Diffraction of cylindrical and plane waves by an array of absorbinghalf-screens

Multiple forward diffraction past an array of many absorbing half-screens whose separation is large compared to wavelength is examined. Starting with the physical optics approximation for half-planes that are equally spaced and of equal height, the field incident on successive edges is represented by a multidimensional Fresnel integral, which is then expanded into a series of functions studied by Boersma (1978). When the angle of incidence with respect to the plane containing the edges is small, each edge is in the transition region of the previous edge, which precludes the use of the geometrical theory of diffraction and related asymptotic theories. The solution obtained applies for incidence either from above or below the plane containing the edges, and is especially suited to the case of near-grazing incidence. This method of solution allows for numerical evaluation of a large number of half-screens and shows how the multiple diffracted fields are influenced by the physical parameters. Both incident plane waves and incident cylindrical waves can be treated     

Scattering by thin dielectric strips

A plane wave incident on a thin dielectric strip with infinite length is considered, letting the incident electric field vector be parallel with the edges of the strip. The field is expanded in the dielectric region as the sum of three plane waves (the forced wave and two surface waves). Thex-axis andy-axis propagation constants are known for each wave, and Galerkin's method is employed to determine the amplitudes of these waves. Finally, the far-zone scattered field is determined by considering the polarization currents radiating in free space. Numerical data are presented to illustrate the scattering properties of lossless and lossy dielectric strips as a function of the angle of incidence and the width of the strip. The calculations show excellent agreement with an earlier moment method using pulse bases and point matching.     

High-frequency scattering from the edges of impedance discontinuities on a flat plane

A high-frequency solution is presented for the scattering of a plane wave at the edges of surface impedance discontinuities on a fiat ground plane. Arbitrary uniform isotropic boundary conditions and a direction of incidence perpendicular to the edges of the discontinuities are considered for both the transverse electric (TE) and transverse magnetic (TM) cases. An asymptotic approximation of the exact solution given by Maliuzhinets and a spectral extension of the geometrical theory of diffraction (GTD) are used. Uniform expressions for the scattered field received at a point on the surface are given, including surface wave contributions. Numerical results are shown and in some examples they are compared with those obtained from a moment method (MM) solution.     

Time Domain Analysis of the Near-Field Radiation of Shaped Electrically Large Apertures

The near-field radiation of several electrically large apertures has been computed based on the aperture field convolution method, including a square aperture, a circular aperture, and a serrated edge aperture. The frequency range is 8.0–12.0 GHz. The aperture diameters are limited to 5.00 m and the corresponding electrical size is 133–200 wavelengths. The time domain spectrum has been obtained by chirp z-transform with a Hamming window and the mechanism for the generation of the aperture near-field radiation in time domain is analyzed. The calculation results demonstrate that the near-field radiation of the aperture can be approximately seen as the synthesis of a plane wave from the aperture, cylindrical waves from the edges, and spherical waves from the corners. By this method, the direction, position, and magnitude of incoming waves in the near-field region can be estimated, and the aperture design can be modified to meet the requirements of the near-field radiation. The near field of the three apertures mentioned above has been compared, and it is shown that the serrated edge aperture has more uniform direct wave and lower diffraction waves, making it a candidate for compact range (CR) aperture design.      

Extensions to the hybrid method of moments/uniform GTD formulationfor sources located close to a smooth convex surface

We present an extension to the uniform geometrical theory of diffraction (GTD) for reflection from smooth curved surfaces. This approach allows the source to be much closer to the reflecting surface than the conventional uniform GTD formulation and does not require a Hertzian dipole source. In essence, the field point is mirrored in the plane tangential to the specular (reflection) point; the incident field is then calculated at the mirror point and the uniform GTD reflection coefficients are used to mirror this field to the original field point. This formulation reduces exactly to the conventional uniform GTD if the incident field is ray optical. The application to a hybrid method of moments (MoM)/GTD code is outlined and results computed using this code are presented for a dipole radiating in the vicinity of a cylinder     

Analysis of nonuniform nonlinear distributed feedback structures using a simple numerical approach

