Modified diffraction coefficients for focusing reflectors

A modification to the halfplane diffraction coefficients is given for a plane wave incident on a curved screen. The result is used to calculate the near field of a parabolic reflector under plane-wave illumination. Comparison with the physical-optics method shows excellent agreement.     

Excitation of a circular loop through a small aperture

The problem of determining the current excited on a circular loop which is placed opposite a small circular aperture in a conducting screen, on which an electromagnetic plane wave is incident, is considered. It is shown that this geometry is unusual in that the resulting integrodifferential equation can be reduced to an explicit expression for the current. The total rate of electromagnetic energy flow in the radiation zone is also calculated, as is the angular dependence of the energy flux. Sample results are presented which demonstrate the resonant behavior of the system when the wavelength of the incident wave is equal to the loop circumference.     

非同轴多层介质椭圆柱体的电磁散射—任意方向斜入射电磁波

本文基于椭圆柱谐基函数展开，对非同轴任意多层介质椭圆柱体的电磁散射进行了严格地电磁场分析，利用Ｍａｔｈｉｅｕ函数及其加法定理，在各层边界面上进行场的匹配，获得了任意方面斜入射电磁波下电磁散射解析解。     

The behavior of the electromagnetic field near the edge of aresistive half-plane

The authors define the behavior of the electromagnetic field near the edge of a resistive half-plane, taken separately, as well as in conjunction with a perfectly conducting halfplane. The efficiency of accounting for the established field behavior while finding the solution of boundary problems of electrodynamics is illustrated by computational results     

A new approach to the electromagnetic diffraction problem of a perfectly conducting half-plane screen

It is well known that the magnetic vector potential of an electromagnetic (EM) field incident upon a perfectly conducting thin plate satisfies, on the plate, an inhomogeneous Helmholtz equation. Rahmat-Samii, Mittra et al., and also Wilton and Dunaway applied this fact to the numerical solution of the thin-plate EM diffraction problem, using a modified version of the electric field integral equation. They pointed out that the Helmholtz equation alone determines the vector potential on the plate only up to an unknown term which expresses the coupling of the current-density components, which is due to the influence of the edge of the plate. It is demonstrated that their method is applicable in the theoretical analysis of some EM plate diffraction problems as well. The method will be applied, in combination with the Wiener-Hopf method, to reproduce the well-known solution of the classical problem of diffraction of plane electromagnetic waves by perfectly conducting half-plane screens. This approach can be done directly in vector form for general, three-dimensional incident waves. The result is applied to the discussion of grazing incidence.     

An asymptotic solution of EM backscattering from a conducting sphere coated with a composite material

An asymptotic high frequency solution for the electromagnetic (EM) backscattered field produced by a plane wave incident on a perfectly conducting sphere coated with a thin composite material is derived in this paper. For the formulation of the incident and the reflected field the characteristics of the wave transformation and the line integral via the stationary phase method were applied, respectively, and the obtained results are cast in the ordinary ray formats of a geometrical optics field. Based on the Watson transform technique, the diffracted field is also formulated from the residue series solution of the problem and presented in a form suitable for the numerical calculation. The numerical results obtained from the derived asymptotic solution show excellent agreement with those from the rigorous eigenfunction solution.     

Diffraction by an anisotropic dielectric half-plane: a uniformasymptotic PO solution

A uniform solution is proposed to, describe the diffraction by a penetrable anisotropic dielectric halfplane illuminated at normal incidence by an electromagnetic plane wave. Resorting to second-order boundary conditions on a sheet simulating a special type of anisotropic dielectric thin layer, a physical optics (PO) approximation for the induced electric and magnetic surface currents is derived. Then, a uniform asymptotic evaluation of the corresponding radiation integral provides the diffracted field in terms of the standard transition function relevant to the uniform theory of diffraction. The effectiveness of the solution is proved by many numerical tests     

Numerical aspects of a dissipative inverse problem

Two numerical aspects of the solution of a one-dimensional electromagnetic inverse problem are considered: the numerical solution of delay integral equations and the sensitivity of the solution of the inverse problem to small changes in the data. In addition a numerical technique is developed for the solution of the direct problem in the time domain. The problem considered is one in which the conductivity and permittivity of the scatterer are continuous functions of depth. The incident field is a transverse electric (TE) plane wave of arbitrary shape, and the inverse problem uses the resulting reflected and transmitted transients to reconstruct the scatterer. For the sake of simplicity, a known scatterer is used to numerically generate the data required for the inverse problem. This is done by using the scattering operators for the problem. The scattering data thus obtained is used to formulate a generalized Gelfand-Levitan integral equation whose solution yields the conductivity and permittivity profiles of the scatterer. The sensitivity of this inversion process is investigated by altering the scattering data.     

