层状媒质上有限长度偶极天线的辐射场

本文采用有耗媒质的复镜象理论与几何光学法相结合的方法计算了两层导电媒质上有限长度偶极天线的辐射场,节省了计算时间,测试值与计算值基本一致。     

色散媒质中的完全匹配层吸收边界

将Ａ．Ｐ．Ｚｈａｏ提出的与媒质无关的完全匹配层吸收边界推广到色散媒质中,推导出了用于色散媒质的无反射完全匹配条件。并对导电媒质进行了数值实验,结果十分理想。与Ｑ．Ｈ．Ｌｉｕ的方法相比,该方法可以应用到更复杂的媒质中。而与Ｓ．Ｄ．Ｇｅｄｎｅｙ的各向异性媒质吸收层相比,在角区的处理要简单得多,更适合于时域有限差分法,对于三维问题更是如此。     

广义柱形坐标系各向异性完善匹配吸收媒质

Ｂｅｒｅｎｇｅｒ提出的完善匹配层只能用于直角坐标系。本文将完善匹配各向异性吸收媒质推广到广义柱形坐标系。推导是在广义柱形坐标系均匀媒质的Ｍａｘｗｅｌｌ方程与直角坐标系各向异性媒质的Ｍａｘｗｅｌｌ方程之间等效基础上进行的。得出了广义柱形坐标系完善匹配层电导率计算公式。     

透层天线电流积分方程的数值解

在已有的三层有耗媒质中的透层天线（ＢＰＡ）电流积分方程的基础上,给出了电流积分方程中的索末菲积分的低频近似解析公式并给出了其物理解释。利用三项式全域电流基函数,求得了透层天线的输入阻抗。采用静态场近似方法,很方便的求出了透层天线在地面的电场分布。     

微带结构Sommerfeld 积分奇异性分析和表面波极点有效提取

提取表面波的DCIM能够准确计算无耗分层媒质中的空域格林函数。然而,多层媒质表面波极点的提取比较困难,并且此方法没有考虑侧面波在远区的作用。本文基于复变函数理论,严格分析了Sommerfeld积分极点和支点奇异性对远场的影响;将PSO(particle swarm optimization)与Newton-Raphson算法相结合,提出一种快速精确定位表面波极点的方法,并将其应用到提取表面波的三级DCIM,实现了分层媒质空域格林函数的准确计算。通过单层无耗介质和四层有耗介质微带结构空域格林函数的计算实例,证明了该方法的正确性和有效性。     

拱形坑道建模及吸收边界性能比较

建立了侧壁拱顶形坑道模型,该模型可根据实际需要在0～π/2范围内选择坑道的弧度.将坑道看成有耗波导,采用TE10模和TMn模进行强迫激励,在此基础上比较了完全匹配层(Perfect Matched Layer,PML)、各向异性完全匹配层(Uniaxial PML,UPML)和卷积完全匹配层(Convolutional PML,CPML)三种在坑道计算模型中常用的吸收边界条件各方面的性能.数值结果表明:CPML边界条件可以有效吸收低频凋落模,克服了PML和UPML用于长时间计算造成的晚时反射,减小了计算域,缩短了计算时间.     

地下三维目标电磁散射特性研究

本文推出集总电阻加载圆柱形天线在时域有限差分（FD-TD）法中的计算公式。对Mur二阶吸收边界条件在三维FD-TD法计算中的稳定性进行了研究，提出保证Mur二阶吸收边界条件稳定所必须注意的问题。在集总电阻加载圆柱形天线激励下，对色激媒质中三维目标的瞬态电磁散射特性进行了计算和分析，并将部分计算结果与实验测试的结果进行了比较，二者具有比较好的一致性。研究了色散媒质和目标特性对目标回波信号的影响，并对有耗媒质中细导线型目标的瞬态散射机理进行了分析。     

广义正交坐标系FDTD算法的准完全匹配层吸收边界条件

本文推导出了广义正交坐标系下的各向异性准完全匹配吸收层(QPML)边界条件,给出了直角坐标、圆柱坐标、保角变换柱形坐标的各向异性准完全匹配吸收媒质.数值计算结果证明了准完全吸收层的吸收效果.     

