Forward-backward iterative physical optics algorithm for computing the RCS of open-ended cavities

The forward-backward methodology is combined with the iterative physical optics (IPO) algorithm to improve convergence for cavity scattering problems. Wave propagation inside elongated cavities, such as jet engine inlet ducts, follows a predominant down-and-back path. The forward-backward method allows the IPO currents on the cavity walls to be updated sequentially (forward) and reverse-sequentially (backward) along the waveguide axis. A relaxation parameter is introduced to help control the convergence characteristics, making the new algorithm mathematically equivalent to the classical iterative method of symmetric successive over-relaxation. The fast far-field approximation (FaFFA) accelerates the matrix-vector products in the IPO formulation, and an equivalent surface impedance is used to characterize thin material linings in the cavity.     

应用谱域迭代算法计算理想导电腔体的RCS

本文介绍了利用谱域迭代方法计算理想导电腔体后向散射和RCS的方法,揭示了波谱在腔体内传播的过程。针对腔体截面形状的不同,给出了应用等效电磁流法解决腔体边缘绕射对口径场影响的方法。最后通过算例验证了方法的正确性。     

A field iterative method for computing the scattered electricfields at the apertures of large perfectly conducting cavities

An iterative method is developed for computing the scattered electric fields at the apertures of large perfectly conducting cavities. The field iterative method (FIM) uses Kirchhoff's approximation to initiate a two stage iterative process (i.e., the method of successive approximations), involving both the magnetic field integral equation and the electric field integral equation, to calculate the electric currents on the internal cavity walls and the electric fields across the aperture of the cavity. The technique combines the flexibility of the boundary-integral method with the speed necessary to efficiently analyze large scale cavity problems. The paper presents the general theory, and applies the technique to the problem of TE scattering from two-dimensional perfectly conducting cavities     

Numerical computation of scattering from a perfectly conducting random surface

A one-dimensionally rough random surface with known statistical properties was generated by digital computer. This surface was divided into many segments of equal length. The moments method was applied to each surface segment assuming perfect conductivity to compute the induced surface current and subsequently the backscattered field due to an impinging plane wave. The return power was then calculated and averaged over different segments. Unlike numerical computations of scattering from deterministic surfaces, problems of stability (as defined by Blackman and Turkey [11]) and convergence of the solution exist for random surface scattering. It is shown that the stability of the numerically computed estimate of the backscattered average power depends onN, the total number of disjoint surface segments averaged;Delta x, the spacing between surface current points;D, the width of each surface segment; andg, the width of the window function. Relations are obtained which help to make an appropriate choice of these parameters. In general, choices ofDelta x, D, andgare quite sensitive to the incident wavelength and the angular scattering properties of the surface.     

IPO-FDTD混合法计算三维复杂腔体的RCS

运用IPO-FDTD混合法计算三维复杂腔体的电磁散射特性。该方法用物理光学迭代法计算腔体前端的缓变部分,用时域有限差分法计算含有复杂结构的腔体终端。在IPO和FDTD之间场耦合中,通过提取“本底噪声”来提高数值精度。数值结果表明该方法是准确有效的。     

An iterative algorithm for the computation of the MVDR filter

Statistical conditional optimization criteria lead to the development of an iterative algorithm that starts from the matched filter (or constraint vector) and generates a sequence of filters that converges to the minimum-variance-distortionless-response (MVDR) solution for any positive definite input autocorrelation matrix. Computationally, the algorithm is a simple, noninvasive, recursive procedure that avoids any form of explicit autocorrelation matrix inversion, decomposition, or diagonalization. Theoretical analysis reveals basic properties of the algorithm and establishes formal convergence. When the input autocorrelation matrix is replaced by a conventional sample-average (positive definite) estimate, the algorithm effectively generates a sequence of MVDR filter estimators; the bias converges rapidly to zero and the covariance trace rises slowly and asymptotically to the covariance trace of the familiar sample-matrix-inversion (SMI) estimator. In fact, formal convergence of the estimator sequence to the SMI estimate is established. However, for short data records, it is the early, nonasymptotic elements of the generated sequence of estimators that offer favorable bias covariance balance and are seen to outperform in mean-square estimation error, constraint-LMS, RLS-type, orthogonal multistage decomposition, as well as plain and diagonally loaded SMI estimates. An illustrative interference suppression example is followed throughout this presentation     