Electromagnetic wave scattering by a system of two spheroids ofarbitrary orientation

An exact solution to the problem of the scattering of a plane electromagnetic wave by two perfectly conducting arbitrarily oriented prolate spheroids is obtained by expanding the incident and scattered electric fields in terms of an appropriate set of vector spheroidal eigenfunctions. The incident wave is considered to be a monochromatic, uniform plane electromagnetic wave of arbitrary polarization and angle of incidence. To impose the boundary conditions, the field scattered by one spheroid is expressed in terms of its spheroidal coordinates, using rotational-translational addition theorems for vector spheroidal wave functions. The column matrix of the scattered field expansion coefficients is equal to the product of a square matrix which is independent of the direction and polarization of the incident wave, and the column matrix of the known incident-field expansion coefficients. The unknown scattered-field expansion coefficients are obtained by solving the associated set of simultaneous linear equations. Numerical results for the bistatic and backscattering cross sections for prolate spheroids with various axial ratios and orientations are presented     

Scattering of an Electromagnetic Pulse by a Moving Wedge

The paper is concerned with the analysis of 2-D scattering of an electromagnetic (EM) pulse by a perfectly conducting wedge moving with a relativistic velocity in a free space. The incident signal is described by a Dirac delta function. Analytical solution to this scattering problem is found, and its physical interpretation is given. The field representation, valid for all scatterer velocities, is then simplified to the case of moderate and low velocities, appropriate for practical applications.     

Electromagnetic coupling through small apertures in a conducting screen

The electromagnetic field coupling through small apertures illuminated by an arbitrary incident plane wave is discussed for general aperture shapes. A set of new integral equations in a form highly amenable to numerical solution techniques is derived. Based on the application of the Rayleigh series method, an analytical solution is obtained for the first few terms of the expansion of the apertureE- field of a circular aperture. Numerical results are also constructed for the aperture field and the diffracted field of small rectangular apertures and compared with those of the circular apertures.     

Scattering of electromagnetic waves by a system of two dielectricspheroids of arbitrary orientation

By means of modal series expansion of the incident, scattered, and transmitted electric and magnetic fields in terms of appropriate vector spheroidal eigenfunctions an exact solution is obtained to the problem of electromagnetic scattering by two dielectric spheroids of arbitrary orientation is obtained. The incident wave is considered to be a monochromatic uniform plane electromagnetic wave of arbitrary polarization and angle of incidence. To impose the boundary conditions at the surface of one spheroid, the electromagnetic field scattered by the other spheroids is expressed as an incoming field to the first one, in terms of the spheroidal coordinates attached to it, using rotational-translational addition theorems for vector spheroidal wave functions. The solution of the associated set of algebraic equations gives the unknown expansion coefficients. Numerical results are presented in the form of plots for the bistatic and backscattering cross sections of two lossless prolate spheroids having various axial ratios, center-to-center separations, and orientations     

Linear and nonlinear EMP diffusion through a ferromagnetic conducting slab

The penetration of an EMP field, such as a plane-wave incident upon and a surge-arrestor current terminated at a shielding plate, through a ferromagnetic conducting slab made of iron or steel is investigated. The diffusion of the electromagnetic field in the highly conducting slab is complicated by the presence of the nonlinear saturation of the ferromagnetic permeabilitymu, due to the large amplitude of the incident EMP. Such a saturation, compared to the no-saturation constantmucase, makes the field diffuse faster in the slab and admits a stronger penetration field inside the shielding if the slab is thin and the pulse duration is long, as expected. On the contrary, if the slab is thick and the puslewidth is short, as the practical cases are, the saturation reduces the penetrated field but maintains its time shape. In this report, first we solve analytically the one-dimension plane-wave incidence problem for a slab with a constantmu. Then we use the results to partly predict and to interpret the numerical values obtained by using a finite difference code for the case of a nonlinearmu. These comparisons reveal excellent agreement. Second, we solve the constantmu, cylindrical-wave incidence problem by an approximate but extremely useful analysis, with its validity parameters clearly established. The results, shown to bear a simple relationship to those for the one-dimension problem, enable one to make use of the one-dimension results and predict easily the penetrated fields caused by a cylindrical incident current. Then for the nonlinear-mucase, we justify and extend that relationship by which the behaviors of the cylindrically diffused fields are obtained from those numerical results of the one-dimensional problem.     

