Diffraction of a cylindrical wave by a perfectly conducting cylinder enclosed in a metamaterial shell with a negative refractive index

The problem of diffraction of a plane (cylindrical) wave by a perfectly conducting cylinder enclosed in a metamaterial shell is rigorously solved. The influence of the geometric dimensions of the shell, the value of the negative refractive index of the metamaterial medium, and the position of a cylindrical wave source on the field structure in the near zone of the scatterer is investigated. It is found that, in the quasi-optical range of the problem parameters, this structure does not exhibit ideal focusing. It is shown that, there are two types of caustics inside the shell. The first type is related with rays reflected by the surface of the interior cylinder and has one cusp point, and the second type is formed by the geometric-optics rays that are refracted by the outer boundary of the shell and do not fall on the surface of the interior perfectly conducting cylinder. The spatial distribution of the total field amplitude and of the equal-amplitude lines in the near zone of the scatterer is reported. The obtained numerical results are correctly interpreted from the physical viewpoint.     

Radiation from one-dimensional dielectric leaky-wave antennas

Dielectric leaky-wave antennas consist of multiple dielectric layers above a ground plane, along with a simple source such as an infinite line current in the one-dimensional case. The layers act as a guiding structure for one or more leaky waves, which are responsible for producing a narrow-beam radiation pattern. Depending on the beam angle, the leaky waves may extend for a considerable distance from the source, and thus experience truncation effects due to a finite-size substrate. One of the purposes of the present investigation is to study efficient methods for calculating the patterns under such conditions. The chirp z-transform (CZT) is shown to be very efficient for this. Simple approximate formulas for calculating the radiation pattern are also investigated. In addition, a study is also made of the different contributions to the total aperture field, including the leaky-wave field, the space-wave field, and the surface-wave field     

Application of complex ray tracing to scattering problems

Representations and geometric constructions associated with complex points, complex lines, and complex rays are introduced. They are applied to the problem of scattering of an evanescent plane wave by a conducting circular cylinder. This problem has an exact solution, which provides a check of the validity of complex ray tracing and suggests more general applications. An important role is played by the transformation that maps the point of reflection, on the complex extension of the scattering surface, onto the trace in real space of the complex reflected ray. For the particular problem considered, the phase and amplitude of the reflected field are computed and the "phase paths" and "phase fronts" are constructed. The reflected field and phase paths obtained in this manner are not to be taken in their entirety because some reflection points are not "illuminated" by the incident wave, and because the reflector may be only part of the cylinder. Tentative selection and truncation rules are used which yield good agreement with the exact solution over some regions. The disagreement, where it occurs, comes-as it does for real rays-from neglecting the diffracted field such as the creeping waves around smooth surfaces and, in the case of truncation, the edge waves from the discontinuity. Some consideration is given to scattering by an arbitrary smooth conductor. Some problems peculiar to the use of complex rays are stated.     

Diffraction of plane and cylindrical waves by a metamaterial cylinder

Rigorous solution of the problem of plane- and cylindrical-wave scattering by a circular cylinder made from a metamaterial is presented. It is shown that, inside a metamaterial cylinder, a symmetric caustic of refracted geometrical-optics rays with one cuspidal point is formed. The influence of the position of a cylindrical-wave source and the absolute value of the refractive index of the metamaterial both on the scattering pattern and the near field structure is studied.     

Shooting and bouncing rays: calculating the RCS of an arbitrarilyshaped cavity

A ray-shooting approach is presented for calculating the interior radar cross section (RCS) from a partially open cavity. In the problem considered, a dense grid of rays is launched into the cavity through the opening. The rays bounce from the cavity walls based on the laws of geometrical optics and eventually exit the cavity via the aperture. The ray-bouncing method is based on tracking a large number of rays launched into the cavity through the opening and determining the geometrical optics field associated with each ray by taking into consideration: (1) the geometrical divergence factor, (2) polarization, and (3) material loading of the cavity walls. A physical optics scheme is then applied to compute the backscattered field from the exit rays. This method is so simple in concept that there is virtually no restriction on the shape or material loading of the cavity. Numerical results obtained by this method are compared with those for the modal analysis for a circular cylinder terminated by a PEC plate. RCS results for an S-bend circular cylinder generated on the Cray X-MP supercomputer show significant RCS reduction. Some of the limitations and possible extensions of this technique are discussed     

金属圆管管壁厚度对瑞利波影响的研究

为了探究管道中瑞利波信号的产生、传播规律,基于热弹激发机制,建立了激光线源在偏心圆管侧面激发超声波的有限元模型,研究了在不同激发源厚度下及不同曲率半径下铝质圆管中瑞利波的产生、传播规律。结果表明,均匀薄管中,管壁厚度在一定程度上决定了所产生的超声信号的成分,并且在一定范围内,超声信号也会随着管壁厚度的变化而变化;在非均匀管道中,激发源处的管壁厚度对超声信号的产生有很大影响,能在一定程度上决定超声信号的成分。这一结果对激光超声在管状材料的无损检测中的进一步应用是有帮助的。     

