应用空域分解法研究复杂物体的散射特性

本文给出了三种空域分解法迭代格式.在入射波非对称而结构对称时.给出了一种处理方法.从而减少计算量和存储量.以园柱体和平板复合体为目标,对TM波正入射时三种迭代格式下的结果及存储量计算量作了比较,然后分别计算了TM波和TE波入射时的散射待性.对入射方向和耦合的影响进行了讨论.     

非均匀电离层中VLF辐射源在海面上产生的场

为了研究甚低频(very low frequency, VLF)电磁波对电离层中目标的近场散射机制,提出了一种VLF电磁波在该环境中对有限长金属和介质柱体近场散射的计算方法。首先,通过定义散射系数和交叉散射系数,分析各向异性电离层中电型波和磁型波激励下的散射矩阵;然后,根据积分方程法和矩量法,计算得到各向异性电离层中有限长金属和介质柱体的近区散射场和表面电流半解析表达式;最后,通过各向异性电离层中VLF波的特征波分析,给出柱体近区散射场中寻常波(O波)和非寻常波(E波)分量空间分布情况。计算结果表明：电离层中VLF波在金属和介质目标表面激励了不同的电流,并在不同的电离层环境参数、柱体尺寸和入射角度下产生相应的近场散射和耦合散射;通过对比O波和E波表面电流分布和散射分量,进一步揭示了VLF波与暴露在均匀等离子体环境中的柱体目标的散射机理。本文的计算方法和结果可为电离层中散射通信和目标探测提供理论依据。

     

电大尺寸多柱体电磁散射问题的一种快速混合算法--MEI+FMM

本文提出了一种改进的快速迭代ＭＥＩ（不变性测试方程）算法用于分析电气大尺寸多柱体的散射问题,在此算法中我们首次将快速多极子技术（ＦＭＭ）用于加速多柱体之间多次散射场的计算。应用本算法计算了柱体周长为几千波长的多柱体散射场。实际计算结果显示,一方法与原有的直接计算方法具有几乎同样的精度,而速度提高了两个数量级。     

一种快速计算多柱体散射问题的区域分解时域有限差分方法

本文提出一种用于计算多柱体散射问题的区域分解时域有限差分算法（DD-FDTD）。当各散射柱体相互间距离比较远时,采用经典的FDTD方法进行计算时,计算域是非常大的,大量的网络浪费在射体之间。文中我们使用区域分解的思想,把各个柱体处理为各个子域。各柱体间大量的网格被省去,代之以二维时域Green函数将各个子域连接起来。采用近远场变换方法,并将柱体间互偶处理为等效柱面波入射场,从而大大压缩了计算时间。由于二维时域Green函数含有关于时间的积分,且此积分是一个反常积分,采用半解析的方法,精确地算出了其中的反常积分值,大幅度提高了计算精度。最终,使该方法得以实现。     

埋地二维柱体的电磁散射

本文利用进域有限差分（ＦＤ－ＴＤ）方法分析计算了正弦平面波照射下埋地二维金属柱体的电磁散射特性，给出了柱体上的感就电流值和地面上方近场区的电磁场值。讨论了有耗介质中差分网格的数值色散特性和吸收边界条件。通过将本文用ＦＤ－ＴＤ计算结果与其它数值结果进行比较，证实了ＦＤ－ＴＤ方法在分析有耗介质中电磁散射问题的有效性。     

BCGs-FFT结合BP神经网络反演金属介质复合柱体目标参数

利用双共轭梯度-快速傅里叶变换方法（BCGs-FFT）结合BP神经网络技术研究了金属介质复合结构柱体目标的电磁逆散射问题.先用BCGs-FFT方法计算了复合结构目标的正散射问题,得到不同目标参数下的多个观测点上的散射电场,以此作为训练样本提供给BP网络,经过适当的离线训练,再以新的散射电场作为网络的输入,实时重构了金属介质复合结构目标的几何、电磁参数.数值结果显示了该方法的有效性.     

