时域有限差分法的收敛性

本文通过计算实例讨论了时域有限差分法的收敛性问题。计算了在平面波照射下无限长介质柱中的电磁场分布。把不同空间和时间步长时的计算结果,与用解析法所获得的结果进行比较,证明时域有限差分法有较好的收敛特性。对具有不同空间分辨率的人体二维非均匀块状模型中吸收电磁能量分布的计算表明,在高度非均匀介质中的场分布,仍然保持较好的收敛性。这些讨论从一个侧面提供了选取网格步长的依据。     

人体吸收电磁能量与入射方向和极化的关系

本文讨论了用时域有限差分法计算在平面电磁波照射下人体对电磁能量的吸收问题。计算了各种入射方向和极化条件下非均匀块状人体模型中的局部比吸收率(SAR)分布。计算结果表明,人体并不总是在正面入射时吸收能量最多;同时还表明,局部比吸收率比平均比吸收率更值得重视,因为局部吸收率的最大值往往比全身平均值大几倍到十几倍。本文的计算结果丰富了人们在人体电磁剂量学方面的知识。     

时域有限差分(FD-TD)法在电磁幅射系统近场计算中的应用

本文讨论了用时域有限差分法计算电磁辐射系统的近场问题。为了验证所用方法的正确性和精确度,计算了几种辐射系统,其中包括结构复杂的环形相控阵列的辐射场。把理论计算所得结果与测量结果进行了比较,表明所用方法是可行的。     

浅层地下目标时域电磁散射问题的数值模拟

本文用时域有限差分法(FD-TD法)计算了浅层地下目标的时域电磁散射。在地下色散媒质的参数r()和r()都是以Debye方程表示时,推出了FD-TD法的迭代分式,并给出了相应的吸收边界条件。通过将FD-TD法计算的结果与其它结果相比较,证实了该方法对计算有耗媒质中电磁场问题的有效性。对瞬态脉冲在色散媒质中的传播特性进行了讨论。分别计算了典型地下目标的时域散射波形和波形堆积图。     

脉冲电磁波对人体作用的研究

用时域有限差分法和非均匀块状人体电磁模型研究了脉冲电磁场对人体的作用问题，首先用解析法计算无限长介质圆中脉冲波所引发的电流以验证计算方法，然后对核电磁脉冲照射下的二维和三维非均匀块状人体模型中产生的电流及所吸收的电磁能量进行了，计算了它们随时间的变化及其在全身的分布，在计算中没有考虑介质色散的影响。     

通过导电壁上窗口透入的电磁波对人体作用的研究

本文用时域有限差分法和人体非均匀电磁模型研究了通过无限大导电壁上窗口透入的电磁场与人体的相互作用。首先对平面电磁波通过导电壁上窗口的透入特性进行了计算,并与解析解进行比较,证明了计算的可靠性。在此基础上对不同极化情况的入射平面波的透入场与靠近窗口的坐姿人体模型的作用进行了研究。计算了人体模型中的电磁场分布和所吸收的电磁能量。     

辛差分格式的Mur吸收边界在电磁场计算中的应用

基于辛(symplectic)差分格式的时域有限差分(FDTD)法,将辛分块Runge-Kutta(SPRK)法引入Mur吸收边界条件(ABC)的推导过程,用辛差分格式代替原来的中心差分格式离散时间变量,推导出了基于辛差分格式的Mur吸收边界条件.通过将辛格式的Mur吸收边界结合辛时域有限差分法,对典型算例进行数值模拟,并将计算效果与传统Mur边界结合时域有限差分法的效果比较,表明辛格式的Mur吸收边界结合辛时域有限差分法计算效果良好,基于辛差分格式的Mur吸收边界条件是正确、有效的,计算精度优于传统方法.     

