带矢量ABC底面的共形完全匹配层

共形完全匹配层是一种有耗各向异性媒质组成的凸且光滑的壳体,其底面一般是PEC面或PMC面,但是PEC面或PMC面会对原散射场产生反射;为了减少底面反射,将CPML原有的PEC（或PMC）底面改为矢量ABC吸收边界,并给出了带矢量ABC底面的CPML泛函公式。通过数值算例证明,这种带矢量ABC底面的CPML边界不仅减少了底面反射,而且吸收效果好,计算精度高。     

A finite‐difference time‐domain approach to waveguide discontinuities using one‐dimensional modal perfectly matched layer based on diagonally anisotropic material

In this paper, one‐dimensional modal perfectly matched layer (PML) is applied to improve the computational efficiency in waveguide discontinuity problems. The proposed PML formulation is based on an anisotropic medium. A WG‐90 rectangular waveguide with a thick asymmetric iris is analyzed via FDTD simulations with Berenger's PML and the proposed one. Numerical results show that the computational efficiency is significantly improved by the new method. © 2000 John Wiley & Sons, Inc. Int J RF and Microwave CAE 10: 264–270, 2000     

Analysis for scattering of conductive objects covered with anisotropic magnetized plasma by trapezoidal recursive convolution finite‐difference time‐domain method

The finite‐difference time‐domain method (FDTD) is extended to three‐dimensional (3D) anisotropic magnetized plasma based on the trapezoidal recursive convolution (TRC) technology. The TRC technique requires single convolution integral in the formulation as in the recursive convolution (RC) method, while maintaining the accuracy comparable to the piecewise linear recursive convolution (PLRC) method with two convolution integrals. In this article, the numerical results indicate that the TRC‐FDTD method not only improves accuracy over the RC‐FDTD with the same computational efficiency but also spends less computational time than the PLRC‐FDTD based on the same accuracy. The 3D TRC‐FDTD formula is provided and the bistatic radar scattering sections of conductive targets covered with anisotropic magnetized plasma are calculated. The results show that magnetized plasma cover layer can greatly reduce echo energy of radar targets, and the anisotropic magnetized plasma cover has better absorption effect than nonmagnetized. © 2010 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2010.     

全波场地震波数值模拟中的边界处理实践研究

在用有限差分法或有限元法模拟无界区域中的波动时,需要对计算区域的边界做特殊处理,以消除由于把地震波的传播设定在有限区域而产生的边界反射。为了这一目的,人们研究出了多种人工边界处理方法,完全匹配层（PML）吸收边界条件就是理想的方法之一,现已被广泛应用。本文将PML吸收边界条件应用于全波场地震波的数值模拟,数值计算实验表明,对qP波,匹配层的厚度为5个网格间距即可达到要求,而对qSV波与qSH波,为达到理想的吸收效果,匹配层的厚度应当增大,当厚度为13个网格间距时达到了理想的吸收效果。     

Time domain electromagnetic scattering using finite elements and perfectly matched layers

We consider a model for the interrogation of a planar Debye medium by a non-harmonic microwave pulse from an antenna source in free space, and we compute the reflected solution using finite elements in the spatial variables and finite differences in the time variable. Perfectly matched layers (PMLs) and an absorbing boundary condition are used to damp waves interacting with artificial boundaries imposed to allow finite computational domains. We present simulation results showing that numerical reflections from interfaces at PML boundaries can be controlled.     

基于正交轨迹的共形PML数值仿真方法

从材料的观点看,完全匹配层是各向异性的介质.场在完全匹配层遵从麦克斯韦方程.通过设计磁导率和介电常数张量参数来实现任意形状的共形完全匹配层.为匹配层动态稳定,可应用二维正交轨迹网格和FDTD方法实现了数值共形完全匹配层仿真,数值仿真结果表明基于正交轨迹网格的共形完全匹配层为一个共形FDTD或FEM方法提供了一个有效地吸收边界条件.     

Asymmetric tri‐band linear‐to‐circular polarization converter in transmission mode

In this article, we propose a metasurface which has function as tri‐band linear‐to‐circular transparent polarization converter based on combining multiple plasmon resonances and Fabry‐Perot‐like resonances. The metasurface is composed of three layers of metallic pattern arrays separated by two‐layer dielectric. The proposed metamaterial can convert incident linearly polarized waves into circularly polarized waves in three frequency bands, and the transmission efficiency reaches up to 90%. When x‐polarized forward wave is normally incident, the transmitted right‐handed circular polarized spectra with axial ratio less than 3 dB are in the range of 18.7 to 21.5 and 46.1 to 50.3 GHz; the left‐handed circular polarized spectrum is 28.7 to 34.3 GHz. The physical mechanism of the polarization converter is analyzed in detail, and an experiment is carried out to verify the proposed polarization converter.     

