Sheet Impedance Research Articles

Investigation of Fano resonance in planar metamaterial with perturbed periodicity

In this paper, we report the formation of sharp Fano resonance in planar metamaterial array with perturbed periodicity. Rigorous sheet impedance theory is given to analyze the electric-magnetic and magnetic-magnetic coupling effects. It is found that periodicity perturbation can provide a general approach for Fano resonance with ultra-strong local field enhancement.

Modeling and Analysis of Printed-Circuit Tensor Impedance Surfaces

Analysis of a printed-circuit tensor impedance surface (PCTIS) is presented. The surface consists of a periodic, subwavelength-patterned metallic cladding printed over a grounded dielectric substrate. First, the dispersion equation for an idealized tensor impedance boundary condition is derived by expressing the field in terms of TE and TM waves. A similar method is then used to find the dispersion equation of the PCTIS consisting of a tensor sheet impedance, which models the metallic cladding, over a grounded dielectric substrate. In addition, a method for extracting the tensor sheet impedance of a periodic, metallic cladding printed over a grounded dielectric substrate, is reported. It involves performing two normal-incidence scattering simulations using a full-wave electromagnetic solver. The method is strictly valid when the ground plane is sufficiently far from the metallic cladding to avoid evanescent-wave interactions. By combining the tensor sheet extraction method with the dispersion equation, the full dispersion characteristics of the PCTIS are analytically predicted in the homogenous limit. The results are verified through full-wave eigenmode simulations.

A Circuit Model for Electrically Small Antennas

A circuit model for electrically small antennas is introduced that is based on their frequency-dependent polarizabilities. This model is useful for straightforwardly analyzing several different small antenna geometries. A negative permittivity sphere, shell, and spheroid are all analyzed. An inductively loaded dipole, a top-hat loaded dipole, and a spherical sheet impedance are also analyzed. The circuit model provides the antenna's radiation quality factor (Q), radiation efficiency (ηrad), and bandwidth. It also offers insight into the operation of the antenna which can aid and simplify design.

HIGH FREQUENCY SCATTERING BY A SECOND-ORDER GENERALIZED IMPEDANCE DISCONTINUITY ON A CYLINDRICALLY CURVED SURFACE

The aim of the present paper is to obtain explicit asymptotic expressions for the \transfer (difiraction) related to the difiraction of high frequency cylindrical waves from the discontinuity occurred in the material properties as well as in the thicknesses of a coated cylindrically curved metallic sheet characterized by the second order GIBCs. Relying on the locality of the high frequency difiraction phenomenon, the angular interval ' 2 (i…;…) is extended to the abstract inflnite space ' 2 (i1;1) wherein the difiracting structure is replaced by a two-part cylindrically curved second order impedance sheet ‰ = a extending from ' = i1 to ' = 1. The resulting boundary value problem is formulated as a Hilbert equation which is solved asymptotically in the high frequency limit. Some graphical results showing the efiects of various parameters on the transfer coe-cients are presented.

Adapting Generally Filled Cylindrical Sheet Impedances for Electromagnetic Compatibility [Measurements Corner

The ability to control the bistatic scattering characteristics from cylindrical objects using a tapered cylindrical impedance sheet is presented. It is shown that with the capability to readily control both the real and imaginary components of a percolated lossy film's effective impedance, specific impedance distributions can be obtained to effectively introduce and direct the overall nulls in the bistatic scattering pattern. The bistatic scattering width calculations are based on a modal analysis and an impedance-sheet boundary condition formulation. Potential uses of tapered cylindrical impedance sheets in bistatic EMC/EMI applications are discussed.

Radio-frequency characteristics of graphene oxide

We confirm graphene oxide, a two-dimensional carbon structure at the nanoscale level can be a strong candidate for high-efficient interconnector in radio-frequency range. In this paper, we investigate high frequency characteristics of graphene oxide in range of 0.5–40 GHz. Radio-frequency transmission properties were extracted as S-parameters to determine the intrinsic ac transmission of graphene sheets, such as the impedance variation dependence on frequency. The impedance and resistance of graphene sheets drastically decrease as frequency increases. This result confirms graphene oxide has high potential for transmitting signals at gigahertz ranges.