FD-TD法分析无载频脉冲探地雷达特性

本文首先利用三维ＦＤＴＤ法对集中电阻加载圆柱形偶极天线的近地面辐射特性进行了计算,给出了天线上电流波形并分析了加载电阻和有耗媒质参数等因素对电流波形的影响;分析并计算了天线“方向图”及其随天线高度和媒质参数变化的规律。其次,利用色散媒质中２．５维ＦＤＴＤ法迭代公式,模拟计算了地下目标雷达回波电平图,并与实际探测结果进行了对比,二者具有较好的一致性;分析了色散媒质参数对雷达探测深度的影响。     

FDTD法分析无载频脉冲探地雷达特性

本文首先利用三维ＦＤＴＤ法对集中电阻加载圆柱形偶极天线的近地面辐射特性进行了计算,给出了天线上电流波形并分析了加载电阻和有耗媒质参数等因素对电流波形的影响;分析并计算了天线“方向图”及其随天线高度和媒质参数变化的规律。其次,利用色散媒质中２．５维ＦＤＴＤ法迭代公式,模拟计算了地下目标雷达回波电平图,并与实际探测结果进行了对比,二者具有较好的一致性;分析了色散媒质参数对雷达探测深度的影响。     

Multidomain pseudospectral time-domain simulations of scattering by objects buried in lossy media

A multidomain pseudospectral time-domain (PSTD) method with a newly developed well-posed PML is introduced as an accurate and flexible tool for the modeling of electromagnetic scattering by 2-D objects buried in an inhomogeneous lossy medium. Compared with the previous single-domain Fourier PSTD method, this approach allows for an accurate treatment of curved geometries with subdomains, curvilinear mapping, and high-order Chebyshev polynomials. The effectiveness of the algorithm is confirmed by an excellent agreement between the numerical results and analytical solutions for perfectly conducting as well as permeable dielectric cylinders. The algorithm has been applied to model various ground-penetrating radar (GPR) applications involving curved objects in a lossy half space with an undulating surface. This multidomain PSTD algorithm is potentially a very useful tool for simulating antennas near complex objects and inhomogeneous media.     

Path loss model for wireless narrowband communication above flat phantom

A new empirical path loss model for wireless communication at 2.4 GHz above a flat, lossy medium, representing human tissue, is presented. The model is valid for dipole antennas for heights up to 5 cm above the phantom and for distances up to 40 cm, and was applied to muscle and brain simulating media. For antennas placed close to the lossy medium, it was found that antenna height has a major influence on path loss. The model has been validated by measurements and simulations, which show excellent agreement     

Application and optimization of PML ABC for the 3-D wave equation in the time domain

A three-dimensional algorithm with the perfectly matched layer (PML) absorbing boundary condition (ABC) for the scalar wave equation in the time domain is presented for general inhomogeneous lossy or loss-free problems. The proposed PML ABC is applicable to practical finite difference schemes treating the time-domain wave equation, such as the time-domain wave-potential (TDWP) technique and the time-domain scalar wave equation approaches to the analysis of optical structures. The time-domain wave equation for lossy media is expressed in terms of stretched coordinate variables. The algorithm is tested for homogeneous and inhomogeneous media. We demonstrate applications to open (radiation) problems and to port terminations in high-frequency circuit problems. New PML conductivity profiles are developed for use with the second order wave equation, which offer lower reflections in a wider frequency band in comparison with the commonly used (in finite-difference time-domain (FDTD) algorithms) profiles. The effect of the termination walls on the overall PML performance is studied and the best choices are singled out.     

有耗介质半空间上线天线的输入阻抗

运用近年来发展起来的矩阵束法得到空间域格林函数的近似解，并结合感应电动势法，给出了有耗介质半空间上垂直放置的线天线输入阻抗的闭合表达式。数值结果表明本文运用的方法对分析垂直于地面以及有耗或无耗介质半空间上的线天线是有效的。     

Heating characteristics of thin helical antennas with conductingcores in a lossy medium. I. Noninsulated antennas