Radiation from a uniaxially anisotropic plasma half space

A line source of magnetic currents is located in a half space with uniaxially anisotropic dielectric constant, with the optic axis inclined at an arbitrary angle to the interface. This two-dimensional, scalar diffraction problem is solved by Fourier integral techniques, and the solution is examined in the asymptotic range of large distance from the source. The radiation field comprises incident, reflected and refracted ray contributions whose properties are determined from a study of the refractive index curves for the medium. In addition, there may exist a lateral wave which is excited by a critically refracted incident ray, and which sheds energy back into the anisotropic medium by refraction. If the medium is a plasma with an externally impressed infinite dc magnetic field, and if the source oscillates below the plasma frequency, there exist shadow regions from which the direct and reflected rays are excluded but which can be penetrated by the lateral waves.     

Scattering of electromagnetic waves by an anisotropic plasma-coated conducting cylinder

This paper considers the scattering of electromagnetic waves by an infinitely long anisotropic plasma-coated conducting cylinder. The source is assumed to be a magnetic current filament which gives rise to an incident magnetic field with only an axial component. Complete expressions for the scattered electric and magnetic fields are obtained. Scattering by an anisotropic plasma column and that by an isotropic plasma-coated conducting cylinder are special cases of the present problem.     

Dispersion and instability of drift waves in a fine-layered semiconductor structure

Drift waves in a fine-layered periodic semiconductor structure subjected to an electric field that induces the drift of charge carriers of different signs are considered. It is shown that the drift waves in this structure can propagate at an angle to the direction of the current, while their properties are governed by the thickness of the layers and the propagation direction. These waves are formed of concentration waves in separate layers and, thus, are “collective” waves. The conditions for origination of instabilities and analytical relations for increments are derived. Origination of instabilities is related to the energy exchange between drift waves in separate layers.     

Numerical Solution of Steady-State Electromagnetic Scattering Problems Using the Time-Dependent Maxwell's Equations

A numerical method is described for the solution of the electromagnetic fields within an arbitrary dielectric scatterer of the order of one wavelength in diameter. The method treats the irradiation of the scatterer as an initial value problem. At t = 0, a plane-wave source of frequency f is assumed to be turned on. The diffraction of waves from this source is modeled by repeatedly solving a finite-difference analog of the time-dependent Maxwell's equations. Time stepping is continued until sinusoidual steady-state field values are observed at all points within the scatterer. The envelope of the standing wave is taken as the steady-state scattered field. As an example of this method, the computed results for a dielectric cylinder scatterer are presented. An error of less than /spl plusmn/10 percent in locating and evaluating the standing-wave peaks within the cylinder is achieved for a program execution time of 1 min. The extension of this method to the solution of the fields within three-dimensional dielectric scatterers is outlined.     

Reflection coefficient of a semiconductor superlattice subjected to a magnetic field

Propagation of magnetoplasma waves in a structure formed by periodically alternating semiconductor and dielectric layers is studied for the case where an external magnetic field is applied parallel to the layers; the waves traveling in the plane normal to the field are considered. Dispersion characteristics are calculated with the finiteness of the speed of light accounted for, and the features of the structure reflectivity are analyzed. It is demonstrated that the physical parameters and thicknesses of the layers composing the superlattice can be obtained from the frequency, angle of incidence, and magnetic-field dependences of the reflection coefficient.     

Excitation of surface waves during diffraction by a circular impedance cylinder

Under certain conditions, surface waves are excited during diffraction by a body with an impedance surface. In the problem on the incidence of a plane wave on a circular cylinder, the total field is decomposed in the fields of such surface waves, and this decomposition is a Watson series constructed similarly to the corresponding series for a metal sphere. The amplitudes of the excited waves and the field structure of these waves are found.     

Diffraction by a concave perfectly conducting circular cylinder

The determination of surface currents induced by a magnetic line source on a perfectly conducting concave cylindrical surface is dealt with in this paper. A "perfect" planar absorber is postulated along the cylinder diameter. The condition that the field be bounded at the origin requires the spectral representation of the characteristic Green's function to possess, in addition to a discrete spectrum comprising a finite number of whispering gallery (WG) modes, also a continuous spectrum. In the asymptotic high frequency limit the latter yields an "intrinsic" diffraction effect that together with the WG mode series correctly reproduces the singularity of the source. The connection between ray optics and the modal series is established by means of a representation comprising modes and rays. This composite representation provides insight into possible generalizations of the results to arbitrary concave cylinders with slowly varying radii of curvature. For circular cylinders with largeka, numerical results are presented for the magnitude of the surface magnetic field distribution.     