一种埋入式腔体的低RCS载体外形设计

根据腔体目标在较大方位角内有较强散射的特点，提出了包覆腔体载体的设计要求。设计了大后掠扁平式后段收尖的载体初步方案。通过头部边缘锐化、头部边缘后掠角增加和对尾部边缘渐变半径处理，获得了符合载体设计期望的“凹坑式”雷达散射截面（RCS）随方位角曲线分布。研究结果显示，在水平极化时，尾部边缘采用渐变半径圆弧过渡曲面可以有效降低边缘散射在入射方向的回波，而在垂直极化时却增强了表面波回波强度。     

Dyadic Green's functions for the perfectly conducting parabolic cylinder

Solutions of the scalar wave equation for parabolic cylinder coordinate system are discussed here. Dyadic Green's functions of the magnetic type for free space and for a perfectly conducting parabolic cylinder are developed. These functions are of fundamental importance for the solution of electromagnetic problems developed in the parabolic cylinder coordinate system, particularly those in the presence of perfectly conducting parabolic cylinders such as that of a parabolic cylinder reflector.     

FINITE DIFFERENCE METHOD FOR ELECTROMAGNETIC SCATTERING FROM PERFECTLY CONDUCTING CYLINDER

A numerical method of solving electromagnetic wave scattering problem is described.Radiation boundary condition(RBC)is applied to confine EM scattering problem in unboundedregion into problem in finite region.Combined with RBC and scatterer surface boundary condi-tion,Helmholtz equation in the finite region is solved numerically by the finite difference method.Thus the distribution of induced surface current on conducting cylinder and near field can beobtained.Computational results for both polarizations for circular,elliptic and square cylindersare presented.These results are in excellent agreement with those obtained by the eigenfunctionexpansion method or moment method and much better then the results of OSRC method.     

Radiation patterns for two monopoles on a perfectly conducting sphere

A general numerical technique for analyzing monopole antennas protruding from a conducting sphere is suggested. Computed results for special cases of antenna locations are compared to known and approximate solutions showing that the method is practical and accurate.     

高阶基尔霍夫法求解导体粗糙表面的散射特性

在分析粗糙表面电磁散射特性的基础上,提出了一种考虑粗糙表面协方差函数Taylor展开的高阶基尔霍夫近似(KA)法,解决了经典KA近似的大逼近误差问题.应用9阶高度的KA和传统的KA,对比研究了不同照射频率和均方根高度下的后向散射系数,并比较了遮挡函数的修正效应.同时应用高、低阶KA计算了典型粗糙面的后向散射系数,并分别与测量值和矩量法的数值解进行了比较.结果表明,9阶表面高度展开的切平面KA不仅拓宽了KA的适用条件,还具有更高的精度范围,从而证明了高阶均方根高度展开的KA近似的有效性.     

Scattering from a periodic array of free-standing arbitrarily shaped perfectly conducting or resistive patches

A fast Fourier transform (FFT)-based iterative approach for computing the fields scattered by an infinite array of free-standing patches is presented. The method is capable of handling patches that are lossy and have arbitrary shape; it is useful for analyzing configurations that may not have been analyzed previously. Though a rectangular FFT is used, the formulation allows the study of the common triangular array periodicities. Results for various geometries are presented and are compared with existing results.     

The inverse electromagnetic scattering problem for a perfectly conducting cylinder

The problem of determining the shape of the cross section of a simply connected perfectly conducting infinite cylinder from a knowledge of the far-field pattern for all angles of observation and small values of the wavenumber is considered. The method proposed relies heavily on conformal mapping techniques. In particular it is shown that if the transfinite diameter is known each Fourier coefficient of the far-field pattern of the electric field determines a Laurent coefficient of the conformal mapping taking the exterior of the unit disk onto the exterior of the unknown cross section. The transfinite diameter is determined by changing the polarization of the incoming wave and measuring the far-field pattern of the resulting magnetic field. Of particular interest is the case when only a finite number of the Fourier coefficients of the far-field pattern are known. In this situation error estimates are obtained by using results on coefficient estimates for univalent functions.     