A simple numerical approach previously proposed for calculating the input-intensity dependence of the reflectivity and transmissivity of a nonlinear dielectric slab for incident plane electromagnetic waves is adopted for analyzing nonuniform nonlinear distributed feedback (NLDFB) structures. The method is first validated by checking with the analytic solution for the bistability characteristics of a strictly periodic, uniform NLDFB structure as well as the reported transmission characteristics of linear structures based on transfer matrix method calculation. The method is then applied to determine the transmission bistability characteristics of linearly tapered and linearly chirped NLDFB structures. The results are found to be different in some cases from those based on a generalized transfer matrix calculation in which the nonuniform structure is approximated by a set of strictly periodic, uniform segments.     

Theory of the directional pattern of a pyramidal horn

A method is presented for calculating the directional pattern of a pyramidal horn antenna from first principles by diffraction theory. It is based on first finding the field in the plane of the aperture, both inside and outside the physical aperture. This is then transformed to find the far-field pattern. The field in the aperture plane is made up of contributions from the primary wave reaching the aperture, whose phase surfaces are assumed to be spherical, edge waves propagating in the plane of the apertures and edge waves that have been reflected from inside the horn. The edge waves traveling in the plane of the aperture are the main object of interest. They propagate not only perpendicular to the edges, but also diffract parallel to the edges because of the finite length of the edges. By taking this fully into account, a directional pattern over the whole forward hemisphere can be predicted for any given frequency from the geometry of the horn. For a particular horn the principal cuts agree with direct measurements made at the National Physical Laboratory, Teddington, UK, to within about 4 dB (except near deep minima)     

Calculation of the high-frequency diffraction by two nearby edges illuminated at grazing incidence

The uniform geometrical theory of diffraction (UTD) together with a generalized spectral extension arc applied to calculate the high-frequency scattering by two nearby edges illuminated at grazing incidence. Several examples arc considered which involve the diffraction by a pair of parallel edges where one edge is illuminated by the shadow boundary field of the other. Expressions for the diffracted field have been obtained for plane, cylindrical, and spherical wave illumination with either the electric or magnetic field perpendicular to the edges. Extensive numerical results are given for a pair of staggered parallel half-planes, a thick screen, and a rectangular cylinder. Comparisons with the results calculated by other techniques are also presented to demonstrate the accuracy of the method.     

Scattering of TE-polarized EM wave by discontinuity in groundeddielectric layer

The two-dimensional problem of EM wave interaction with a dielectric discontinuity in an infinite grounded dielectric layer is studied. An electric field integral equation (EFIE) for TE illumination has been derived based on the Green's function for the electric field produced by induced polarization currents in the discontinuity region. Impressed electric fields consist of either plane waves incident from space above the dielectric layer or surface waves supported by that layer. Method of Moments (MoM) numerical solutions for the induced electric field in the discontinuity region are implemented. The amplitudes of surface waves excited by excess discontinuity-region polarization currents are calculated, as well as the pattern of the scattered field and the associated scattering width. It is observed that the excitation of a surface-wave mode reduces the back scattered radiation for TE-polarized plane wave incidence. The accuracy of the theory is verified by comparison of numerical results with those of existing studies     

A new representation for the Green's function of multilayer media based on plane wave expansion

In this paper, a new representation for the space domain Green's function of general multilayer media is presented. This approach is based on an efficient plane wave expansion of the source incident field. Using the transmission line model for the multilayer medium, the effect of the layered medium on the incident plane waves is determined by a transmission coefficient, and the amplitude and phase of each plane wave at the field point are obtained. The total field is evaluated by summing the resultant plane waves at the field point. The main advantage of the proposed method is that the spatial domain Green's function of multilayer media can be obtained easily as a summation of simple exponential functions, without the need for Sommerfeld integration or complex image approximation. The plane wave approximation is independent of the parameters of the medium and is valid over a wide frequency range. Furthermore, the exponential form of the plane wave solution makes it possible to compute the method of moments matrix elements analytically for most important types of basis functions in multilayer problems.     