Analysis of far-infrared horns, lightpipes, and cavities containing patterned conductive films

A scheme is described for calculating the scattering parameters of patterned conductive films in waveguide. The films can have non-uniform, non-isotropic, and non-local sheet impedances. Once the scattering parameters are known, they can be combined with the scattering parameters of paths, dielectric slabs, and waveguide steps to build up models of complicated components comprising patterned films in profiled lightpipes and cavities. It is then straightforward to calculate the Stokes fields of the total reception pattern, the natural optical modes to which the component is sensitive, the Stokes fields of the individual natural modes, and the spatial state of coherence. The method is demonstrated by modeling an absorbing pixel in a length of shorted multimode waveguide. The natural optical modes change from being those of the waveguide to those of a free-space pixel as the size of the absorber is reduced.

Negative refraction by a uniaxial wire medium with suppressed spatial dispersion

Here, we show that a structure formed by an array of metallic wires loaded with metallic patches may enable strong negative refraction even when the conductivity of the metal is very large, e.g., in the microwave regime. It is shown that the composite structure may be regarded as a uniaxial indefinite material loaded with impedance sheets, and that the negative refraction effect may be characterized using homogenization methods.

A Novel Method for Determining the R-Card Sheet Impedance Using the Transmission Coefficient Measured in Free-Space or Waveguide Systems

Free-space and rectangular waveguide techniques for determining the effective complex permittivity and, ultimately, the effective sheet impedance of an R-card using the forward transmission coefficient are presented. The advantage of using a transmission coefficient method instead of a more traditional reflection-based technique is discussed. The exact transcendental expressions relating the transmission coefficient and effective complex permittivity are derived and approximated using the Maclaurin series for sine and cosine. It is shown that the Maclaurin series expansion leads to simple closed-form solutions to the effective complex permittivity and avoids the use of sensitive and often unstable root search algorithms, which are necessary to solve transcendental equations. The accuracy of the approximations is directly related to the R-card's thickness and wavenumber. Free-space (4-16 GHz) and waveguide (8.2-12.4 GHz) measurements are made using two R-cards of differing thicknesses and impedances to demonstrate the method and regimes of validity. An uncertainty analysis is also performed to demonstrate the robustness of the technique.

Superconducting Films for Absorber-Coupled MKID Detectors for Sub-Millimeter and Far-Infrared Astronomy

We describe measurements of the properties, at dc, gigahertz, and terahertz frequencies, of thin (10 nm) aluminum films with 10 Omega/square normal state sheet resistance. Such films can be applied to construct microwave kinetic inductance detector arrays for submillimeter and far-infrared astronomical applications in which incident power excites quasiparticles directly in a superconducting resonator that is configured to present a matched-impedance to the high frequency radiation being detected. For films 10 nm thick, we report normal state sheet resistance, resistance-temperature curves for the superconducting transition, quality factor and kinetic inductance fraction for microwave resonators made from patterned films, and terahertz measurements of sheet impedance measured with a Fourier Transform Spectrometer. We compare properties with similar resonators made from niobium 600 nm thick.

Subwavelength Imaging Using Conducting Sheets

Here we demonstrate that, paradoxically, subwavelength imaging can be produced by purely resistive means. Space acts like a low pass filter for highly evanescent field components, and if a sheet or thin layer of imperfectly conducting material is placed adjacent to a source, such that the layer overcomes the larger impedance of the spatial low pass filter, no relative attenuation of evanescent components is experienced at the location of the sheet. This results in near-field subwavelength imaging, which also holds for reactive sheets. The conducting layer enables us to trade definition for amplitude. Impedance sheets are commonplace in radio frequencies or microwaves, hence the phenomenon identified here is widespread, and can be extended into the IR or optical region, as well as to other areas of physics where wave motion exists.

