时域有限差分法在有耗介质电磁能量吸收问题中的应用

本文论述了用时域有限差分法计算平面电磁波照射下有耗介质中电磁能量的吸收问题。讨论了影响计算精度的各种因素。计算了均匀和分层均匀球体中能量的吸收和分布,并把计算结果与精确的解析分析进行了比较。还讨论了理想导电面的模拟问题,并把结果与镜象理论进行了对比。所有的结果都说明,在一定条件下,时域有限差分法能给出满意的结果。     

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。     

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.     

一种解介质体散射问题的新方法

本文提出一种解非均匀介质体电磁散射问题的新方法。由测量到的介质体外的散射场,按照递推关系,可以求出全部介质体内的场分布。文章详细地推导了介质体内场分布的递推关系,并给出了计算机模拟计算的结果。     

THE RELATIONSHIP BETWEEN THE EM ENERGY ABSORPTION OF HUMAN BODY AND THE INCIDENT DIRECTION AND POLARIZATION OF PLANE WAVE

The problems of EM energy absorption of human body irradiated by plane wave arediscussed by the Finite-Difference Time-Domain(FD-TD)method.The local Specific AbsorptionRates(SARs),the whole-body average SARs and the layer average SARs for the inhomogeneousblock model of human body with different incident direction and different polarization of theincident waves are calculated.The results show that the appearance of maximum EM energyabsorption is not always at the situation of the front incidence and the local SARs are moreimportant for the interaction of the EM fields with human body.All results provide more infor-mation about the electromagnetic dosimetry for human body.     

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。     

FD—TD法分析埋地目标对基带脉冲波的电磁散射特性

本文利用ＦＤ－ＴＤ法分析了埋了埋地目标对基带脉冲波的电磁散射问题。在推出有耗媒质中ＦＤ－ＴＤ法代公式和吸收边办条件的基础上，对基带脉冲波在有耗媒质中的传播特性和埋地目标的电磁散射特性分别进行了较为详细地讨论。给出了部分目标的回波堆积图，并对探地雷达的探测性能与媒质特性、目标特性的关系进行了分析。     

有损耗左手材料模型的时域电磁分析

分析了电磁(EM)场在有损耗左手材料(LHM)模型中能量与损耗因子间的关系,推导了有耗LHM模型的时域有限差分法(FDTD)计算公式,数值模拟了EM场在该有耗LHM材料模型中的传播;对比无耗与有耗LHM模型的EM聚焦特性,分析EM场在不同损耗因子LHM材料中的传播.研究表明,数值结果与理论分析得到较好吻合.     

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.     

Resonance Absorption in Nonsymmetrical Lossy Dielectric Inserts in Rectangular Waveguides

Features of resonance power absorption in a lossy dielectric resonator shaped as a rectangular trapezium in a rectangular waveguide are studied. It is found that such a resonator can provide strong absorption (or reflection) of the TE₁₀-mode input from the straight (or inclined) interface port. The maximum absorption is observed when the sum of the coupling coefficients equals unity. Simple formulas relating the scattering and absorption coefficients with the coupling coefficients at the resonance frequency are presented. The physical mechanism of resonance absorption is discussed.     

Three-dimensional finite, boundary, and hybrid element solutions ofthe Maxwell equations for lossy dielectric media

Finite, boundary, and hybrid element approaches are presented as numerical methods for computing electromagnetic (EM) fields inside lossy dielectric objects. These techniques are implemented as computer algorithms for solving the Maxwell equations in heterogeneous media in three dimensions. Algorithm verification takes the form of comparisons of test cases with analytic solutions. Computed results for each technique are in good agreement with exact solutions, especially in light of the coarse computational grid resolutions used. Implementation was in FORTRAN on a moderate-sized computer (MicroVax II). The basic problem formulation is quite general; however, it has direct application in hyperthermia as a cancer therapy where the EM fields produced inside the patient by external sources are of interest. An example of the application of these numerical methods in a three-dimensional clinical setting is shown     

New approximate formulations for EM scattering by dielectric objects

The extended Born approximation (ExBorn) has been shown an efficient formulation in the electromagnetic (EM) scattering by dielectric objects in both free-space and air-Earth half-space problems. In most cases, ExBorn is much more accurate than the conventional Born approximation at low frequencies. When the frequency is high or the contrast of dielectric objects is large, however, the ExBorn approximation becomes inaccurate. In this paper, new approximations are proposed for the EM scattering by dielectric objects buried in a lossy Earth, which are also suitable for the case of free space. It has been shown that the zeroth-order form of new approximations is completely equivalent to ExBorn. Hence, high-order approximations can be regarded as high-order ExBorn. Closed-form formulations are derived for the new approximations. Using the fast Fourier transform (FFT), these formulations can be implemented efficiently at a cost of CNlogN, where N is the number of unknowns and C is a small number. Numerical simulations show that high-order ExBorn approximations are much more accurate than the ExBorn approximation.     

