基于ILDC理论的平面结构电磁散射计算

增量长度绕射系数理论(ILDC)是对电大尺寸复杂目标进行边缘绕射计算的有效方法之一。本文基于散射总场、表面光学场和边缘绕射场三者之间的关系，针对几何建模的特点，从另一个角度推导出了ILDC在平表面结构中的应用形式，并验证了该方法的正确性，为目标电磁特性的研究提供了一种可行的计算方法。     

基于ILDC理论的平面结构电磁散射计算

增量长度绕射系数理论(ILDC)是对电大尺寸复杂目标进行边缘绕射计算的有效方法之一。本文基于散射总场、表面光学场和边缘绕射场三者之间的关系,针对几何建模的特点,从另一个角度推导出了ILDC在平表面结构中的应用形式,并验证了该方法的正确性,为目标电磁特性的研究提供了一种可行的计算方法。     

旋转目标的电磁特性分析与计算

分析了一类旋转目标对雷达的电磁特性,并提出了一种基于复射线法和几何绕射理论来计算翼体的边缘绕射场,用复射线法计算夹角的反射场,计算结果说明方法在工程上是可行的,能够满足需要。     

Ka 波段紧缩场天线的设计

设计了一个Ka 波段紧缩场天线。馈源设计结合角锥喇叭在反射面边缘照射电平下降快与波纹喇叭波瓣宽度宽的特点,通过给角锥喇叭加扼流套的改进设计来实现对反射面的照射。反射面采用锯齿型边缘来降低边缘的绕射作用,并进一步在无法加工锯齿部位贴附三角形吸波材料来减小绕射对静区特性的影响。暗室内采用平面测量结果表明,Ka 波段紧缩场天线静区特性满足设计指标要求。     

分析大口径凹形结构散射的一种混合射线方法

在大口径凹形结构散射的特点及有关分析方法的基础上,提出了一类基于复射线方法和几何绕射方法的分析处理大口径凹形结构散射的混合射线方法,即用复射线法计算反射场,以几体绕射法来计算边缘绕射场。基于所设计的便于工程上实现的计算流程,有关分析及相关结果说明了这一方法的可行性和可靠性。     

表面具有分形结构金属劈边缘绕射系数的提取

本文提出了一种计算数值绕射系数有有效方法，并将之用于提取表面具有分形结构金属劈的绕射系统，利用时域有限差分法提供的散射场，根据几何绕射理论建立含有绕射系数未知量的方程组，通过联立方程组的求解，得到散射体边缘的数值绕射系数。具体计算例子表明，该算法是有效的。     

平面波谱法分析电磁波在管道中的传播

通过分析电磁波从管道开口耦合进入管道的绕射机理及场在腔体内的射线特征，将进入管道的绕射场展开为平面波谱，利用平面波在自由空间的传播特性分析了电磁波沿管道的传输和反射特征。通过算例验证了该算法的有效性。     

压缩天线测试场反射器的设计

应用几何绕射理论的结果,分析、介绍了两种压缩天线测试场(简称压缩场)反射器—滚边反射器和锯齿边反射器。导出了锯齿边反射器的边缘轮廓线方程,并根据J.B.凯勒的口径角点绕射系数公式,介绍了角点绕射场的计算式。给出了平面反射器模型的实验结果。     

大宽高比S形二元喷管电磁散射特性研究

基于研究大宽高比对S形二元喷管的电磁散射特性的影响的目的.在S形二元喷管进出口面积、偏心距、面积变化规律及中心线变化规律不变的条件下,采用自适应超椭圆方法设计了出口宽高比为4.5-8.0的S形二元喷管.采用等效边缘电磁流法与迭代物理光学法分别分析了8种S形二元收敛喷管的出口边缘绕射场及总散射场的电磁散射特性.研究结果表明：在水平、垂直两种极化方式下,喷管出口宽高比的变化对S形二元收敛喷管边缘绕射场的RCS影响较小.喷管出口宽高比的变化对总散射场的RCS影响较大;在水平极化下,宽高比为5.5的S形二元喷管散射场的RCS在大的探测范围内有较小的RCS     

任意多边形导体板的高频散射分析

结合微分长度绕射系数和一致性等放电磁流推导得到了微分边缘绕射系数，将任意观察方向上的远区散射场与任意方向入射的平面波以绕第三者数形式联系起来，得到一种实用简便的高频散射计算方法。并以多边形平板为例验证了该方法。     

Significance of image representation for face verification

In this paper we discuss the significance of representation of images for face verification. We consider three different representations, namely, edge gradient, edge orientation and potential field derived from the edge gradient. These representations are examined in the context of face verification using a specific type of correlation filter, called the minimum average correlation energy (MACE) filter. The different representations are derived using one-dimensional (1-D) processing of image. The 1-D processing provides multiple partial evidences for a given face image, one evidence for each direction of the 1-D processing. Separate MACE filters are used for deriving each partial evidence. We propose a method to combine the partial evidences obtained for each representation using an auto-associative neural network (AANN) model, to arrive at a decision for face verification. Results show that the performance of the system using potential field representation is better than that using the edge gradient representation or the edge orientation representation. Also, the potential field representation derived from the edge gradient is observed to be less sensitive to variation in illumination compared to the gray level representation of images.     

