电波传播

0100421一维随机粗糙面电磁散射的小波矩量解[刊]/郭立新//西安电子科技大学学报.—2000.27(5).—585～589(D)粗糙面的电磁散射问题可以归结为一个电场积分方程。采用小波基函数将积分算子展开,利用矩量法求解矩阵方程,得到一维随机粗糙面的等效电流分布,并进一步得到散射场,所得结果与有关文献的结果基本一致。参8     

二维多导体柱电磁散射特性的特征基函数法分析

特征基函数法是近两年提出来的一种求解电磁散射问题的有效方法,该方法使用的特征基函数不受传统矩量法离散尺寸的限制,因而可以大大减小要求解的矩阵方程。应用特征基函数法分析了二维多导体柱的电磁散射特性,计算了多个无限长导电椭圆柱和方柱的雷达散射截面,结果表明特征基函数法的计算结果与传统矩量法的计算结果吻合良好,而计算量却大为减少。     

双站散射模式响应的分析与应用

基于电磁散射积分方程求解的矩量法,将入射和散射的平面波分解成含有Bessel函数的模式叠加,提出了目标双站散射模式响应的概念,得到目标远场双站雷达散射截面(RCS)与模散射响应之间成傅立叶变换的关系.根据基函数的性质阐明了模散射响应矩阵具有带限低通特性、互易对称性和近似成对角分布稀疏特性.模式散射响应是与外在观测角无关...     

线天线的小波矩阵变换法求解

本文研究天线的小波矩阵变换法求解。首先采用三角插值基函数和线天线的一种精确计算模型,利用矩量法得到矩阵方程,然后通过Daubechies离散小波变换进行快速求解。计算结果表明,该方法能够使阻抗矩阵稀疏化,从而提高了求解速度。     

离散小波变换加速导体目标散射计算的分析

如今矩量法被广泛地应用于解决电磁场问题中,但是应用矩量法所得到的矩阵是稠密阵,如果直接求解无疑耗费大量的时间,若能将矩阵稀疏化,那么求解起来就会省时得多.针对这个问题,本文将给出离散小波变换方法稀疏化矩量矩阵,并以Daubechies三阶尺度函数为例,阐述小波的一些基本性质以及滤波系数的构造,论述小波变换原理和详细推导了变换矩阵的构造.并给出一个全新的算例--二维导电平面上的平行双槽线上的散射问题.     

结合P-FFT算法和特征基函数分析大型阵列散射

特征基函数方法利用特征值分解提取目标散射特征,构造基于特征向量的基函数可以高效的缩减矩量法分析所需的未知量数目,有利于分析有限周期阵列电磁散射或辐射问题。然而,对于电大尺寸电磁阵列散射问题,直接求解由特征基函数组成的矩阵方程,仍然面临着计算量较大等问题,难以适用于单机计算。本文结合特征基函数和预修正傅里叶快速算法求解体面结合积分方程,分析了大型金属介质混合有限周期阵列的散射特性,该算法有效减少了计算量和计算时间,并且改善了迭代求解收敛性能。     

导体目标瞬态电磁散射的时空矩量法研究

时域积分方程的矩量法是求解瞬态电磁散射的方法之一。研究了基于加权Lagurre函数和RWG基函数分别作为时间、空间基函数的时空矩量法,给出了时域磁场积分方程时空矩量法的全部计算公式,编制了相应串行和并行计算程序。计算结果表明：该方法具有很好的时域稳定特性,为宽带电磁散射分析提供了可能,同时也指出了其应用的局限性,为改进其方法提供了参考。     

金属-各向异性介质体组合目标频域体表积分方程矩量法

利用体表积分方程矩量法求解了具有任意的介电常数张量和磁导率张量的各向异性介质与金属的组合目标的电磁散射问题.给出了基于RWG面基函数和SWG体基函数的体表积分方程阻抗矩阵元素表达式并详细推导了阻抗矩阵元素所涉及的各种积分运算的计算方法;通过数值计算实例与解析解或其它数值方法的详细对比分析,证明了计算公式的正确性.     