Transient and time-harmonic dyadic Green's functions for a perfectly conducting cone

New representations for the time-dependent dyadic Green's functions for a perfectly conducting semi-infinite cone are presented. For the special case of small cone angles and an on-axis source, simplified expressions are given for both the time-dependent and time-harmonic regimes.     

基于MATLAB的电大尺寸目标RCS计算系统研究

在分析物理光学法(PO)、等效电磁流法(MEC)、几何光学物理光学法(GOPO)等算法的基础上开发了基于MATLAB的电大尺寸目标RCS计算软件系统.应用MATLAB外部接口与FORTRAN语言混合编程提高了计算效率.最后利用该软件系统计算了典型目标和某大型舰艇的RCS,典型目标的RCS计算结果与测量值比较,吻合良好.某大型舰艇目标的RCS计算结果经分析,计算结果合理.     

Microwave density imaging of perfectly conducting objects in thenear-field region

Analytical and numerical studies of microwave diversity imaging of continuous and discrete conducting objects in the near-field region are presented. Analytical results show that the image of the scattering object can be reconstructed by Fourier inversion of the data acquired from the recorded scattered field using angular and frequency diversity techniques. Different feature information of the scattering object can be obtained using a polarization diversity technique. Various scattering arrangements are studied and compared on the basis of the reconstructed image quality and practical considerations. Numerical results show that the described frequency, angular, and polarization diversity techniques in the backward scattering arrangement can result in a cost-effective approach in near-field microwave imaging systems     

Full-wave analysis of coupled perfectly conducting wires in amultilayered medium

A full-wave analysis of coupled perfectly conducting cylindrical wires in a multilayered dielectric medium is presented. The analysis is based on a Fourier series expansion of the unknown surface currents on each wire and on an integral equation for the longitudinal field on the wires. The calculations are not restricted to the propagation constants of the different modes, but explicit results are presented for the impedances associated with each wire and each eigenmode as a function of frequency. Propagation constants, longitudinal currents on the wires, and impedances lead to a complete equivalent circuit for the structures being considered     

Performance of iterative computation in self-timed rings

The computation of iterative functions need not be limited by the rate at which external signals, such as a clock, can be supplied to an on-chip circuit. Instead, self-timed structures can compute without clock or latch delays. In particular, a self-timed ring is a loop of logical stages that, after initialization with operands, computes multiple cycles of an iterative computation without further external handshaking. Viewed as a whole, a self-timed ring has a total latency and throughput dependent not only on the individual stages' latencies and cycle times, but also on the total number of stages, tokens, and extra bubbles in the ring. This article derives the performance characteristics of self-timed rings, illustrates them with graphs, and discusses the implications for designing rings with optimal performance. Certain suggested ring configurations allow iteration with no latches and zero delay overhead, achieving a total latency equal to just the sum of the raw function-block delays. This property has been verified by measurements on a chip that demonstrates a self-timed ring for the example function of floating-point division. Fabricated in 1.2 CMOS, the ring occupies 7 mm² and generates a quotient bit every 2.8 ns.     

New equations for electromagnetic scattering by small perturbationsof a perfectly conducting surface

New equations are derived for the electromagnetic scattering produced by a small perturbation of a general perfectly conducting surface. These equations explicitly incorporate the magnetic field boundary condition on the general surface, which implies that the new Born term by itself leads to conventional backscatter formulas. The accuracy of the new equations is demonstrated by an example     

Numerical calculation of the diffraction coefficients for anarbitrary shaped perfectly conducting cone

A method for numerical calculation of the diffraction coefficients for electromagnetic diffraction by arbitrarily shaped perfectly conducting cones is proposed. The approach makes an extensive use of the analytic formulas of Smyshlyaev [1993] in combination with further developments, including a use of the potential theory adapted to the Laplace-Beltrami operator on a subdomain of unit sphere. This reduces the problem to a Fredholm integral equation on the closed curve of the unit sphere (defining the cone's geometry) which can be solved numerically. This strategy permits us to implement a numerical code for calculation of the diffraction coefficients for cones of rather general cross sections. Results of sample calculations for the circular and elliptic cones are given