates With Surface PEEC Method for Lightning Transient Analysis

The round steel plate is the basic component of the external floating roof tank structure. Modeling large-size round steel plates is essential for analyzing the lightning response of external floating roof tanks. This article builds time-domain equivalent circuit models for round steel plates excited by the lightning current. First, a meshing scheme is presented to generate sector and cylindrical surface cells, which make use of the distribution characteristics of the currents and voltages on the round steel plates. Then, the surface partial element equivalent circuit (PEEC) approach is applied to model the plate. A method of calculating the equivalent surface impedance is proposed, which converts the frequency dependence of the skin effect into impedance based on the time domain. Finally, the time-domain surface PEEC approach is validated by the indirect time-domain solution based on the Fourier transform and the commercial software using the finite-difference time-domain technique, respectively. The comparisons show that the calculation results of different methods are consistent with each other. However, the proposed method in this article can greatly enhance the computational efficiency while meeting the accuracy requirements. It has great significance to the study of lightning protection for external floating roof tank.

A new leaky wave antenna using wire‐loaded metagratings: The theoretical analysis and implementation

SummaryA new leaky wave antenna (LWA) based on wire‐loaded metagratings (MGs) is proposed, with the capability of successfully employing MGs for leaky modes. Periodic capacitive‐wired and inductive‐wired MGs are located on a dielectric substrate in the LWA framework and backed by a perfect electric conductor (PEC) layer. These MGs are partial radial surfaces to radiate energy using equivalent surface impedance with a local value of the complex wavenumber. A theoretical analysis has created a model for the equivalent electromagnetic fields produced via loaded‐wire MGs and generated the Z‐matrix and S‐matrix for a two‐port structure from the electromagnetic fields. As a result, a beam‐scanning range of 53° for the operating bandwidth of 9.9–11.9 GHz is achieved with a scanning sensitivity of 26.5°/GHz. Meanwhile, the realized gain is obtained up to 12.9 dB. The measurement and simulation results indicated that the MGs LWA was consistent. The proposed antenna performed admirably in beam‐steering communication systems.

On the surface impedance modeling of metasurfaces composed of graphene-coated spherical nanoparticles

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.

Analysis of an electrically reconfigurable metasurface for manipulating polarization of near-infrared light

Integrating plasmonic metasurfaces with transparent conductive oxide (TCO) materials provides a new, to the best of our knowledge, opportunity to dynamically manipulate all degrees of freedom of light beams. Applying external stimuli to TCOs changes the plasmonic response of the nanostructure as a result of establishing an inhomogeneous optical property with deeply subwavelength films of epsilon-near-zero media, which is a challenge for better theoretical understanding of the dynamical tuning functionality. In this paper, we propose a dynamically reconfigurable reflective metasurface with the capability of electrical manipulation of the polarization state of near-infrared light. The structure behavior is investigated by numerical analysis and described based on the coupled-mode theory concept using the parameters of resonance wavelength, absorption, and radiation quality factors, altered by an external voltage. Variation of the applied bias voltage from − 0.7 to 4.2 V for p -polarized light leads to a wide range of absorption quality factors from 43 to two. This change causes a dramatic decrease in the equivalent surface impedance of the metasurface in the ratio 4.9:0.2, which may pave the way to tailor polarization-manipulating functions such as circular-to-linear polarization conversion and cross-polarization conversion.

Scattering Evaluation of Equivalent Surface Impedances of Acoustic Metamaterials in Large FDTD Volumes Using RLC Circuit Modelling

Most simulations involving metamaterials often require complex physics to be solved through refined meshing grids. However, it can prove challenging to simulate the effect of local physical conditions created by said metamaterials into much wider computing sceneries due to the increased meshing load. We thus present in this work a framework for simulating complex structures with detailed geometries, such as metamaterials, into large Finite-Difference Time-Domain (FDTD) computing environments by reducing them to their equivalent surface impedance represented by a parallel-series RLC circuit. This reduction helps to simplify the physics involved as well as drastically reducing the meshing load of the model and the implicit calculation time. Here, an emphasis is made on scattering comparisons between an acoustic metamaterial and its equivalent surface impedance through analytical and numerical methods. Additionally, the problem of fitting RLC parameters to complex impedance data obtained from transfer matrix models is herein solved using a novel approach based on zero crossings of admittance phase derivatives. Despite the simplification process, the proposed framework achieves good overall results with respect to the original acoustic scatterer while ensuring relatively short simulation times over a vast range of frequencies.

Tensor Holographic Metasurface Using Addition Theory to Independently Manipulate Orthogonally Polarized Bessel Beams

AbstractA tensor holographic metasurface is proposed, to independently manipulate nondiffractive beams with orthogonal polarization and flexible propagation direction. To theoretically analyze the proposed topology, addition theory is developed to modulate surface impedance using tensor cells. Moreover, a polynomial formula is derived to express the equivalent surface impedance of tensor unit cells for pattern generation. To validate this concept, a tensor holographic metasurface is designed, fabricated, and measured. The well‐matched simulation and measurement results show that, for the first time, the proposed holographic metasurface with tensor unit‐cells, simultaneously generates two Bessel beams, not only with orthogonal polarization, but also with expected propagation direction rather than fixed along the broadside. Moreover, the proposed topology is promising to manipulate multiple beams not limited to two Bessel beams.

