Oblique scattering by a pair of conducting half planes: TM case

An exact series solution for the oblique scattering by a pair of infinitely long parallel edges of perfectly conducting half planes is formulated for a TM plane wave using the mode-matching technique (MMT). The scattered and guided fields are represented in terms of an infinite series of radial waveguide modes. By applying the appropriate boundary conditions, the coefficients of the scattered field are obtained. The diffraction coefficient of double edges is subsequently derived from the scattered field     

Analysis of a Longitudinal Gyroelectric Discontinuity Inside a Fiber Waveguide

The scattering of guided electromagnetic waves from a finite-length longitudinal gyroelectic discontinuity inside a fiber waveguide is treated analytically. An integral equation approach is employed to formulate the corresponding boundary-value problem. The induced field inside the gyroelectic discontinuity region is expanded into a Fourier-type series in terms of the well-known cylindrical waves M and N plus a purely longitudinal wave Q. Then the method of moments is applied to decouple the basic integral equation. The resulting infinite coupled system of equations is truncated and solved numerically. After determining the field inside the discontinuity, the scattered far field inside the dielectric-rod waveguide is computed by employing a steepest descent integration technique. Numerical results for the scattering coefficients of an incident HE/sub 11/ dominant mode are obtained. Finally, design principles are discussed for practical components based on the treated longitudinal gyroelectric discontinuity.     

用特征散射方向图预估电大平面阵的散射

利用平面照射波在无限大周期阵列上感应电流的周期特性，对无限阵列格林函数加窗的方法得到了有限阵列的格林函数，从而将电大有限阵列的分析缩减到单个的阵元上。进而由矩量法得到单个阵元上的电流分布和散射场，并定义为单元的特征散射方向图。仿照阵列综合理论，整个阵列的散射归结为特征散射方向图和阵因子的乘积，极大的简化了电大有限阵列的散射分析，并通过算例验证了该方法的有效性。     

TM scattering by a wedge with concaved edge

The TM scattering problem by a perfectly conducting wedge with concaved edge is formulated for a line source excitation using the mode-matching technique. The scattered and guided fields are represented in terms of an infinite series of radial waveguide modes with unknown coefficients. By applying the appropriate boundary conditions, the coefficients of scattered field are obtained. For small ka, the diffraction coefficient of concaved edge is derived from the scattered field     

Wave propagation on helical antennas

The current distribution, the near field, and the far field of helical antennas are examined. It is found that the application of the helix as an endfire radiator is dependent on two higher Order modes of the helix wave functions. This differs from the application of the helix as a slow wave structure in a traveling wave tube where the lowest order mode of the helix wave functions is utilized.     

Matrix formulation of electromagnetic scattering

A new method is proposed for the computation of the radar cross section and other associated field quantities arising when a smooth, perfectly conducting obstacle is illuminated by an incident electromagnetic wave. The scattered wave is first represented by a distribution of electric dipoles over the surface in question, with the response from any dipole proportional to the induced surface current density at that point. The surface current is then determined by the "boundary condition" that the scattered wave, through interference, precisely cancels the incident wave inside the obstacle. One obtains in this mariner a pair of coupled (infinite) matrix equations for the surface current. Green's identity permits decoupling of the equations, reducing the problem to roughly the equivalent of two independent scalar problems. The equations have been specialized to axially symmetric obstacles and then solved numerically on the IBM 7094 for several examples of interest. Reciprocity and energy conservation are also examined and the resonant mode (interior) problem set up explicitly in matrix form.     

Scattering of Guided Modes Caused by an Arbitrarily Shaped Broken End in a Dielectric Slab Waveguide

Electromagnetic scattering of guided modes in a dielectric slab waveguide caused by an arbitrarily shaped broken end is analyzed theoretically by using the integral equation method. By solving the integral equations iteratively, the tangential components of the electric and magnetic fields on the broken end surface are determined, from which the reflected mode power, the radiation wave power and field patterns, and the total scattered power are obtained. Numerical results are presented for the plane-perpendicular, plane-tilted, and arc-shaped end surfaces. Both TE and TM modes are assumed as an incident wave.     

