三维介质频率选择表面结构的多模网络分析

该文采用多模网络与严格模匹配相结合的方法分析了介质周期结构在电磁波斜入射情况下的散射特性。定量地分析了这种三维频率选择表面结构的频率选择特性随入射波的频率、入射角度、周期层和均匀层的厚度与介电常数等结构参数的变化关系,从而为三维介质频率选择表面的设计和应用提供了依据。     

三维频率选择结构的研究进展

频率选择结构是一种对空间电磁波具有选择特性的人工周期结构.传统FSS在单元结构、分析理论及设计方法上的研究日趋成熟,但这类FSS基于二维或2.5维的周期单元对电磁波实施调控时,其工作性能和设计灵活性受到严重限制.基于波导腔或传输线的多模周期单元经过周期排布而构成的三维FSS可在空间维度上扩展,使得三维FSS在设计上可提供更多的自由度,从而表现出优越的电磁波调控性能.综述了三维FSS最新的研究进展,并重点介绍了三维FSS设计的最新技术及其在电磁兼容中的应用前景.     

FEM/BEM混合法计算各向异性不均匀介质柱电磁散射

应用有限元-边界元(FEM/BEM)混合法计算二维各向异性不均匀介质柱电磁散射,对介质柱内、外区域分别采用有限元和边界元法进行分析,然后应用边界条件建立部分稀疏部分满填充的矩阵方程.应用内观法结合多波前法求解该矩阵方程,分别计算了均匀分布和不均匀分布的各向异性介质柱的雷达散射截面.数值计算表明,有限元-边界元混合法在分析和计算不均匀开放域电磁问题时有一定的优势.     

介质周期结构频率选择特性的多模网络分析

本文采用多模网络与严格模匹配相结合的方法分析了平面波斜入射时介质周期结构的频率选择反射特性。讨论了这种结构全反射的频率、带宽和在确定频带内出现全反射的个数随周期层厚度、附加介质层厚度及介电常数的变化关系,为介质频率选择表面结构的设计提供了依据。最后用平板介质波导理论说明了这种波现象与多层平板介质结构导模色散特性的关系。     

双层错位频率选择表面的电磁特性分析

针对双层频率选择表面(FSS)在曲面应用中容易出现单元错位问题,基于耦合积分方程建立错位FSS结构的理论分析模型,分析了双层错位FSS的电磁特性.采用谱域矩量法并结合周期性边界条件,将各层单元表面电流的计算范围限定在一个周期内;对于错位层FSS单元被周期边界截断的情况,考虑了周期边界上导体表面电流的连续性;选用RWG(Rao-Wilton-Glisson)基函数描述导体表面的感应电流,以实现对任意形状FSS单元的数值计算.以方环形贴片单元为例验证了算法的准确性,并考察了周期边界的电流连续性对FSS电磁特性的影响,分析了层间距、入射极化方向和错位量等参数对错位FSS结构散射特性的影响.     

介质周期结构色散特性的严格分析：一种三维边值问题的求解方法

本文以耦合模理论和Ｆｌｏｑｕｅｔ理论为基础，把本征模展开与空间谐波展开巧妙地结合起来，严格分析了斜入射情况下介质周期结构的色散特性－三维边值问题。这种新方法既避免了通常耦合模方法分析此类问题所引入的微扰近似，又比Ｆｌｏｑｕｅｔ方法简单方便。通过对斜入射情况下一些介质结构滤波特性的数值分析，证实了本文方法的有效性，精确性和实用性。     

斜入射情况下介质周期结构三维散射问题的严格求解

本文采用多模网络与严格模匹配相结合的方法,以介质周期结构在平面波沿主平面二维斜入射情况下散射特性的分析为基础,经过巧妙的数学处理,严格求解了三维斜入射情况下介质周期结构的散射问题,从而为毫米波和光集成电路中有关介质周期结构的精确分析和奠定了基础.     

由左手媒质构成的新的频率选择表面

采用多模网络理论与严格模匹配相结合的方法,详细分析了新的由左手媒质构成的周期结构的频率选择特性.着重研究了结构参数对表面选择特性的影响.对左手和右手周期结构的频率选择特性作了比较,并对一些现象做了解释.分析结果表明:左手周期结构具有比右手周期结构大得多的全反射谱带宽.该研究结果对于精确设计新的毫米波频率选择表面有一定的参考价值.     