Excitation of electromagnetic waves in a gyroelectric cylinder

The coupling of electromagnetic waves originating from a dipole to an infinite gyroelectric cylinder is treated analytically. The anisotropy axis of the gyrotropic medium is assumed to coincide with the cylinder axis. An electric dipole type primary excitation, with an arbitrary axis of orientation, is considered and the dyadic Green's function is derived for the gyroelectric cylinder. The field inside the anisotropic medium is expressed as a superposition of transverse magnetic (TM) and transverse electric (TE) type waves and a purely longitudinal wave. Excitation of guided modes along the gyroelectric guide and radiation patterns of dipoles radiating in the proximity of the gyroelectric cylinder are analyzed and computed for several cases.     

Transformation of surface waves in homogeneous absorbing layers

Surface waves in homogeneous absorbing layers are studied. The transformation of surface waves into other types of guided waves with frequency or layer's parameters variations is analyzed. It is found that in absorbing layers the standing damped surface waves do not exist and continuous transformation of surface waves into leaky waves is forbidden. Surface waves can only transform continuously into nonphysical waves with field strengths increasing exponentially in both the direction of propagation and in normal direction away from the layer into free-space. The frequency at which surface waves transform into nonphysical waves can be considered as cutoff frequency of surface waves. New and more general definitions previously proposed by the authors for the phase and energy velocities of guided waves are analyzed. It is shown that these velocities are identically equal to each other and never exceed the lightspeed in free-space. Several new physical phenomena are discovered. These are the upper frequency cutoff, its shifting to higher frequencies for materials with lower losses and the merging of high TE wave modes     

On the application of the Sommerfeld representation in atwo-dimensional rotationally invariant anisotropic medium

A.J.W. Sommerfeld's (1959) bundle of rays field representation is applied in a fictitious two-dimensional isotropic space that is mapped into a real rotationally invariant anisotropic region via a polarization-dependent coordinate transformation selected so as to obtain a field solution in the anisotropic region. Two elementary transformations are found, and the resulting representations (in the form of a modal angular spectrum or in terms of nonperiodic anisotropic ray bundles of complex trajectories) are analyzed. Field singularities are encountered and discussed in the context of their relation to the isotropic space rays. As an application, the solution to the canonical scattering problem of an anisotropically coated (ten material parameters, five for each polarization) circular cylinder is presented. Only H polarization is treated explicitly since the other (E) is obtainable via duality     

The relation between creeping waves, leaky waves, and surface waves

In this paper, we examine the creeping wave propagation wavenumbers, modal impedance, and field behavior on a dielectric coated circular cylinder. The physical interpretation is assisted by comparing the cylinder's pole waves with the leaky waves and surface waves that occur on a flat, grounded dielectric slab. The propagation wavenumbers and modal impedance are computed in the complex wavenumber plane. The cylinder propagation wavenumbers come from a transcendental equation involving Hankel functions, which are entire functions of complex order, whereas for the slab, branch-point singularities are present. This difference is examined, so that one can better understand how a coated cylinder behaves like a flat slab, when the cylinder radius is large. It is found that for the cylinder, the Stokes line for the asymptotic expansion of the Hankel function plays a role that is similar to the planar slab branch cut.     

Fields of a curved plasma layer excited by a slot

An analysis of the fields of a plasma coated conducting cylinder of infinite extent, excited by an infinite axial slot, is presented for large radius cylinders. The saddle point evaluation of the radiation fields is discussed for uniform low-loss plasma layers of arbitrary thickness and index of refraction. Patterns are presented and compared with the flat layer case, and the effect of the curvature on the pattern is discussed. The residue series evaluation is considered and a method of determining the poles is discussed. The locus of the first pole for thin lossless dielectric layers is presented and the transition from leaky waves to surface waves is discussed as a function of the dielectric constant and radius of curvature.     

非平行分层等离子体鞘套电波传播的复射线方法

基于损耗媒质的复Snell定律提出采用复射线方法研究电磁波在非均匀且非平行分层的等离子体媒质中的传播特性.该方法考虑非均匀平面电磁波的复射线(包括等幅度面的传播射线和等相位面的传播射线), 追踪电磁波复射线在每层媒质中的传播路径以及它们透过媒质分界面时的折射射线, 同时根据复射线的传播方向计算电磁波在每层媒质中的传播衰减, 数值累计整个传播过程中的传播衰减即可获得电磁波穿过等离子体鞘套的总衰减.由钝锥体仿真流场数据简化出非平行分层几何模型并采用复射线方法进行计算分析.计算结果表明:飞行器头部至尾部的传播衰减相差巨大且呈现迅速减小趋势, 非平行分层非均匀等离子体媒质存在某特定入射角, 能使传播衰减达到最小值.