We report a combined theoretical and experimental study of the heating characteristics of helical antennas in lossy dielectric media. Proposed biomedical application of such antennas include angioplasty, hyperthermia, and catheter ablation of tissue. The study focuses on helical antennas, operated in the normal mode (wavelength greater than antenna diameter but comparable to antenna length), that are terminated at one end by a short circuit and at the other by a coaxial feedpoint. The analytical model is based on the helical sheath approximation, extended to the case of lossy media. In addition, experimental studies were performed on helical antennas immersed in aqueous electrolyte of various conductivity. The antennas show two distinct modes of propagation: a slow mode similar to that observed in helical antennas in loss-free media, and a faster mode. The analytical/numerical results are in good agreement with experimental data, thus demonstrating the validity of the model     

Simulations of ground-penetrating radars over lossy andheterogeneous grounds

The versatility of the three-dimensional (3D) finite-difference time-domain (FDTD) method to model arbitrarily inhomogeneous geometries is exploited to simulate realistic ground-penetrating radar (GPR) scenarios for the purpose of assisting the subsequent designs of high-performance GPR hardware and software. The buried targets are modeled by conducting and dielectric prisms and disks. The ground model is implemented as lossy with surface roughness, and containing numerous inhomogeneities of arbitrary permittivities, conductivities, sizes, and locations. The impact of such an inhomogeneous ground model on the GPR signal is demonstrated. A simple detection algorithm is introduced and used to process these GPR signals. In addition to the transmitting and receiving antennas, the GPR unit is modeled with conducting and absorbing shield walls, which are employed to reduce the direct coupling to the receiver. Perfectly matched layer absorbing boundary condition is used for both simulating the physical absorbers inside the FDTD computational domain and terminating the lossy and layered background medium at the borders     

有耗平面和三维目标复合散射的FDTD分析

采用FDTD方法计算了有耗地面与三维目标的复合散射,对吸收边界条件,连接边界条件和近远场变换作了细致讨论.吸收边界使用了广义PML吸收层,它对电磁波有较好的吸收效果.连接边界处则利用解析入射波和三波迭加技术,上半平面用入射波和反射波、下半平面用透射波作为对目标的外加场进行计算.得到近场数据后,为避免出现复杂的Sommerfeld积分,用互易原理简化了外推过程.FDTD算法与矩量法和快速多层多极子相比,具有节省内存,计算时间短等优点.通过地上物体和地下物体的计算验证了FDTD方法的精确性,讨论了散射体离地高度对后向RCS的影响.     

Physical limitations of antennas in a lossy medium

The dissipated power and the directivity of antennas in a homogeneous, lossy medium are systematically analyzed in this paper. The antennas are ideal and located inside a lossless sphere. In the lossy space outside the sphere, the electromagnetic fields are expanded in a complete set of vector wave functions. The radiation efficiency, the directivity, and the power gain are defined for antennas in a lossy medium, and the optimal values of these quantities are derived. Simple relations between the maximal number of ports, or channels, an antenna can use and the optimal directivity and gain of the antenna are presented.     

A new anisotropic perfectly matched layer medium for mesh truncation in finite difference time domain analysis

In this paper an unsplit anisotropic perfectly matched layer (PML) medium, previously utilized in the context of finite element analysis, is implemented in the finite difference time domain (FDTD) algorithm. The FDTD anisotropic PML is easy to implement in the existing FDTD codes, and is well suited for truncating inhomogeneous and layered media without special treatment required in the conventional PML approach. A further advantage of the present approach is improved performance at lower frequencies. The applications of the unsplit anisotropic PML/FDTD method are illustrated by considering the problems of a plane wave propagation and an open microstrip line.     

Analysis of the numerical One-Step method for the study applied on bio electromagnetics

The development of wireless technologies arises important questions about the effects of the wave propagation in the human body. To study accurately these effects, we have to use rigorous numerical methods. In this paper, we present and analyze the One-Step time domain method. This method, which was proposed by De Raedt [Phys Rev E 67(056706):1–12, 2003] for lossless media, is known to be unconditionally stable and so it can be used for applications for which the Courant–Friedrich–Levy (CFL) stability condition can be a limiting factor, e.g., for bioelectromagnetic applications. The numerical dispersion and the insertion of lossy media in the One-Step method are evaluated. The perfectly matched layer (PML) absorbing conditions are also introduced in our study.