DIFFRACTION BY A WEDGE OR BY A CONE WITH IMPEDANCE-TYPE BOUNDARY CONDITIONS AND SECOND-ORDER FUNCTIONAL DIFFERENCE EQUATIONS

DIFFRACTION BY A WEDGE OR BY A CONE WITH IMPEDANCE-TYPE BOUNDARY CONDITIONS AND SECOND-ORDER FUNCTIONAL DIFFERENCE EQUATIONS

An Analytical Characterizzation of Metal Foams for Shielding Applications

The preliminary electromagnetic shielding behavior of metal foams, a novel class of materials, is developed and discussed. The use of metal foams in electromagnetic applications is a new fleld of scientiflc investigation. More speciflcally, the realization of electromagnetic shields has been considered and experimental Shielding Efiectiveness (SE) measurements have been performed, demonstrating very good performance. In order to allow the design of metal foam EM shields, a double 3D wire-mesh screens, obtained as a development of previously 2D laminated shield, has been present, compared the relative results with experimental measure. The good agreement between the preliminary results, encourages the development of an e-ciency analytical model of the complex electromagnetic behavior of metal foams. The analysis of the shielding properties of difierent kinds of Duocelr aluminium foams slabs obtained by varying porosity and apparent density, has been investigated, performed through ex- perimental measurements, and their shielding properties has been shown and discussed (2). Metal foams are complex an random structures which requiring sophisticated analytical mod- els. Moreover, because of its versatility and its capability to deal with heterogeneous media, the Variable-Mesh Finite Difierence Time Domain (VM-FDTD) method, is naturally the most ap- propriate approach (2), though unfortunately, it is computationally onerous. The 2D laminated wire-mesh screen model, developed by Casey (3) and compared with commercial aluminium shield perforated periodically with apertures (4), was a flrst step to solve the EM problem of the rigor- ous evaluation of the metal foam's shielding efiectiveness (5). Encouraged by the results, we have improved the previously 2D model, developing a preliminary analytical 3D models. Therefore the electromagnetic shielding behavior of a slab of metal foam, has been investigated considering a model with a double wire-screens mesh, separated by an air space. The single screen, whose meshes are assumed to be square, is described by an equivalent sheet impedance operator as mentioned in (3). The agreement of both experimental and theoretical data is a challenging to optimize this model in 3D.

Effect of standing stubble on radar backscatter from harvested rice fields

The radar backscatter from standing rice stubble in a harvested rice field is computed numerically using the method of moments with the equivalence principle and impedance sheet theory. The radar backscatter from a bare surface is also computed based on an empirical model, and combined with the numerical results from the standing rice stubble. Based on the analyses of the radar backscatter for various conditions of standing stubble, it is found that the effect of the standing stubble in a typical harvested rice field is negligible when the water content of the stubble is less than about 40%.

Analysis of Acousto-Electromagnetic Wave Interaction Using Sheet Boundary Conditions and the Finite-Difference Time-Domain Method

Acousto-electromagnetic wave interaction occurs when an electromagnetic wave scatters from an object under seismic or acoustic illumination. The vibration of the object under acoustic excitation gives rise to a frequency modulated scattered field which depends on both the object and electromagnetic and acoustic source parameters. The objective of this study is to accurately calculate the Doppler spectrum of the bistatic scattered field which is usually orders of magnitude smaller than the fundamental component of the scattered field (the stationary target response). In this analysis the recently developed sheet boundary conditions are used to set up a duplicate problem of a stationary object having time varying sheet impedance and admittance accounting for the object vibration. The problem is formulated using the two-dimensional finite-difference time-domain (FDTD) method in an iterative scheme in order to find the broadband response of the scattered field Doppler component. Two-dimensional analytical solutions for a canonical geometry are used to verify the FDTD simulation results.