有限元法结合周期边界条件分析介质光栅衍射

应用周期边界条件建立了分析介质周期结构散射问题的有限元格式。求解域仅为周期结构的一个栅单元尺寸。分析计算了位相型透射光栅的衍射效率,栅单元结构分别为矩形和直角三角形。由于有限元方法的离散网格可以很好地匹配介质边界,该方法可以分析任意栅单元截面形状的介质栅形结构。     

介质损耗对左手媒质周期结构频率选择特性的影响

采用多模网络与严格的模匹配相结合的方法研究了介质损耗对左手媒质构成的周期结构频率选择特性的影响;并对其中的新现象给予了解释.讨论了损耗对左手和右手周期结构的频率选择特性影响的差异.本文得到的结果对于精确设计新的毫米波频率选择表面有一定的指导意义.     

A numerical absorbing boundary condition for finite difference andfinite element analysis of open periodic structures

In this paper we present a novel approach to deriving local boundary conditions, that can be employed in conjunction with the Finite Difference/Finite Element Methods (FD/FEM) to solve electromagnetic scattering and radiation problems involving periodic structures. The key step in this approach is to derive linear relationships that link the value of the field at a boundary grid point to those at the neighboring points. These linear relationships are identically satisfied not only by all of the propagating Floquet modes but by a few of the leading evanescent ones as well. They can thus be used in lieu of absorbing boundary conditions (ABCs) in place of the usual FD/FEM equations for the boundary points. Guidelines for selecting the orders of the evanescent Floquet modes to be absorbed are given in the paper. The present approach not only provides a simple way to derive an accurate boundary condition for mesh truncation, but also preserves the banded structure of the FD/FEM matrices. The accuracy of the proposed method is verified by using an internal check and by comparing the numerical results with the analytic solution for perfectly conducting strip gratings     

A combined FEM/MoM approach to analyze the plane wave diffractionby arbitrary gratings

The diffraction of TE- and TM-polarized plane waves by planar gratings is numerically analyzed using a combined finite-element-method/method-of-moments (FEM/MoM) algorithm based on the generalized network formulation. The interior region, treated using the FEM, is truncated to a single unit cell with the introduction of an exact periodic boundary condition, which is enforced as a natural boundary condition. Using the FEM to compute the fields within the periodic structure allows gratings of arbitrary cross section and material composition to be efficiently modeled     

Phased array antenna analysis with the hybrid finite element method

A new analysis technique for infinite phased array antennas was developed and demonstrated. It consists of the finite element method (FEM) in combination with integral equation radiation conditions and a novel periodic boundary condition for 3-D FEM grids. Accurate modeling of rectangular, circular and circular-coaxial feeds is accomplished by enforcing continuity between the FEM solution and several waveguide modes across an aperture in the array's ground plane. The radiation condition above the array is enforced by a periodic integral equation in the form of a Floquet mode summation, thus reducing the solution to that of a single array unit cell. The periodic boundary condition at unit cell side walls is enforced through a matrix transformation. That mathematically “folds” opposing side walls onto each other with a phase shift appropriate to the array lattice and scan angle. The unit cell electric field is expanded in vector finite elements. Galerkin's method is used to cast the problem as a matrix equation, which is solved by the conjugate gradient method. A general-purpose computer code was developed and validated for cases of open-ended waveguides, microstrip patches, clad monopoles and printed flared notches, showing that the analysis method is accurate and versatile     

Impedance boundary conditions in a hybrid FEM/MOM formulation

When numerically modeling structures with imperfect conductors or conductors coated with a dielectric material, impedance boundary conditions (IBCs) can substantially reduce the amount of computation required. This paper incorporates the IBC in the finite-element method (FEM) part of a FEM/method of moments (FEM/MoM) modeling code. Properties of the new formulation are investigated and the formulation is used to model three practical electromagnetic problems. Results are compared to either measured data or other numerical results. The effect of the IBC on the condition number of hybrid FEM/MoM matrices is also discussed.     