Pulsed field diffraction by a perfectly conducting wedge: aspectral theory of transients analysis

The canonical problem of pulsed field diffraction by a perfectly conducting wedge is analyzed via the spectral theory of transients (STT). In this approach the field is expressed directly in the time domain as a spectral integral of pulsed plane waves. Closed-form expressions are obtained by analytic evaluation of this integral, thereby explaining explicitly in the time domain how spectral contributions add up to construct the field. For impulsive excitation the final results are identical with those obtained previously via time-harmonic spectral integral techniques. Via the STT, the authors also derive new solutions for a finite (i.e., nonimpulsive) incident pulse. Approximate uniform diffraction functions are derived to explain the field structure near the wavefront and in various transition zones. They are the time-domain counterparts of the diffraction coefficients of the geometrical theory of diffraction (GTD) and the uniform theory of diffraction (UTD). An important feature of the STT technique is that it can-be extended to solve the problem of wedge diffraction of pulsed beam fields (i.e., space-time wavepackets)     

Reflection and transmission of electromagnetic waves normally incident on a plasma slab moving uniformly along a magnetostatic field

Power reflection and transmission coefficients are found for linearly and circularly polarized plane electromagnetic waves, normally incident on a plasma slab, moving uniformly along a magnetostatic field, normal to the slab boundaries. The solution is found by applying the boundary conditions in the rest frame, and then using relativistic transformations for the fields and the plasma parameters to find the reflection and transmission coefficients observed in the laboratory frame. The results for the circularly polarized incident waves are found in closed form. Numerical results are presented for linearly polarized incident waves. It is found that with an increase in the magnetostatic field, the absolute maximum of the reflection coefficient increases at different velocities. An increase in the magnetostatic field makes the slab more transparent at velocities for which the transmission coefficient with no magnetostatic field is very small. A dielectric-like behavior is observed for large magnetostatic fields. The sum of the power reflection and power transmission coefficients is found to be no longer equal to unity for velocity different from zero.     

A concise conjugate gradient computation of plate problems withmany excitations

One of the major difficulties in the application of the conjugate gradient algorithm for the analysis of electromagnetic scattering problems is the necessity to carry out the calculation separately for each incident wave. In the approach suggested, rather than handling the incident waves directly, a class of possible excitations is represented by a set of strip-type basis functions. For these functions, convergence is predictable and rapid because the majority of the strips are located away from the edges of the scatterer. This choice also facilitates the use of the physical optics approximation as a good initial guess. Once the solutions for all the unit basis functions over the body are known, they can be combined to synthesize the solution for any excitation using the weighting coefficients associated with the expansion of the incident field. Numerical examples are given, and they demonstrate the substantial savings achieved by adopting this approach for the analysis of multiple excitations     

Diffraction of Obliquely Incident Plane Waves by Resistive and Conductive Planar Surfaces

The multiple diffraction of obliquely incident plane waves by the three-part resistive and conductive plane is evaluated via uniform asymptotic high frequency analysis. This analysis is based on a spectral iteration technique consisting of employing the Fourier Integral representation of the diffracted field through the solution of a pair of uncoupled Wiener-Hopf equations. By studying these equations iteratively, the double and triple diffracted fields are obtained through the saddle-point technique. Some numerical examples are also presented.     

An Approximate Solution to Some Ferrite Filled Waveguide Problems with Longitudinal Magnetization

An approximate solution for the field structure and propagating modes in parallel plane, circular, and coaxial ferrite filled waveguide is presented. Bundles of plane waves are assumed to propagate in these structures which bounce back and forth along the guide. The solutions are classified into two types depending on the negative or positive equality of the incident and reflected waves. In the case of the circular guide the waves form a cone, and in the coaxial guide they form a frustum of a cone about the axis. The elemental plane waves are also assumed to satisfy Polder's relation and the boundary conditions at the guide walls. Simple relations are obtained with this equivalence for the propagation constant and the field. Comparison to rigorous theory is made in the case of the parallel plane and circular guide. Some experimental verification is presented for the completely filled coaxial waveguide.     