一种旋绕型频率选择表面的传输特性分析

利用基于矩量法的电磁分析软件Ansoft Designer研究了一种新的旋绕型双方环频率选择表面(new convoluted double square loop frequency selective surface,简称NCDSL FSS)的传输性能.结果表明,该结构对入射电磁波的入射角、极化方式等不灵敏,比经典CDSL FSS受栅瓣影响小;通过调节其凹槽深度,两频带间隔可围绕单元尺寸、阵列周期均相同的传统同心双方环频率选择表面(double square loop frequency selective surface,简称DSL FSS)的频带间隔双向调节,这是CDSL FSS无法做到的;而且NCDSL FSS高频谐振点处的-0.5 dB相对反射带宽也总是比CDSL FSS和DSL FSS的大.因此该结构是一种简单且性能较优的频率选择器件.     

Pulsed EM Field Response of a Thin, High-Contrast, Finely Layered Structure With Dielectric and Conductive Properties

The response of a thin, high-contrast, finely layered structure with dielectric and conductive properties to an incident, pulsed, electromagnetic field is investigated theoretically. The fine layering causes the standard spatial discretization techniques to solve Maxwell's equations numerically to be practically inapplicable. To overcome this difficulty, an approximate method is proposed that models the interaction of the layer with an incident electromagnetic field via a boundary condition that expresses the in-plane conduction and contrast electric polarization currents in terms of the exciting incident field by relating the jump in the tangential component of the magnetic field strength across the layer in terms of the (continuous) tangential component of the electric field strength in the layer. In the pertaining layer admittance coefficient, the integrated values of the conductance and the contrast permittivity profiles across the layer occur. The model is applied to the scattering of an incident plane wave with pulsed time signature by a layer of infinite extent. Expressions for pulse shapes of the scattered field are obtained. In them, the layer properties and the direction of incidence and polarization of the incident wave occur as parameters. Numerical results are presented for reflected and transmitted wave pulse shapes for some parameter values.      

ESPRIT-based 2-D direction finding with a sparse uniform array ofelectromagnetic vector sensors

Aperture extension (interferometry baseline extension) is achieved in this novel ESPRIT-based two-dimensional (2-D) arrival angle estimation scheme using a sparse (a.k.a., thin or thinned) uniform rectangular array of electromagnetic vector sensors spaced much farther apart than a half-wavelength. An electromagnetic vector sensor is composed of six spatially co-located, orthogonally oriented, diversely polarized antennas, distinctly measuring all six electromagnetic-field components of an incident multisource wavefield. Each incident source's direction of arrival (DOA) is estimated from the source's electromagnetic-field vector component and serves as a coarse reference to disambiguate the cyclic phase ambiguities in ESPRIT's eigenvalues when the intervector sensor spacing exceeds a half wavelength. Simulations demonstrate the significant performance gain realizable by this method for radar and wireless mobile fading-channel communications     

On the interaction of electromagnetic fields with wire cagestructures

The authors address the problem of the interaction of electromagnetic fields with wire cage structures. The two canonical shapes considered are cubes and cylinders. The motivation for this work comes from a need to investigate electromagnetic field coupling (transient or broadband CW) to communication facilities or buildings with metallic rebar elements in the perimeter walls. Coupling calculations are performed by using NEC-2 computer codes and the experiments were performed at the Swiss MEMPS facility. MEMPS is a hybrid type of nuclear electromagnetic pulse (NEMP) simulator consisting of an impedance loaded elliptical structure with either a transient or a broadband CW excitation. Both types of excitations have been used in experimentally studying the coupling characteristics. In the case of cylindrical wire cage structures, both polarizations, i.e., incident electric field and incident magnetic field parallel to the cylinder axis, have been investigated. Good agreement is seen between the computed and measured result     

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.     

Propagation and depolarization of an arbitrarily polarized waveobliquely incident on a slab of random medium

The problem of how a slab of random medium affects the propagation and polarization of an arbitrarily polarized obliquely incident electromagnetic wave is investigated. The general formulation is given by using vector radiative transfer theory. The multiple scattering solution is compared with the analytical first-order solution when the optical distance is small and the comparison shows that the results are consistent with each other. The multiple scattering results for a left-handed circularly polarized incident wave show that the transmitted wave is still right-handed near the backscattering direction. The scattering signature obtained shows that for normal incidence one can obtain a maximum backscattered power if an arbitrarily oriented linearly polarized incident wave is chosen and that for an incident angle of 30°, a minimum backscattered power can be obtained if a linearly polarized incident wave is oriented at 45° or 135°     

Effect of switching a magnetoplasma medium on a traveling wave:longitudinal propagation

The interaction of electromagnetic radiation with a time-switched magnetoplasma is considered. It is shown that for longitudinal propagation within the so-called radio approximation (neglecting ion motion), the original wave splits into three waves whose frequencies are different from that of the incident wave. One of the waves is strongly influenced by the static magnetic field, and its frequency can be controlled by the strength of this field. Numerical results are presented for the cases of right-hand and left-hand circular polarization of the incident waves