一种节省内存的降维时域有限差分法及其在散射计算中的应用

介绍一种节省内存的降维时域有限差分方法(R-FDTD)．该方法理论上可节省约33％的场分量存贮量。本文首次把R-FDTD用于散射计算，提出了一种更为简单直观的新的导体处理方法，并阐明R—FDTD中连接边界、吸收边界和近一远场变换的实现方法。以TE波入射时二维柱体散射为例的计算结果表明，R—FDTD在节省约33％内存的基础上，计算精度与标准FDTD相当。     

用保角变换法计算任意源激励的任意截面柱体的辐射场

利用保角变换方法，将奇异性边界变换为简单光滑的边界，从而使点匹配法计算表面电流时能自动控制边界的划分和匹配点的选取，因而它能处理任意截面导电柱体的散射问题。本文进一步将保角变换方法与互易定理结合起来，这一方法可以处理任意截面柱体旁天线辐射的一般问题。本文计算了无限长正四棱柱对平面波的散射并与圆柱的散射进行了比较；并计算了无限长导电正四棱柱分周向半波振子的辐射并用矩量法的结果进行了验证。     

电磁格点理论在有耗介质涂层导体薄片上TE波散射中的应用

在ＴＥ波入射下，对于计算二维封闭导体柱散射问题所常用的磁场积分方程法，当用于导体薄片时将会失效．本文采用电磁格点理论和电场方法研究这个问题．结合二维介质柱的电磁格点方程，解决了有耗介质涂层导体薄片的ＴＥ波散射．文中给出计算实例．     

FD—TD法求解有耗柱体电磁散射的近场

本文讨论了用时域有限差分法计算E平面波入射到任意截面柱体时的二维电磁散射近场问题。先计算了完纯介质柱体的几个典型的例子,与文献[1]中的实验结果及其它数值结果相比较,证明所用方法的有效性。然后,计算了有耗体和有耗介质覆盖导电体的电磁散射近场。     

圆柱面分层介质结构的微波成像

讨论研究了圆柱面分层介质结构的微波成像问题。设计了一种基于源型积分方程的迭代成像算法。为改善算法的数值稳定性，设计采用了平面波在目标区域中心处的级数展开式中的各次柱面波作为入射波组合的处理方法，该处理方法同时可简化算法的分析计算过程，为验证算法的可靠性和有效性，文中对不同的介质结构分别进行了数值模拟，结果表明该文所设计的算法应用于不同介质结构时均能保持较高的准确性，并具有良好的数值稳定性。     

Sub-threshold analysis and drain current modeling of polysilicon thin-film transistor using Green's function approach

An analytical analysis for a poly-crystalline silicon thin-film transistor is presented. The Green's function approach is adopted to solve the two-dimensional Poisson's equation using Neumann's boundary conditions at the silicon-silicon di-oxide interface. The developed model gives an insight of device behavior due to the effect of traps and also grain-boundary effect. The analysis of threshold voltage depicts short-channel effects and drain-induced barrier lowering. The model is extended to analyze the transfer characteristics and obtain the transconductance of the device. The results obtained show good agreement with the numerical model and with simulated results, thus proving the validity of our model.     

A macroscopic view of self-replication

In 1953, Crick and Watson published their landmark paper revealing the detailed structure of the DNA double helix. Several years earlier, von Neumann embedded a very complex configuration, a universal interpreter-copier, into a cellular array. Astoundingly, the structure of this configuration, able to realize the self-replication of any computing machine, including a universal Turing machine, shares several common traits with the structure of living cells as defined by Crick and Watson's discovery. To commemorate the 100th anniversary of von Neumann's birth, this paper presents a macroscopic analysis of self-replication in computing machines using three examples. After describing self-replication in von Neumann's universal interpreter-copier, we will revisit the famous self-replicating loop designed by Langton in 1984. In order to overcome some of the major drawbacks of Langton's loop, namely, its lack of functionality and the fact that it is ill-adapted for a realization in electronic circuits, we present a novel self-replicating loop, the Tom Thumb loop. Endowed with the same capabilities as von Neumann's interpreter-copier, i.e., the possibility of replicating computing machines of any complexity, our loop is moreover specifically designed for the implementation of self-replicating structures in programmable digital logic.     

An analysis of new and existing FDTD methods for isotropic coldplasma and a method for improving their accuracy

Over the past few years, a number of different finite-difference time-domain (FDTD) methods for modeling electromagnetic propagation in an isotropic cold plasma have been published. We have analyzed the accuracy and stability of these methods to determine which method provides the greatest accuracy for a given computation time. For completeness, two new FDTD methods for cold plasma, one of which is based on the concept of exponential fitting, are introduced and evaluated along with the existing methods. We also introduce the concept of cutoff modification which can be easily applied to most of the FDTD methods, and which we show can improve the accuracy of these methods with no additional computational cost. Von Neumann's stability analysis is used to evaluate the stability of the various methods, and their accuracy is determined from a straightforward time-and-space harmonic analysis of the dispersion and dissipation errors. Results of numerical experiments to verify the accuracy analysis are presented. It is found that for low-loss plasma, the piecewise linear recursive convolution method (PLRC) method is the most accurate, but the method of Young (see Radio Sci., vol.29, p.1513-22, 1994) can use less memory and is nearly as accurate. In this low-loss plasma regime, cutoff modification can significantly reduce the error near cutoff at the expense of slightly greater error at lower frequencies. For strongly collisional plasmas, the PLRC method also provides the most accurate solution     