大型阵列结构电磁散射的P-FFT算法

提出利用预校正的快速傅立叶变换(P-FFT)和矩量法(MoM)快速分析电大阵列结构的电磁散射特性.研究了适合于电大尺寸结构近区和远区场计算的格林函数的快速算法,同时,利用Rao-Wilton-Glisson(RWG)函数作为基函数和测试函数,可以考虑阵列单元任意方向的表面电流或磁流分布,并利用P-FFT加速矩量法的矩阵求解,减少内存需求和计算时间.计算结果表明该方法特别适合于大型阵列电磁散射特性的分析.     

小波矩量法计算导电平面上的三维腔体散射

该文采用小波矩量法求解导电平面上的三维腔体散射,利用小波变换稀疏化矩量矩阵,加速其求逆,减少了腔体散射的总体计算时间。通过具体的算例,表明小波矩量法在计算三维散射问题中是非常有效的。     

A hybrid BEM/WTM approach for analysis of the EM scattering fromlarge open-ended cavities

An effective hybrid boundary-element method (BEM) and wavelet-transform method (WTM) is proposed to analyze electromagnetic scattering from three-dimensional (3-D) open-ended cavities with arbitrary shapes. This hybrid technique formulates the original cavity problems by a magnetic field integral equation. The BEM is employed to establish the mapping between the original complex integral surface and the unit square. The WTM is used to reduce the density of the moment matrix. Since a surface integral equation has to be solved, the WTM requires a two-dimensional (2-D) wavelet basis to implement the numerical computation. The previous fast iterative algorithm for 2-D wavelets has been extended for efficiently constructing various 2-D wavelet basis functions by a tensorial product from two one-dimensional (1-D) regular multiresolution analyses. Unlike the conventional method of moments, the proposed hybrid technique can always obtain sparse moment matrix equations, which can be efficiently solved by sparse solvers. As the level scales for numerical discretization of cavities increase, larger compression rates can be obtained, which makes it possible for the hybrid BEM/WTM technique to efficiently solve scattering from large open-ended cavities with complex terminations. Numerical results are presented to demonstrate the merits of the proposed method     

基于离散小波标架的信号降噪

本文提出了基于小波标架的软门限压缩降噪方法,它是根据小波标架表示系数在不同的尺度有不同的方差,在各尺度分别模拟理想选择估计器来选择门限对小波系数进行压缩。该方法能有效地减少［３,４］中基于小波基方法的振荡效应,并对仿真结果作了理论分析。     

基于Morlet小波的水下声信号频率识别

为准确获取水下目标的发声频率,建立了激光干涉法探测水下声信号的实验系统。提出一种基于Morlet小波的水声信号处理方法,将Morlet小波母函数的频率取定值,改变尺度因子,利用两者比值与水下声信号频率的关系分析小波系数模值,实时获取水下声信号的频率信息。实验结果表明:小波系数图可以反映出水下目标在某一时刻的发声频率,实验系统能够实时探测出1～15kHz的水下声信号。     

基于双正交小波变换的矩不变量

寻找相对于尺度、平移、旋转不变的小波不变量是多尺度分析在模式识别中的关键问题.矩是一种理论和应用上比较成熟的方法,本文将矩与多尺度小波分解的近似系数联系起来,利用空间基函数的双正交性推导得到了双正交小波矩不变量,并用实验验证了结果的正确性.同时以Haar小波为例对结论中的限制条件进行了理论分析和实验验证,结果表明可以计算高于平滑阶数的小波矩,且计算精度符合要求.由此获得了比较完善的理论和实验结果,最后指出了它在实际应用中所需注意的问题.     

Adaptive multiscale moment method (AMMM) for analysis of scatteringfrom perfectly conducting plates

Adaptive multiscale moment method (AMMM) is presented for the analysis of scattering from a thin perfectly conducting plate. This algorithm employs the conventional moment method and a special matrix transformation, which is derived from the tensor products of the two one-dimensional (1-D) multiscale triangular basis functions that are used for expansion and testing functions in the conventional moment method. The special feature of these new basis functions introduced through this transformation is that they are orthogonal at the same scale except at the initial scale and not between scales. From one scale to another scale, the initial estimate for the solution can be predicted using this multiscale technique. Hence, the compression is applied directly to the solution and the size of the linear equations to be solved is reduced, thereby improving the efficiency of the conventional moment method. The basic difference between this methodology and the other techniques that have been presented so far is that we apply the compression not to the impedance matrix, but to the solution itself directly using an iterative solution methodology. The extrapolated results at the higher scale thus provide a good initial guess for the iterative method. Typically, when the number of unknowns exceeds a few thousand unknowns, the matrix solution time exceeds generally the matrix fill time. Hence, the goal of this method is directed in solving electrically larger problems, where the matrix solution time is of concern. Two numerical results are presented, which demonstrate that the AMMM is a useful method to analyze scattering from perfectly conducting plates     