Effects of external magnetic field on propagation of electromagnetic wave in uniform magnetized plasma slabs

A simple method is proposed to describe the propagation of electromagnetic waves in magnetized uniform plasma slabs. Using this method, the reflection, absorption and transmission coefficients of such plasmas for right-hand circularly waves are studied and the effects of the continuously changing external magnetic field on the power of the electromagnetic waves propagated in magnetized plasma slabs with fixed parameters are presented. Our method enables more detailed numerical analyses which are useful in practical applications pertaining to the control of the reflection or absorption coefficients of electromagnetic wave through a uniform magnetized plasma slab by adjusting the external magnetic field.     

Perfectly matched layers for time-harmonic elastodynamics of unbounded domains: theory and finite-element implementation

One approach to the numerical solution of a wave equation on an unbounded domain uses a bounded domain surrounded by an absorbing boundary or layer that absorbs waves propagating outwards from the bounded domain. A perfectly matched layer (PML) is an unphysical absorbing layer model for linear wave equations that absorbs, almost perfectly, outgoing waves of all non-tangential angles-of-incidence and of all non-zero frequencies. This paper develops the PML concept for time-harmonic elastodynamics in Cartesian coordinates, utilising insights obtained with electromagnetics PMLs, and presents a novel displacement-based, symmetric finite-element implementation of the PML for time-harmonic plane-strain or three-dimensional motion. The PML concept is illustrated through the example of a one-dimensional rod on elastic foundation and through the anti-plane motion of a two-dimensional continuum. The concept is explored in detail through analytical and numerical results from a PML model of the semi-infinite rod on elastic foundation, and through numerical results for the anti-plane motion of a semi-infinite layer on a rigid base. Numerical results are presented for the classical soil–structure interaction problems of a rigid strip-footing on a (i) half-plane, (ii) layer on a half-plane, and (iii) layer on a rigid base. The analytical and numerical results obtained for these canonical problems demonstrate the high accuracy achievable by PML models even with small bounded domains.     

Rigorous analysis of uniaxial bi‐anisotropic dielectric resonators by the mode matching method

Rigorous analysis of uniaxial bi‐anisotropic dielectric resonators is carried out by the mode matching method. To validate our numerical model, results are satisfactorily compared with the literature for a uniaxial electric anisotropic dielectric resonator (DR). Then bi‐anisotropy effects of DRs dielectric materials on their resonant frequencies are studied in detail. ©1999 John Wiley & Sons, Inc. Int J RF and Microwave CAE 9: 385–393, 1999.     

Single‐layer high‐gain flat lens antenna based on the focusing gradient metasurface

A single‐layer transmitting focusing gradient metasurface (F‐GMS) has been proposed that can realize high gain increment at 10 GHz. The unit of F‐GMS is composed of two identical structures placed on the top and bottom of one dielectric layer, which can have high transmitting efficiencies that over 0.8 and achieve [0, 2π] phase range in X‐band. The F‐GMS can convert the spherical waves into plane waves. A patch antenna working at 10 GHz is positioned as the focus of the proposed F‐GMS as the feed source to develop an ultrathin flat lens antenna system. It achieves a simulated gain of 19.6 dBi which is 12.9 dB greater than that of the single patch antenna at 10 GHz. Lastly, the F‐GMS and the patch antenna are manufactured and then measured in an anechoic chamber. A good agreement was demonstrated between experimental and simulated results.     

Long Time Behavior of the Perfectly Matched Layer Equations in Computational Electromagnetics

We investigate the long time behavior of two unsplit PML methods for the absorption of electromagnetic waves. Computations indicate that both methods suffer from a temporal instability after the fields reach a quiescent state. The analysis reveals that the source of the instability is the undifferentiated terms of the PML equations and that it is associated with a degeneracy of the quiescent systems of equations. This highlights why the instability occurs in special cases only and suggests a remedy to stabilize the PML by removing the degeneracy. Computational results confirm the stability of the modified equations and is used to address the efficacy of the modified schemes for absorbing waves.     

On the Accuracy and Stability of the Perfectly Matched Layer in Transient Waveguides

Energy transmitted along a waveguide decays less rapidly than in an unbounded medium. In this paper we study the efficiency of a PML in a time-dependent waveguide governed by the scalar wave equation. A straight forward application of a Neumann boundary condition can degrade accuracy in computations. To ensure accuracy, we propose extensions of the boundary condition to an auxiliary variable in the PML. We also present analysis proving stability of the constant coefficient PML, and energy estimates for the variable coefficients case. In the discrete setting, the modified boundary conditions are crucial in deriving discrete energy estimates analogous to the continuous energy estimates. Numerical stability and convergence of our numerical method follows. Finally we give a number of numerical examples, illustrating the stability of the layer and the high order accuracy of our proposed boundary conditions.     