Gaussian Beam Summation Representation of a Two-Dimensional Gaussian Beam Diffraction by a Half Plane

A Gaussian beam summation (GBS) representation for half plane diffraction of an incident two dimensional Gaussian beam (GB) that hits arbitrarily close to the edge is presented. The scattered field is expanded into an angular spectrum of GBs that emerge from the edge. We derive asymptotic expressions for the diffracted beams amplitudes, that are valid uniformly as a function of the distance of the incident beam from the edge and of the scattering beams angles. In the limiting cases when the incident GB hits far from the edge, these expressions reduce uniformly to a geometrical optics model plus a weak edge diffractions due to the off-axis beam field that hits the edge. Applications to GBS modeling of indoor propagation are discussed.     

Stochastic Theory of Edge Diffraction

We introduce an original formulation for the electromagnetic field diffraction by a knife edge with random roughness: the formulation, based on the asymptotic physical optics approach, leads to closed form evaluations of the statistics of the diffracted field. The edge roughness is described by a stationary zero-mean Gaussian stochastic process with standard deviation sigma and correlation length L. The physical optics approximation is used to evaluate surface currents; mean and variance of the diffracted field are evaluated by means of asymptotic techniques under the hypotheses lambda/r rarr 0, where lambda is the wavelength and r is the distance from the edge, sigma not large with respect to lambda, and sigma/r Lt 1. The main advantages of the proposed method are its simplicity and the fact that, differently from other approaches, the obtained total field is explicitly written as the sum of incident, reflected and diffracted fields. A very interesting conclusion is that, for moderate edge roughness, the diffracted field propagation can be described in terms of the same ray congruence as in the straight (smooth) edge case, with the only difference that the field associated to each ray is a random variable whose statistics are, in this paper, computed in closed form. The presented approach can be then considered as a first step toward a general stochastic theory of edge diffraction. The proposed theoretical results are amenable of interesting practical applications: for instance, the obtained diffracted field statistics can be used to predict the maximum accuracy that can be expected for ray-tracing algorithms that are based on the straight edge assumption.     

Asymptotic analysis of edge-excited currents on a convex face of aperfectly conducting wedge under overlapping penumbra region conditions

The edge-excited surface currents on a convex face of a perfectly conducting curved wedge are investigated in the asymptotic high-frequency limit for the case where the penumbra regions of the edge and surface diffractions overlap. The edge of the wedge is assumed straight, and the incident electromagnetic wave locally plane and normal to the edge. Both polarizations are considered. The surface field induced by the edge diffraction is synthesized in the spirit of the spectral theory of diffraction (STD): the solution for the edge-diffracted field is interpreted as a spectrum of inhomogeneous plane waves, and the surface field excited by each spectral plane wave is obtained by analytical continuation of the Fock (1965) functions into complex space. The main purpose of this work is to prove the reciprocity of a solution deduced previously for the problem of line source radiation from the wedge in question. As a by-product, useful identities for an incomplete Airy function and an Airy-Fresnel integral are developed     

基于分级边缘方位场匹配的人脸特征定位

针对大范围姿态角度问题,提出了一种分级边缘方位场匹配(HEOFM)的人脸特征定位算法.提出了结构Hausdoff距离作为边缘方位场匹配的测度;在此基础上,先根据人脸图像的整体边缘方位场匹配(GEOFM)进行了姿态预估计,得到特征的粗略定位;然后根据特征边缘方位场匹配(FEOFM)在局部区搜索,实现特征的精确定位.实验结果表明:针对小姿态角度人脸图像,与一般的特征检测算法相比,该算法所需训练样本个数较少而定位精度相当;针对中、大姿态角度人脸图像,该算法具有良好的特征定位性能.     