A new FEM approach for open boundary Laplace's problem

An efficient improved finite element method (FEM) is presented for electromagnetic Laplace's problems with open boundary. The whole infinite domain is divided into a set of infinite elements instead of ordinary finite elements. Since a special FEM discretization and FEM solving procedure are used, it can not only take much less computer memory than that the conventional FEM needs, but also avoid the calculation error introduced by the truncated boundary or absorbing boundary condition used in conventional FEM     

Finite-element method coupled with method of lines for the analysis of planar or quasi-planar transmission lines

A full-wave analysis incorporating the finite-element method (FEM) and the method of lines (MoL) is presented in this paper to investigate a planar or quasi-planar transmission-line structure containing complex geometric/material features. For a transmission-line structure being considered, the regions containing complex media are modeled by the FEM while those consisting of simple media with simple geometry are analyzed using the MoL. From the field solutions calculated by MoL, the boundary conditions are constructed. The boundary integrals involved in finite-element analysis are then carried out using these boundary conditions. Since the finite-element analysis is employed only in the complex parts of the structures, while other parts are handled by the MoL, this approach not only retains the major advantage of the FEM in simulating complex structures but also becomes more efficient than the conventional finite-element analysis. Good agreement between the calculated results and those reported in the available literature is obtained and thus validates the present approach. Furthermore, proficient computational efficiency of this method is demonstrated by examining its convergence property. Finally, a number of relevant transmission-line structures are analyzed to illustrate the applications of this approach.     

Vector one-way wave absorbing boundary conditions for FEMapplications

In the paper a derivation is presented which leads to a new and general class of vector absorbing boundary conditions (ABCs) for use with the finite element method (FEM). The derivation is based on a vector one-way wave equation and a polynomial approximation of the vector radical. It is shown that wide-angle absorbing boundary conditions, as proposed in Halpern and Trefethen (1988) for optimal absorption of out-going waves, can be obtained in vector form. Vector plane waves are used to evaluate the accuracy and the reflection performance of these boundary conditions in a wide range of incidence angles. The implementation of the vector ABCs in a FEM formulation is also provided to show how up to the fifth-order absorbing accuracy can be achieved with derivatives only up to the second-order. A possible formulation is described which not only yields a third-order accuracy with first-order derivatives, but also retains the symmetry of the FEM matrix     

波导介质不连续性问题的FEM/PML方法分析

将完全匹配层(PML)与矢量有限元方法(FEM)相结合对波导介质不连续性问题进行分析。利用PML对计算区域进行截断，大大节省了FEM分析问题的内存消耗。对波导的单介质块加载问题的S参数进行了分析计算，数值结果与文献结果一致。在此基础上，计算了多介质块加载波导的散射参数。数值结果证实了应用FEM／PML方法对波导介质不连续性问题进行分析的正确性与有效性。     

Integral equation modeling of multilayered doubly-periodic lossy structures using periodic boundary condition and a connection scheme

This paper presents a boundary integral formulation to analyze multilayered doubly-periodic lossy structures with arbitrary geometry. The formulation is based on the moment method using first-order triangular patch basis functions. Each individual layer is analyzed separately using the simple free-space Green's function. After discretization, periodic boundary conditions are imposed on each region and a connection scheme is used to connect the regions. Metallic patches between layers or on the periodic boundary are also included in the model. Several examples are presented showing both the flexibility and the accuracy of the method.     

Ground effects for VHF/HF antennas on helicopter airframes

In this paper, the finite element method (FEM) is used to predict the space and surface wave radiation patterns of VHF/HF antennas mounted on a helicopter in the presence of a lossy ground. The equivalent sources of the radiation system are obtained by solving an FEM problem in conjunction with an absorbing boundary condition (ABC) or an impedance boundary condition (IBC). From the equivalent sources, the total radiated field is calculated using the equivalence principle and superposition; the original problem is converted into a set of properly combined Hertzian dipoles referred to as the Sommerfeld problem. Instead of evaluating the Sommerfeld integral rigorously, Norton's approximation is used to improve the overall computational efficiency. The validation of this method is accomplished in two steps: first, the FEM is compared with the finite-difference time-domain method (FDTD) in the absence of a lossy ground; second, the Hertzian dipole problem is solved in the presence of a lossy ground and the results are compared with analytic solutions. Finally, this technique is extended to analyze an antenna on a helicopter above a lossy ground