Equivalent Surface Impedance of an Infinite Periodic Array of Slot Impedance Loads Based on an Equilateral Triangle Cross-Section Cavity with a Dielectric Coating

In this paper, we consider a two-dimensional problem of determining the equivalent surface impedance of an infinite array of slot impedance loads with a dielectric layer. Each element of the array contains three regions: the first region occupies the entire half-space above the interface and the primary field is excited in this region by a plane wave; the second region does not contain excitation sources and is a dielectric layer; and the third region also does not contain exciting sources and is bounded by the walls of the cavity with a equilateral triangle cross section. The second and third regions are connected through one or several slots in a perfectly conducting screen located at their border. A strip conductor is located in the opening of each slot. The problem was solved by the method of integral equations, for the numerical implementation of which the Krylov–Bogolyubov method was used. Numerical results are presented as the dependences of the equivalent surface impedance on the geometric dimensions of the structure at fixed values ​​of the array period, the incidence angle of the electromagnetic wave, and the thickness of the dielectric layer. A comparative analysis of the obtained dependences with the characteristics of a similar array without a dielectric layer is performed.

Bandwidth limit and synthesis approach for single resonance ultrathin metasurfaces

Metasurfaces have emerged as a promising technology for the manipulation of electromagnetic waves within a thin layer. In planar ultrathin metasurfaces, there exist rigorous design methods, based on the equivalent surface impedance of patterned metallic layers on dielectric substrates. In this work, we derive a limit on bandwidth achievable in these metasurfaces, based on constraints that their meta-atoms should be passive, causal and lossless and that they should obey the time-bandwidth product rules of a single resonance structure. The results show that in addition to elementary design parameters involving variation of the surface impedance, the bandwidth is critically limited by the dielectric substrate thickness and permittivity. We then propose a synthesis method for broadband ultrathin metasurfaces, based on an LC resonance fit of the required surface impedance and experimentally verify a broadband dispersive structure at millimeter-wave frequencies. This results in a bandwidth enhancement of over 90%, relative to a reference metasurface created with the narrowband design process.

Electromagnetic characteristics of a metasurface composed of uncoated and coated spherical particles

A general theory for the reflection, transmission and equivalent surface impedance of a symmetric and non-reciprocal metasurface composed of dipole scatterers has been developed under oblique incidence. The proposed theory is then used for metasurfaces composed of uncoated and coated spheres under normal illumination. The materials considered for uncoated and coated spheres are realistic, i.e., magnetodielectric and Gallium Arsenide. Some specific designs of anti-reflector, reflector and absorber metasurfaces with the free space background have been discussed. It is also observed that isorefractive background media can be used to enhance the reflection magnitude from a metasurface composed of Gallium Arsenide coated magnetodielectric spheres as compared to the free space background under some specified conditions.

A Physical Optics Approach to the Analysis of Metascreens

Artificial screens based on metasurfaces (MTSs), also called metascreens (MetSs) are becoming a very popular tool for electromagnetic field manipulation. While significant research efforts have been devoted to the development of synthesis methodologies, less work has been done for the accurate modeling of real structures based on this concept. This paper presents a method based on a Physical Optics (PO) for the efficient description of the scattered field of metascreens consisting of a stack of MTS separated by dielectric layers. The derivation of the PO currents is based on the definition of proper transmisssion coefficients for the electric and magnetic fields which, in turn, relies on an equivalent transmission line model of the multilayer structure. In this model, the MTSs are represented through homogenized equivalent surface impedances. The proposed model takes into account the non-local transmission properties and the finite thickness and size of the MetS. The accuracy in the scattered field prediction has been verified through comparison with full-wave simulations.

THE MODAL EXPANSION THEORY APPLIED TO 3-D METAMATERIAL WAVEGUIDES CHARACTERIZATION

In this article, the Modal Expansion Theory (MET) is applied to 3-D metamaterial waveguides. The equivalent surface impedances of the metamaterial are computed thanks to an open software: GetDP, based on a 3-D Finite-Element-Method (FEM). This program is called during the MET algorithm, which allows considering the frequency and incidence angle dependency of the surface impedances of the metamaterial to compute the dispersion diagrams and the field cartography. To validate the dispersion diagrams obtained with this technique, another FEM commercial software (HFSS) is used as a reference.

Mixed Numerical Methodology for Evaluation of Low-Frequency Electric and Magnetic Fields Near Power Facilities

This paper develops a mixed finite-element, analytical solutions, equivalent surface impedance methodology for the computation of electric and magnetic field distributions near power facilities. The proposed method enables efficient consideration of conductor sag, ground surface irregularities, proximity effects of metallic components such as transmission line towers, and transformer tank walls as well as human presence. Application examples of this methodology are given, illustrating that under a single-circuit 400 kV transmission line, the maximum electric field intensity at 1.5 m height from the ground is generated at a distance of 10 m from the line center while the maximum current density induced in a human standing on the ground is developed in the ankle; the computed values are practically half of the admissible limits of 5 kV/m and 2 mA/m2, respectively, foreseen for general public exposure by international standards.