Integrated method for boundary delineation of agricultural fieldsin multispectral satellite images

Most agricultural statistics are calculated per field, and it is well known that classification procedures for homogeneous objects produce better results than per-pixel classification. In this study, a multispectral segmentation method for automated delineation of agricultural field boundaries in remotely sensed images is presented. Edge information from a gradient edge detector is integrated with a segmentation algorithm. The multispectral edge detector uses all available multispectral information by adding the magnitudes and directions of edges derived from edge detection in single bands. The addition is weighted by edge direction, to remove "noise" and to enhance the major direction. The resulting edge from the edge detection algorithm is combined with a segmentation method based on a simple ISODATA algorithm, where the initial centroids are decided by the distances to the edges from the edge detection step. From this procedure, the number of regions will most likely exceed the actual number of fields in the image and merging of regions is performed. By calculating the mean and covariance matrix for pixels of neighboring regions, regions with a high generalized likelihood-ratio test quantity will be merged. In this way, information from several spectral bands (and/or different dates) can be used for delineating field borders with different characteristics. The introduction of the ISODATA classifier compared with a previously used region growing procedure improves the output. Some results are compared with manually extracted field boundaries     

Scattering by right angle dielectric wedge

An asymptotic solution of electromagnetic waves scattered by a right-angled dielectric wedge for plane wave incidence is obtained. Scattered far fields are constructed by waves reflected and refracted from dielectric interfaces (geometric-optical fields) and a cylindrical wave diffracted from the edge. The asymptotic edge diffracted field is obtained by adding a correction to the edge diffraction of physical optics approximation, where the correction field in the far-field zone is calculated by solving a dual series equation amenable to simple numerical calculation. The validity of this result is assured by two limits of relative dielectric constantvarepsilonof the wedge. The total asymptotic field calculated agrees with Rawlins' Neumann series solution for smallvarepsilon, and the edge diffraction pattern is shown to approach that of a perfectly conducting wedge for largevarepsilon. Calculated far-field patterns are presented and the accuracy of physical optics approximation is discussed.     

Design rules for field plate edge termination in SiC Schottkydiodes

Practical design of silicon carbide (SiC) Schottky diodes incorporating a field plate necessitates an understanding of how the addition of the field plate affects the performance parameters and the relationship between the diode structure and diode performance. In this paper, design rules are presented for SiC Schottky diodes that incorporate field plate edge termination. The use of an appropriate field plate edge termination can improve the reverse breakdown voltage of a SiC Schottky diode by a factor of two. Reverse breakdown voltage values can be obtained that are up to 88% of the theoretical maximums     

Radio-Wave-Propagation Modeling in the Presence of Multiple Knife Edges by the Bidirectional Parabolic-Equation Method

Fast full-wave computation of fields is the main reason for a radio system planner to apply the parabolic-equation method (PEM) for radio-coverage prediction. In this paper, the capability of PEM for the determination of backward electromagnetic-field scattering by single and multiple knife edges is investigated. For the wave equation, a suitable solution is considered. Contrary to the available formulations for the PEM, the proposed method includes a backward-propagating term. For both the forward and backward terms, the split-step algorithm is employed. Similar to the terrain-masking method, the effect of edges is exerted by the Fresnel-Kirchhof approximation into the forward-propagating wave. For the backward field, a similar algorithm is derived. Then, an iterative marching algorithm is developed for modeling radio wave propagation over multiple knife edges. Numerical results are compared with those of Uniform Theory of Diffraction (UTD) and finite-difference time-domain methods for the single-knife-edge problem. In addition, the effect of the backward-propagating field on radio wave propagation is investigated. The effect of edge locations and heights on the backward scattered field and number of computational iterations is studied. It is seen that, for the single knife edge, the backward scattered field depends on the height and location of the edge. The intensity of the backscattered field is a monotonic increasing function of the edge height. In addition, by increasing the edge height, the beam width of the backward scattered field increases. In ranges far from the knife edge in front of it, the field spreads, and its amplitude decreases. For multiple knife edges, it is seen that, in addition to the single backward scattered field, radio-wave-propagation modeling requires multiple forward-backward terms between successive edges. The geometry of the problem determines the required number of terms for sufficient accuracy     

基于近场时域谱的紧缩场口径优化设计

根据口径近场的时域谱,可以在全频段评估口径的绕射特征,从而建立紧缩场口径设计的一般准则.在X波段8～12GHz,计算了几种限度为5m×5m口径的近场时域谱.计算结果表明,在紧缩场设计中,为降低边缘绕射场电平,方口径优于圆口径,理想多项式锥削优于余弦锥削和均匀分布.提出两种工程化的边齿设计方法,在口径场均匀分布条件下,近场时域干扰谱分量均低于-25dB.口径近场计算均采用口径场卷积法.