Fast estimation of continuous Karhunen-Loeve eigenfunctions usingwavelets

This paper develops a new wavelet method for the fast estimation of continuous Karhunen-Loeve eigenfunctions. The method of snapshots is modified by projecting the ensemble functions onto orthogonal or biorthogonal interpolating function spaces. Under well-behaved piecewise smooth polynomial ensemble functions, the size of the covariance matrix produced is greatly reduced, without sacrificing much accuracy. Moreover, the covariance matrix C˜ may be easily decomposed such that C˜ = A^T A, and thus, the more stable singular value decomposition (SVD) algorithm may be applied. An interpolating scheme that reduces the computation of projecting the ensemble functions onto the biorthogonal subspace to a single sample is also developed. Furthermore, by projecting the ensemble functions onto wavelet spaces, the covariance matrix may be sparsified by a multiresolution decomposition. Error bounds for the eigenvalues between the sparsified and nonsparsified covariance matrix are also derived     

使用M进制小波对尺度与旋转不变的图像特征点进行检测

基于小波变换和尺度空间理论,提出了一种尺度与旋转不变的图像特征点的检测方法。首先对图像进行多尺度的M进制小波变换,然后根据M进制小波变换系数的能量构造对应的二阶矩阵,最后通过考察该矩阵的特征值确定图像的特征点。实验表明,与目前最常用的方法相比,本文方法对图像的旋转变化与尺度变化均具有更好的不变性,在光照变化和噪声干扰等外部条件影响下也能保持较高的鲁棒性。     

Analysis of Millimeter Wave Scattering by an Electrically Large Metallic Grating Using Wavelet-Based Algebraic Multigrid Preconditioned CG Method

An effective wavelet based multigrid preconditioned conjugate gradient method is developed to solve electromagnetic large matrix problem for millimeter wave scattering application. By using wavelet transformation we restrict the large matrix equation to a relative smaller matrix and which can be solved rapidly. The solution is prolonged as the new improvement for the conjugate gradient (CG) method. Numerical results show that our developed wavelet based multigrid preconditioned CG method can reach large improvement of computational complexity. Due to the automaticity of wavelet transformation, this method is potential to be a block box solver without physical background.     

An introduction to discrete finite frames

The frame concept was first introduced by Duffin and Schaeffer (1952), and it is widely used today to describe the behavior of vectors for signal representation. The Gabor (1946) expansion and wavelet transform are two special well-known cases. The goal of this article is to describe the frame theory and introduce a simple tutorial method to find discrete finite frame operators and their frame bounds. An easily implementable method for finding the discrete finite frame and subframe operators has been presented by Kaiser (1994). We introduce the method of Kaiser to compute the discrete finite frame operator. Using subframe operators, the biorthogonal basis and projection vectors in a subspace can be easily calculated. Gabor and wavelet analysis are two popular tools for signal processing, and they can reveal time-frequency distribution for a nonstationary signal. Both schemes can be regarded as signal decompositions onto a set of basis functions, and their basis functions are derived from a single prototype function through simple operations. Therefore, the basis functions used in Gabor and wavelet analysis can be regarded as special frames. For completeness we also make some simple introductions on the results of special frames such as discrete Gabor and wavelet analysis     

用计算全息制作二维Haar小波变换滤波器的研究

提出了用计算全息制作三类Haar子波滤波器的方法。将一维Haar子波函数组合,将得到三种类型的二维Haar子波函数,利用这三类二维Haar子波可以分别对图像进行角、水平边和垂直边的提取。采用罗曼Ⅲ型迂回位相编码法对三类Haar子波函数进行编码,绘出计算全息图。为了提高全息图的质量,采用显示器分屏抓图的办法,将全息图分割输出。在4f系统中实现对二维图像的三类Haar子波变换,给出了提取二值图像特征的模拟结果。