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.     

Gaussian beam scattering from a conducting cylinder partially buried in a ground plane

The problem of scattering from an infinitely long conducting cylinder that is partially buried in a perfectly conducting ground plane due to an obliquely incident gaussian beam is solved by an exact procedure based on the method of images by first adopting a simplification originally proposed by Kozaki. The incident and the specularly reflected fields are expressed in terms of cylindrical vector wave functions multiplied by a weighting function which involves the beam parameters like the radial distance of the source and beam width. The scattered fields originating from the cylinder and its image in the ground plane are expressed in terms of cylindrical vector wave functions. The boundary conditions on the surface of the cylinder are then imposed and this procedure leads to a coupled infinite system of equations for the even and odd mode expansion coefficients of the scattered field. These equations are solved numerically for the case of cylinders having electrical radius in the Rayleigh and resonance regions. Both the transverse magnetic and transverse electric polarizations of the incident beam wave are considered and some representative numerical results for the scattered far-field are presented in graphical form. The magnitude of the induced current for the TM polarization is calculated and compared with the corresponding case of plane wave incidence.     

Scattering and Mode Conversion of Guided Modes by a Spherical Object in an Optical Fiber

The scattering and the mode conversion of the guided modes due to a spherical object in a step-index optical fiber is analyzed theoretically. The incident fiber mode is expanded in terms of the spherical vector wave functions, and the scattered fields are obtained by applying the boundary conditions on the surface of the object with the aid of these expansions. The expression for the total scattered power and the mode conversion coefficients are given. As an example, the scattering and mode conversion caused by a spherical air bubble are evaluated numerically.     

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     

螺旋带径向厚度对色散和耦合阻抗的影响

该文将有限厚度的实际螺旋带等效为存在无限薄螺旋带的等厚真空层,从场分析和软件模拟两个方面研究了等效慢波结构的色散特性和耦合阻抗.将理论计算、软件模拟和实验测试结果进行了比较,结果表明,用真空层等效有限厚度的实际螺旋带时,将无限薄螺旋带置于等厚真空层的中央,其螺旋慢波结构的色散特性和耦合阻抗的理论计算、软件模拟和实验测试结果三者取得了很好的一致.     

Plane wave coupling to multiple conductor transmission lines abovea lossy earth

The current induced on an infinite multiple conductor transmission line located above a lossy homogeneous medium due to a transient plane wave is discussed. An exact solution is formulated in the frequency domain using a spatial transform technique. The widely utilized quasi-TEM approximation is derived directly from the exact solution with emphasis on the physical consequences of the assumptions made. Both frequency domain and time domain numerical results are presented for typical transmission structures and documented electromagnetic pulse (EMP) excitations. Comparison of the quasi-TEM approximation to the exact solution is made in order to study the validity of its application in EMP coupling problems. The modeling of the EMP source as an incident plane wave is examined by comparing the induced current due to a dipole source with its steepest-descent contribution     

A Fast Computation of the Start-Oscillation Current of Backward Wave in MMW HTWT's

Backward wave oscillation can be troublesome in high power helix TWT, especially millimeter wave helix traveling wave tubes (MMW HTWT's). To suppress these oscillations, start-oscillation current of backward wave has to know when MMW HTWT's are designed. A fast computation of the start-oscillation current for MMW HTWT's is presented. It is shown that the computed values are agreed with the experimental results in Ka band.     