Numerical analysis of electromagnetic fields in biological issues

The feasibility of using integral equation techniques to predict EM fields in biological tissues is demonstrated by analyzing the scattering from arbitrary cylinders composed of lossy dielectric materials. For the circular cylinder case, good agreement is obtained between moment method solutions and exact solutions. To show the applicability to arbitrary geometries, an elliptical cylinder is also considered.     

The Role of Chirality and Resonance in Synthetic Microwave Absorbers

Microwave absorption by a lossy dielectric material containing thin metal wires is considered. The wires are bent to create either chiral, non-chiral or racemic unit cells. No physical mechanism is found to support patents which were granted between 1990 and 1993, and related claims in the engineering literature, that chirality is the key to improved microwave absorbers. Instead, in synthetic composites which employ thin metal wires in a lossy dielectric host, half-wave resonance of the inclusions – not their geometric shape – is identified as the mechanism responsible for enhanced absorption. It is also found that – even in an essentially lossless host – resonant steel wires, whether chiral or not, can strongly absorb electromagnetic waves.     

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     

Inorganic–Organic Hybrid Dielectrics for Energy Conversion: Mechanism,Strategy, and Applications

Dielectric materials with higher energy storage and electromagnetic (EM) energy conversion are in high demand to advance electronic devices, military stealth, and mitigate EM wave pollution. Existing dielectric materials for high-energy-storage electronics and dielectric loss electromagnetic wave absorbers are studied toward realizing these goals, each aligned with the current global grand challenges. Libraries of dielectric materials with desirable permittivity, dielectric loss, and/or dielectric breakdown strength potentially meeting the device requirements are reviewed here. Regardless, aimed at translating these into energy storage devices, the oft-encountered shortcomings can be caused by either of two confluences: a) low permittivity, high dielectric loss, and low breakdown strength; b) low permittivity, low dielectric loss, and process complexity. Contextualizing these aspects and the overarching objectives of enabling high-efficiency energy storage and EM energy conversion, recent advances in by-design inorganic–organic hybrid materials are reviewed here, with a focus on design approaches, preparation methods, and characterization techniques. In light of their strengths and weaknesses, potential strategies to foster their commercial adoption are critically interrogated.     

磁偶极子激励的脉冲电磁波在损耗介质中的传播

本文用技坐标系中的时域有限差分法（FD-TD）模拟了磁偶极子激励的脉冲电磁波在均匀和轴对称分层均匀损耗介质中的传播问题，所得结果与文献中对类似问题的研究结果一致，从而相互得到了验证。并进一步证明Maxwell方程完全适应于脉冲电磁波在损耗介质中的传播问题。所用方法可用于电磁脉冲测并问题的进一步研究。     

Electromagnetic coupling of coplanar waveguides and microstriplines to highly lossy dielectric media

The spectral-domain technique is utilized to analyze the coupling characteristics of coplanar waveguides and microstrip lines coupled with multilayer lossy dielectric media. Numerical results illustrating the dispersion characteristics of coplanar and microstrip lines, as well as the various electric field components coupled to highly lossy dielectric media, are presented. It is shown that the presence of a superstrate of lossless dielectric between the coplanar waveguide and the lossy medium plays a key role in setting up an axial electric field component that facilitates leaky-wave-type coupling to the lossy medium. The thickness of the superstrate relative to the gap width in the coplanar waveguide is important in controlling the magnitude of this axial electric field component. The coupling characteristics of the microstrip and coplanar lines are compared, and results generally show improved coupling if coplanar waveguides are utilized. Values of the attenuation constant α are higher for coplanar waveguide than for microstrip line, and for both structures α decreases with frequency     

Use of the Finite-Difference Time-Domain Method in Calculating EM Absorption in Human Tissues

Although there are acceptable methods for calculating whole body electromagnetic absorption, no completely acceptable method for calculating the local specific absorption rate (SAR) at points within the body has been developed. Frequency domain methods, such as the method of moments (MoM) have achieved some success; however, MoM requires computer storage on the order of (3N) 2 and computation time on the order of (3N) 3 where N is the number of cells. The finite-difference time-domain (FDTD) method has been employed extensively in calculating the scattering of metallic objects, and recently is seeing some use in calculating the interaction of EM fields with complex, lossy dielectric bodies. Since the FDTD method has storage and time requirements proportional to N, it presents an attractive alternative to calculating SAR distribution in large bodies. This paper describes the FDTD method and evaluates it by comparing its results to analytic solutions in two and three dimensions. The utility of the FDTD method is demonstrated by a 3D scan of the human torso. The results obtained demonstrate that the FDTD method is capable of calculating internal SAR distribution with acceptable accuracy. With the availability of supercomputers, such as the CRAY II, the calculation of SAR distribution in a man model of 50 000 cells (1.27 cm per cell) appears to be feasible.