Electromagnetic scattering from vibrating penetrable objects using a general class of time-varying sheet boundary conditions

Calculation of electromagnetic (EM) scattering from vibrating penetrable cylinders of arbitrary cross-section is presented using a general class of time-varying sheet boundary conditions (SBCs) in conjunction with the method of moments (MoM). Sheet impedance and admittance expressions are first derived fromthe exact scattering solution for a penetrable circular cylinder with perturbed radius. Then, using the SBCs, integral equations are derived and solved numerically so that vibrating cylinders with arbitrary cross-section can be treated. Cylinder vibrations are assumed to be non-relativistic, allowing a simplified calculation of the scattered Doppler spectrum. A critical factor in the calculation of the potentially small Doppler components is that the time-varying nature of the cylinder boundary, contained within the sheet impedance and admittance expressions, can be isolated from the unperturbed terms in the scattered field. Comparison with exact and analytical perturbation solutions are presented to demonstrate the accuracy of the numerical solution.

Finite-difference time-domain modeling of frequency selective surfaces using impedance sheet conditions

Novel finite-difference time-domain (FDTD) models of frequency-selective surfaces (FSS) based on impedance sheet conditions are developed. The analytical basis of the models lies in impedance sheet conditions with general reactive grid impedances applying to a great variety of grid realizations. New models for periodic arrays of metal particles and for the complementary structures of slots in metal screen are formulated for FDTD in the case of normal incidence. The properties of the FSS are included in the grid impedance, which is implemented into FDTD, considerably simplifying the otherwise extremely cumbersome modeling task. The convergence and the accuracy of the models are assessed with numerical simulations by comparing with analytical and measured reference results.

Diffraction of a Normally Incident Plane Wave by an Impedance Wedge With Its Exterior Bisected by a Semi-Infinite Impedance Sheet

This paper reports an application of a previously proposed procedure to diffraction of a normally incident, arbitrarily polarized plane electromagnetic wave by a canonical structure which consists of a wedge with different face impedances and a semi-infinite impedance sheet bisecting the exterior of the wedge. The use of the Sommerfeld-Malyuzhinets technique converts the original boundary value problem into a system of linear equations for two coupled spectral functions. Eliminating one of them, we get a second-order difference equation for the other spectral function. From this function and the boundary condition on the upper wedge face we construct an even and in the basic strip regular new spectral function. Then we transform the second-order equation into a simpler one by means of a generalized Malyuzhinets function /spl chi//sub /spl Phi//(/spl alpha/), and express the solution to the latter in an integral form with help of the so-called S-integrals. Solving a Fredholm equation of the second kind for points on the imaginary axis of the complex plane, which follows from the integral representation, enables one to compute the sought-for function. The second spectral function is obtained via its dependence upon the first one. We present a first-order uniform asymptotic solution, as well as numerical results.

FDTD Modeling of Arrays and Grids of Lossy Conductors Based on Impedance Sheet Conditions

Fast finite-difference time-domain (FDTD) modeling techniques for arrays of parallel wires and wire meshes are developed. The analytical basis of the models lies in the use of impedance sheet conditions (Kontorovich conditions) for dense wire arrays and grids, connecting the tangential electric fields and the averaged currents in the plane of a wire array or grid. The proposed methods allow us to avoid the direct discretization of the fields inside the wires, greatly simplifying the modeling task. The conductor losses in the wires are accounted for assuming a strong or weak skin effect. The impedance sheet conditions are formulated in the time-domain using the convolution integral, which is evaluated recursively in FDTD. Numerical examples and comparisons with analytical results are provided to verify the proposed techniques.

Enhanced transmission through two-period arrays of subwavelength holes

A two-period array of subwavelength holes in a perfect electrically conducting plate permits enhanced transmission of incident plane waves with wavenumbers near those of leaky waves supported by the array. A simple periodic impedance sheet model explains this phenomenon and provides an approximate expression for the transmission peak location. Transmission through thin perforated plates is studied using a full-wave solver to verify model-theoretic observations. Transmission coefficient peak splitting and narrowing phenomena, associated with thick perforated plates, are explored.