有耗电磁场问题中复宗量整数阶圆柱函数的计算

本文介绍了复宗量整数阶圆柱函数数值计算的递推算法。在计算机允许的数值范围内对圆柱函数的阶数及宗量范围均没有限制。试算结果经加法定理和Wronskian关系式的验算表明准确度很高。     

A Real Cloud Computer

Attempts to calculate the weather numerically have a long history. The first effort along these lines took place not in some cutting-edge university or government lab but on what the lone man doing it described as "a heap of hay in a cold rest billet." Lewis Fry Richardson, serving as an ambulance driver during World War I and working with little more than a table of logarithms, made a heroic effort to calculate weather changes across central Europe from first principles way back in 1917. The day he chose to simulate had no particular significance-other than that a crude set of weather-balloon measurements was available to use as a starting point for his many hand calculations. It's no surprise that the results didn't at all match reality. Three decades (and one world war) later, mathematician John von Neumann, a computer pioneer, returned to the problem of calculating the weather, this time with electronic assistance, although the limitations of the late-1940s computer he was using very much restricted his attempt to simulate nature. The phenomenal advances in computing power since von Neumann's time have, however, improved the accuracy of numerical weather forecasting and allowed it to become a routine part of daily life. Will it rain this afternoon? Ask the weatherman, who in turn will consult a computer calculation.     

Comments on “Bistatic specular scattering from roughdielectric surfaces” [and reply]

The author comments on the paper of De Roo and F. T. Ulaby (see ibid., vol.42, no.2, p. 220-231, 1994) which described experimental work on scattering by dielectric rough surfaces, and investigated the shift of the Brewster angle when a plane interface becomes randomly modulated. As mentioned by the authors, the numerical evidence of the phenomenon has been given where rigorous computations have shown that the minimum of the reflected intensity is shifted toward lower incidence angles. This conclusion, concerning one-dimensional surfaces under p polarization, is in agreement with the aforementioned experimental work. Later, with the help of the perturbation methods, a more detailed study has been achieved by several authors for both one-dimensional surfaces and two-dimensional surfaces. The main point is that although the theories are not the same, they all lead to the same conclusion and confirm the previous results. Therefore, the author was very surprised to read in the introduction that a shift toward grazing angles was predicted. The DE Roo and Ulaby reply that the theoretical and numerical calculations predict that the Brewster angle shifts in the direction of normal incidence (negative shift) as the surface roughness increases, and their experimental results indeed support the prediction. They have no disagreement with this comment and are in fact pleased that their data confirms the theory     

FDTD编程模拟光波导光场分布的数值不对称的校正方法

以条形光波导为例,研究了用时域有限差分(FDTD)法编程模拟波导光场分布时的数值不对称性,及其对模拟计算造成的影响,提出了两种合理的解决方法,成功消除了数值不对称现象对正常模拟的干扰,通过在波导光场FDTD模拟中的实际应用,分析了每种方法的特点及欠缺,分别论证了它们在数值模拟中的应用价值,并阐明这两种方法对于任意形状光波导光场的FDTD模拟是普遍适用的。     

Computational and experimental analysis of the scattering byrotating fans

The electromagnetic scattering of rotating blades is investigated both theoretically and experimentally. We have developed an analytical method based on physical optics (PO) and the method of equivalent currents (MEC), in conjunction with the quasi-stationary method. We have predicted the bistatic field scattered by a multiple skew-plated rotating fan. Comparisons with a numerical calculation based on the method of moments and with measurements have given very satisfactory results. A comparison with previously published results has also been carried out. Physical interpretations are given both in time and frequency domains. Our analytical model correctly predicts the spreading and magnitude of the frequency response as a function of the scatterer's skew angles, its rotation frequency, and the directions of incidence and scattering     

有限口径亚波长衍射光学器件的严格矢量分析

利用二维时域有限差分(FDTD)方法作为严格电磁计算模式,分析、比较了连续面型和经过Farn方法得到的亚波长结构面型的衍射微柱透镜在TE极化波和TM极化波入射情况下的聚焦特性。对不同F数衍射微柱透镜的严格矢量分析表明,亚波长结构器件的聚焦特性对输入光波的极化情况有更强的敏感性。并简要讨论了FDTD方法的数值色散和计算空间吸收边界的设置等问题。