3D‐printed cylindrical Luneburg lens antenna for millimeter‐wave applications

A 3D‐printed cylindrical Luneburg lens antenna working at 26 GHz is proposed in this article. The antenna consists of a feeding waveguide, a 3D‐printed cylinder, and a pair of printed metal grids which are stuck on the side faces of the cylinder. 3D‐printed structure ensures the convenience for processing and structural integrity of the Luneburg lens. Hole drilling technology is utilized for the design of the cylindrical lens. In the E‐plane, conversion of spherical waves into planar waves is achieved based on the gradient refractive index which is realized by the gradient equivalent relative dielectric constant. The main part of the lens contains a hole drilling region to realize the desired equivalent permittivity from 1.23 to 2, while another gradual‐thickness region realize the permittivity ranges from 1.23 to 1. H‐probe method is utilized for the optimization of the gradual‐thickness region in this article. And for the H‐plane, with the grids, H‐field distribution is optimized compared with the Luneburg lens antenna without the loading grids. Thus, the side lobe level (SLL) in H‐plane could be reduced. Meanwhile, a narrower half power beamwidth (HPBW) in H‐plane will be obtained due to the metal grids. Experiment results illustrate the feasibility and validity of the proposed 3D‐printed cylindrical Luneburg lens antenna.     

FDTD方法吸收边界条件的研究及应用

用时域有限差分法(FDTD)求解电磁散射问题中,吸收边界条件的设置起着关键性作用.通过时间和空间上的递推算法对时域有限差分法中的两种吸收边界条件:Mur吸收边界条件和完全匹配层(PML)的吸收效果进行了比较和分析.同时,引入参数对PML的差分方程进行了优化,避免了将电磁场分裂为两个分量进行计算,进而降低了计算内存开销.实验结果证明PML具有更优越的吸收性能.最后,在FDTD算法中应用PML吸收层对一圆柱形导体的雷达散射截面积(RCS)进行数值仿真,验证了FDTD算法在计算雷达散射截面积(RCS)上的有效性.     

Enhanced waveguide bandpass filters using S‐shaped resonators

This article presents a new solution for stopband performance improvement of rectangular waveguide bandpass filters using S‐shaped resonator loaded waveguide configurations at microwave and millimeter‐wave frequencies. The proposed filter structure is compact in size when comparing with the standard E‐plane counterpart. Compactness is achieved by taking advantage of the properties of slow wave effect in half wavelength resonators. Periodicity is readily imposed upon cascading the S‐shaped resonators within the rectangular waveguide. The structure is simple and compatible with E‐plane technology. This type of bandpass filters can be easily realized with a single metallo‐dielectric insert within a standard rectangular waveguide. Simulation and experimental results are presented to validate the argument along with some design guidelines. © 2009 Wiley Periodicals, Inc. Int J RF and Microwave CAE 2009.     

Mutual coupling reduction in planar Yagi antenna array using bidirectional absorbing metasurface

An effective method for reducing the mutual coupling between the antenna arrays using a bidirectional absorbing metasurface is proposed in this article. This is achieved by embedding the bidirectional absorbing metasurface between the two planar Yagi antennas. The metasurface is mainly composed of 3 × 6 unit cells. Each unit cell includes three‐layer metal rings and two‐layer dielectric slabs. Since the front and back of the metasurface are completely consistent, a bidirectional absorptivity can reach to 86% or more. Using the proposed metasurface, the mutual coupling at the resonance point can be reduced to about 38 dB. And it has little effect on other performances of the Yagi antennas. Moreover, the proposed metasurface can improve the radiation performance of the antenna array, and make it have a smaller back lobe and a larger front‐to‐back ratio.     

Coaxial probe‐fed open‐ended waveguide antenna based on sand casting process with filtering characteristics and all‐in‐one structure

In this article, a coaxial probe‐fed open‐ended waveguide filtering antenna with all‐in‐one structure is proposed and fabricated by adopting sand casting process. It is known that for waveguide component with complex internal space, detached parts, and post‐processing assembly are often necessary, while unavoidably resulting in error, air gap and loss. In this context, sand casting process is here utilized to fabricate the whole waveguide structure directly, thus achieving a fully integrated antenna. Besides, by controlling the external couplings from probe feed and free space, the proposed coaxial probe‐fed antenna can provide similar impedance matching to a waveguide‐fed three‐order waveguide filter. Both the frequency‐ and time‐domain reflection coefficients of filter and antenna have been compared and analysed. An antenna prototype is fabricated and measured to validate the proposed design, which provides a successful example of all‐in‐one waveguide filtering antenna.     

Miniaturized rectangular waveguide filters

Two types of miniaturized rectangular waveguide filters are presented. Miniaturization is achieved using the slow‐wave effect of electromagnetic bandgap (EBG) surfaces and the left‐handed properties of split ring resonators (SRRs). The proposed EBG waveguide bandpass filter performs passband in the frequency range, which corresponds to the waveguide with the lower recommended operating band consequently enabling significant miniaturization of the structure. The SRR‐loaded bandstop filter makes use of the effect imposed by left‐handed medium (LHM), which is created by a combination of SRRs and wireline on the dielectric slab. Both filters are designed, simulated, and tested. Experimental results of the SRR‐loaded bandstop filter are presented to demonstrate feasibility of the proposed structures. © 2007 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2007.