书讯

正由北京大学王长清、祝西里教授编著的"电磁场计算中的时域有限差分法"一书的第二版已于2014年2月由北京大学出版社作为"中外物理学精品书系"的前沿系列出版。该书全面系统地论述了时域有限差分(FDTD)法的基本原理及其在广泛领域的应用方法,并反映了最新发展成果。前三章介绍了时域有限差分法的基本原理,包括差分格式的建立,数值色散和稳定性以及网格的剖分方法。第四章系统地介绍了应用于开域问题的各种吸收边界条件。第五至七章讨论了时域有限差分法在散射、辐射、微波和光波线路分析中的应用。第八章专门讨论     

通过导电壁上窗口透入的电磁波对人体作用的研究

本文用时域有限差分法和人体非均匀电磁模型研究了通过无限大导电壁上窗口透入的电磁场与人体的相互作用。首先对平面电磁波通过导电壁上窗口的透入射性进行了计算，并与解析解进行比较，证明了计算的可靠性。在此基础上对不同极化情况的入射平面波的透入场与靠近窗口的姿人体模型的作用进行了研究，计算了人体模型中的电磁场分布和所吸收的电磁能量。     

填充左手介质矩形波导的截止频率研究

提出用时域有限差分法(FDTD)分析填充左手介质矩形波导的截止频率,给出了直角坐标系下无耗均匀左手介质中的二维时域有限差分法(FDTD)的差分公式.用FDTD法分析计算了填充无耗均匀左手介质的厚度和折射率对矩形波导TE模截止频率的影响,并与右手均匀介质填充进行比较,结果表明二者对TE模截止频率的影响正好相反,展示了左手介质所具有的奇特的电磁特性.     

USE OF FINITE-DIFFERENCE TIME-DOMAIN METHOD FOR CALCULATING EM ABSORPTION IN LOSSY DIELECTRIC SCATTERER

The problem for calculating EM energy absorption by lossy dielectric scatterer ir-radiated by plane wave are discussed.The factors affecting the accuracy of computation arediscussed.The calculated results of EM energy absorption and its distribution in homogeneousand layered homogenous lossy dielectric spheres are presented,and a comparison of these resultswith analytical solution is given.The calculation is carried out for dielectric cylinder on conduct-ing ground as well,and the results are compared with the image theory.All the computationsshew that the finite-difference time-domain method can give satisfactory results.     

Fast-Fourier-Transform Method for Calculation of SAR Distributions in Finely Discretized Inhomogeneous Models of Biological Bodies

The paper describes a novel iterative approach for calculations of specific absorption rate (SAR) distributions in arbitrary, lossy, dielectric bodies. To date, the method has been used for 2-D problems where its accuracy has been confirmed by comparison with the analytic solutions for homogeneous and layered, circular, cylindrical bodies. With computation times that are proportional to N log/sub 2/N rather than N/sup 2/ to N/sup 3/ for the method of moments, the present approach should be extendable to 3-D bodies with N= 10/sup 4/to 10/sup 5/ cells allowing, thereby, details of SAR distributions that are needed for EM hyperthermia, as well as for assessing biological effects.     

The dependence of electromagnetic energy absorption upon human-headmodeling at 1800 MHz

The authors of a previously published paper on the dependence of electromagnetic (EM) energy absorption concluded that homogeneous modeling of the human head is suited for assessing the spatial-peak absorption for transmitters operating at 900 MHz or below. Additional studies became necessary for the frequency bands utilized by new mobile communications systems (i.e., 1.5 and 2.5 GHz) since some peripheral tissue layers have a thickness of the range of λ/4-λ/2. The results of the simulations combined with worst-case considerations confirmed the anticipated and more complex relationship between absorption and anatomical details at these higher frequencies. Nevertheless, a homogeneous representation of the head is suited for assessing the maximum specific absorption rate (SAR) in the head of the user of mobile telecommunication equipment (MTE) if the appropriate dielectric parameters are chosen     

Electromagnetic scattering by arbitrary shaped three-dimensional homogeneous lossy dielectric objects

The recent development and extension of the method of moments technique for analyzing electromagnetic scattering by arbitrary shaped three-dimensional homogeneous lossy dielectric objects is presented based on the combined field integral equations. The surfaces of the homogeneous three-dimensional arbitrary geometrical shapes are modeled using surface triangular patches, similar to the case of arbitrary shaped conducting objects. Further, the development and extensions required to treat efficiently three-dimensional lossy dielectric objects are reported. Numerical results and their comparisons are also presented for two canonical dielectric scatterers-a sphere and a finite circular cylinder.     