Equivalent Surface Impedance of an Infinite Array of Slot Impedance Loads Based on a Cavity Having the Cross-Section in the Form of an Equilateral Triangle

The two-dimensional problem about the excitation of an infinite array of slot impedance loads by a plane wave is considered. The aim is the determination of the equivalent surface impedance. Every array element is a slot in a perfectly conducting screen. The slot is loaded by a cavity having the cross-section in the form of an equilateral triangle. The problem is solved using the integral equation method, which is numerically realized with the use of the Krylov—Bogolyubov method. The dependences of the impedance on the slot dimensions, the cavity dimensions, and the plane wave incidence angle are calculated for the case of the H polarization of the incident wave. The dependences obtained for the considered construction of the impedance load contained in an infinite array are analyzed and simultaneously compared with the characteristics of a single impedance load.

Theoretical study of a metasurface-based sound absorber

A metasurface-based sound absorber panel is proposed to achieve high sound absorption with a deep sub-wavelength thickness and a relatively broad bandwidth. The panel composed of different periodically-arranged Helmholtz resonators has been designed by a radiation impedance model which takes the mutual radiation impedance into account. Numerical simulations based on COMSOL Multiphysics show that the radiation impedance model yields more accurate results compared with the traditional equivalent surface impedance method. Broad bandwidth of sound absorption is achieved under proper coupling by carefully tuning the geometry and distance between Helmholtz resonators.

Compact High-Efficiency Broadband Metamaterial Polarizing Reflector at Microwave Frequencies

A compact highly efficient broadband microwave polarizing reflector, employing a quasi-2-D metamaterial composed of a single layer of square split-ring resonators, is presented and its performance is reported. Numerical and experimental results show a cross-polarization average polarization conversion ratio of greater than 90% for normal incident linearly polarized waves over a frequency range of 8.2–23 GHz. Due to multiple resonances occurring over this frequency range, the polarizing reflector presents both broadband performance and high polarization conversion efficiency. Transfer matrix and equivalent surface impedance theory and modeling are discussed to explain the physical underpinnings of the reflector’s multiresonance behavior. These physical mechanisms are further analyzed by examination of simulated surface current distributions. The reflector is expected to be employed in applications where the control of the signal polarization is of central importance.

Analysis of coupling between magnetic dipoles enhanced by metasurfaces for wireless power transfer efficiency improvement

In this paper, we investigate the possibility of improving efficiency in non-radiative wireless power transfer (WPT) using metasurfaces embedded between two current varying coils and present a complete theoretical analysis of this system. We use a point-dipole approximation to calculate the fields of the coils. Based on this method, we obtain closed-form and analytical expressions which would provide basic insights into the possibility of efficiency improvement with metasurface. In our analysis, we use the equivalent two sided surface impedance model to analyze the metasurface and to show for which equivalent surface impedance the WPT efficiency will be maximized at the design frequency. Then, to validate our theory, we perform a full-wave simulation for analyzing a practical WPT system, including two circular loop antennas at 13.56 MHz. We then design a metasurface composed of single-sided CLSRRs to achieve a magnetic lensing based on the calculated equivalent surface impedance. The analytical results and full-wave simulations indicated non-radiative WPT efficiency improvement due to amplifying the near evanescent field which can be achieved through inserting the proposed metasurface.

Broadband Wireless Optical Nanolink Composed by Dipole-Loop Nanoantennas

In this paper, a broadband wireless optical nanolink with plasmonic optical nanoantennas is theoretically proposed and analyzed. The nanolink is formed by linear dipole-loop nanoantennas for transmitter and receiver. The analysis is performed using the linear method of moments with equivalent surface impedance, where we apply a voltage source in the transmitting antenna and connect a load in the receiving antenna. The power received in the load is investigated as a function of frequency and distance between transmitter and receiver. In addition, a comparison is made between this wireless nanolink with a bifilar optical transmission line. The results show that the proposed nanolink, with dipole-loop nanoantenna, can increase the operating bandwidth in the range of 179.1–202.5 THz, when compared with conventional nanolink based only on dipole antennas. In addition, wireless nanolinks, based on dipole or dipole-loop antennas, are more suitable than wired nanolink for distances above approximately 22 μm.

Equivalent Surface Impedance of an Infinite Periodic Array of Slot Impedance Loads with Dielectric Coating

The problem of plane-wave excitation of an infinite array of slot impedance loads with a dielectric layer is considered in order to determine the equivalent surface impedance. The solution of the problem is found by the method of integral equations, solved numerically by the Krylov–Bogolyubov method, which reduces the integral equation to a system of linear algebraic equations. The numerical results are presented in the form of dependences of the equivalent surface impedance on the geometric dimensions of the structure at fixed values of the period of the array, the angle of incidence, and the thickness of the dielectric layer. It is shown that the equivalent surface impedance can be controlled by varying the width of the slot and the strip conductor. The dependences obtained are compared with the characteristics of an array of slot impedance loads without a dielectric layer.