Microwave-induced auditory effect in a dielectric sphere

The acoustic pressure wave generation inside an electromagnetically lossy dielectric sphere from an incident microwave pulse is analyzed rigorously. The pressure wave equation derived using the first-order approximation of a thorough formulation on microwave-induced thermoacoustic effect in dielectrics, is used. The inhomogeneous hyperbolic-type pressure wave differential equation is solved by a Green's function theory approach. The boundary conditions on the dielectric sphere-air interface are taken into account. The power is computed by applying the exact Mie theory solution for the dielectric sphere. Two types of acoustic waves are derived inside the sphere: (a) a transient burst type pressure wave, corresponding to the free-space contribution of Green's function; and (b) an infinite set of damped oscillations related to the normal acoustic modes of the spherical resonator. Numerical results are computed and presented for several cases     

Finite element analysis of weakly guiding fibers with arbitrary refractive-index distribution

It is shown, with numerical examples, that fundamental quantities that characterize the guided modes of an optical fiber, such as propagation constant, field pattern, fraction of power in the fiber core, group delay, and waveguide dispersion, can be easily and accurately computed by a simple finite element method that incorporates an efficient recurrence algorithm for modeling the infinite cladding of the fiber. With this method, propagation characteristics of single-mode fibers with different refractive-index distributions are calculated and compared.     

Scattering from apertures in infinite ground planes using FDTD

A scattered field version of FDTD for scattering from an aperture in an infinite ground plane is presented. In this formulation the fields reflected from the infinite ground plane are computed analytically, not as FDTD scattered fields. This is necessary to eliminate scattering from the edges of the ground plane, where it is terminated at the FDTD outer boundary. Also, the fields scattered by the ground plane are usually of much higher amplitude than the desired aperture-scattered fields. In this formulation these fields need not be absorbed by the FDTD outer boundary. This provides more accurate calculation of low amplitude scattering from the aperture. The formulation can include materials in the aperture and on both sides of the infinite ground plane. For example, scattering from an aperture antenna with a dielectric cover backed by an aperture filled with lossy dielectric can be computed with this formulation     

Scattering and mode conversion of guided modes by an arbitrary cross-sectional cylindrical object in an optical slab waveguide

The scattering and mode conversion of guided modes caused by a cylindrical scatterer of arbitrary cross section embedded in, or placed on, a dielectric slab waveguide are investigated theoretically. The method employed for the analysis is the surface current integral equation technique. The total scattered power, mode-conversion coefficients, and scattering patterns for the typical numerical examples are shown.     

Scattering by an arbitrary cylinder at a plane interface: broadsideincidence

The problem of the determination of the fields scattered by an infinite dielectric cylinder of arbitrary cross section located at the interface between two semi-finite dielectric media is reduced to the solution of integral equations for unknown functions defined on the boundaries. These boundary functions are chosen so as to minimize their number. The incident field is that of a plane monochromatic wave. The derivation of the integral equations is given for the transverse electric (TE) mode for a dielectric cylinder and for a perfectly conducting cylinder. The exact electromagnetic fields are obtained from the solutions of the integral equations by integration, and the radar cross section can be computed from the far-field approximation. Sample outputs of the computer programs that implement this solution are shown     

Radiation pattern synthesis techniques as applied to the study of perfectly conducting periodic structures of arbitrary profile: TE-polarization

Previous works [1]-[5] on planar perfectly conducting periodic structures had their mathematical formulation developed primarily with basis on the boundary conditions imposed on the conducting surface of the structure thus requiring near-field computation to be somehow performed. In the present work it is shown that the induced currents, and from that the resulting scattering properties of periodic structures, can also be determined exclusively from its scattered farfield characteristics. Firstly, the structure is regarded as an uniform infinite periodic array excited with a progressive phase-shift resulting from illumination by a plane-wave. From boundary conditions on the "shadow" side of the structure one can easily obtain the directions of the nulls of the array pattern which in fact are the nulls of the far-field array element-pattern. Synthesis techniques are then used to obtain the current on the structure from which the scattered field can be computed. Numerical results obtained with the present method compare quite favourably with previously published numerical results and also with experimental data.