矩形涂层板的电磁散射

本文讨论了涂覆有耗介质的矩形平板的电磁散射。涂层介质的参数可以任意。表达式中涉及的Maliuzhinets函数的计算,由于采用了多项式近似,避免了复平面内的积分,从而使计算时间大为减少。最后将理论计算结果与实验值相比较,两者吻合得很好。     

An absorber tip diffraction coefficient

The UTD corner diffraction solution for a perfectly conducting corner is empirically modified for the case of a dielectric corner. The dielectric may be lossless or lossy, but is assumed to be homogeneous. This modified solution is used to calculate the bistatic scattering from the tip of a dielectric pyramid. Sample calculations display some features of the scattering from a single lossy dielectric pyramid. To verify the solution, calculations are compared with backscatter measurements of a single pyramid that is cut from a homogeneous lossless dielectric (polyethylene). Calculations are then compared with measurements for the more pertinent case of bistatic scattering from a wall of pyramidal radar absorbing material     

Scattering by arbitrarily cross-sectioned layered, lossy dielectric cylinders

The scattering properties of TM or TE illuminated lossy dielectric cylinders of arbitrary cross section are analyzed by the surface integral equation techniques. The surface integral equations are formulated via Maxwell's equations, Green's theorem, and the boundary conditions. The unknown surface fields on the boundaries are then calculated by flat-pulse expansion and point matching. Once the surface fields are found, scattered field in the far-zone and radar cross section (RCS) are readily determined. RCS thus obtained for circular homogeneous dielectric cylinders and dielectric coated conducting cylinders are found to have excellent agreements with the exact eigenfunction expansion results. Extension to arbitrary cross-sectioned cylinders are also obtained for homogeneous lossy elliptical cylinders and wedge-semicircle cross-sectioned cylinders, with and without a conducting cylinder in its center. RCS dependences on frequency and conductivity as well as the matrix stability problem of this surface integral equation method are also examined.     

Angularly propagating waves in a radially inhomogeneous, lossy dielectric cylinder and their connection with the natural modes

A physical interpretation is proposed for the natural modes of a radially inhomogeneous, lossy dielectric circular cylinder embedded in a homogeneous, lossless dielectric. These modes are either of the creeping-wave or of the whispering-gallery type. With the aid of a generalized Wentzel-Kramer-Brillouin (WKB) asymptotics, an asymptotic characteristic equation is derived for each type, which can be explained physically. Numerical results obtained from the exact characteristic equation and both approximate equations are presented and discussed.     

A moment method solution for the shielding properties of three-dimensional objects above a lossy half space

In this paper, a moment method (MM) solution for analyzing the electromagnetic (EM) shielding properties of three-dimensional (3-D) lossy dielectric and magnetic objects over a lossy half space is presented. An MM, based on a volume formulation and a special Green's function in the spectral domain, is developed. Plane waves with TMz and TEz polarizations incident upon 3-D lossy material structures are demonstrated for the shielding effects of those bodies in the presence of a lossy ground. Some of the results are compared with those evaluated by applying the finite difference time domain method, and good agreements are obtained. The MM solution can be used to study the shielding problems for 3-D objects located above a lossy ground.     

A NEW METHOD FOR SOLVING THE SCATTERING PROBLEM OF DIELECTRIC BODIES

A new approach to the EM scattering problem of an inhomogeneous lossy dielectric body is pro-posed.It is shown that the entire interior electric field distribution can be obtained from the measured exteri-or field distribution by simple recurrence relations.Detailed derivations of these recurrence relations for thefield distribution inside the scattering body are presented,and the results obtained by computer simulationsare given.