左手媒质的阻抗边界条件

从Maxwell方程出发,研究了平面波入射到空气和左手媒质的分界面上时,界面附近的电磁场,并将电场和磁场的关系表示为阻抗边界条件表达式,然后与右手媒质的阻抗边界条件进行对比,指出其形式上的统一和计算公式的区别。在此基础上,对多层媒质引用传输线法进行分析,并采用其给出的多层媒质的反射系数计算公式,计算了多层媒质内包含左手媒质时的表面反射系数,并对多层媒质包含左手媒质和右手媒质的情形进行了对比,研究了其反射系数幅度和相位的异同。最后将边界条件应用到阻抗半平面的散射场的计算中,探讨了平面波入射到单面涂敷左手媒质的阻抗半平面上时的散射场,并对场的计算公式作了简单分析。     

垂直入射均匀平面波的多层平板屏蔽效能分析

本文基于电磁场方法,详细导出多层平板屏蔽体对垂直入射均匀平面波的屏蔽效能计算公式。此外,提出计算屏蔽效能时,集肤深度和屏蔽体厚度的约束关系,给出用作计算机机箱的几种常用材料的屏蔽效能计算实例。     

A SPICE model for multiconductor transmission lines excited by anincident electromagnetic field

This paper describes a SPICE model that may be used for predicting the time-domain or frequency-domain voltages and currents induced at the terminations of a multiconductor transmission line (MTL) by an incident electromagnetic held. Explicit results for the entries in the SPICE circuit model are obtained for an incident uniform plane wave that may represent sources such as radio and television transmitters, radars, lightning, etc. The result relies on the transformation of the MTL equations into uncoupled modal lines by similarity transformations. The entries in the similarity transformations are provided for lossless lines. The model is implemented using controlled sources to implement the modal transformations and delay lines to implement the modal lines. If the model is implemented as a SPICE subcircuit model, the time-domain form of the incident field can be implemented as a source external to that subcircuit model so that changes in the line responses due to changes In the incident field waveform can be simulated without changing the subcircuit model. In order to avoid negative line delays, the result is restricted to incident waves having components of the propagation vector in the positive direction along the line. This restriction can be removed by simply reversing the line. The paramount advantages of the model are that both time-domain and frequency-domain results can be easily obtained with the existing SPICE code, and nonlinear loads, such as transistors and digital devices, as well as dynamic loads, such as inductors and capacitors, may be easily incorporated using the existing elements in the SPICE code. Predicted results for MTL's using the method are compared to those of the time-domain to frequency-domain transformation and finite difference-time-domain (FDTD) methods     

A uniform geometrical theory of diffraction for an edge in a perfectly conducting surface

A compact dyadic diffraction coefficient for electromagnetic waves obliquely incident on a curved edse formed by perfectly conducting curved ot plane surfaces is obtained. This diffraction coefficient remains valid in the transition regions adjacent to shadow and reflection boundaries, where the diffraction coefficients of Keller's original theory fail. Our method is based on Keller's method of the canonical problem, which in this case is the perfectly conducting wedge illuminated by plane, cylindrical, conical, and spherical waves. When the proper ray-fixed coordinate system is introduced, the dyadic diffraction coefficient for the wedge is found to be the sum of only two dyads, and it is shown that this is also true for the dyadic diffraction coefficients of higher order edges. One dyad contains the acoustic soft diffraction coefficient; the other dyad contains the acoustic hard diffraction coefficient. The expressions for the acoustic wedge diffraction coefficients contain Fresenel integrals, which ensure that the total field is continuous at shadow and reflection boundaries. The diffraction coefficients have the same form for the different types of edge illumination; only the arguments of the Fresnel integrals are different. Since diffraction is a local phenomenon, and locally the curved edge structure is wedge shaped, this result is readily extended to the curved wedge. It is interesting that even though the polarizations and the wavefront curvatures of the incident, reflected, and diffracted waves are markedly different, the total field calculated from this high-frequency solution for the curved wedge is continuous at shadow and reflection boundaries.