Study of Wideband,Wide Angle,Polarization Independent Metamaterial Hilbert Curve Absorbing Screen for Terahertz Bolometers

A wideband, wide angle metamaterial Hilbert curve array terahertz absorbing screen is reported. The absorber consists of array of an Aluminum first order Hilbert curve array over a copper ground plane separated by dielectric spacer. 100% fractional bandwidth (-3dB absorption bandwidth) for normal incidence and more than 90% in case of oblique incidence of 45° and 60° for both TE and TM polarizations have been verified numerically. The operating bandwidth of the structure proposed here is several times of those reported to date. Peak absorption is 98% for normal incidence and more than 86% for 60° angle of incidence for either polarization. Furthermore, effects of variations in Hilbert curve dimensions, spacer thickness, spacer permittivity and ground plane conductivity are also investigated. The structure is shown to have high tolerance to variations in the aforementioned parameters.     

等效模型误差对蜂窝吸波结构设计结果的影响

使用等效模型可以大大简化蜂窝吸波结构的设计复杂度,但等效模型的误差会对设计的结果产生影响。基于单轴各向异性介质中的电波传播理论和粒子群优化算法,通过定义更符合实际的随频率变化的相对误差,对不同情况下Hashin鄄Shtrikman(HS)模型的误差对蜂窝吸波结构优化设计结果的影响进行研究。结果表明,HS 模型的误差会在一定程度上影响蜂窝吸波结构的优化结果,特别是当电磁波斜入射时,由于纵向传播常数受到横向等效模型误差的影响较大,而受到纵向等效模型的影响较小,所以优化结果也主要受到横向等效模型误差的影响。     

电磁波多层媒质传播的[C]网络新理论

本文讨论电磁波在多层媒质中传播的反射和折射。将电磁波斜入射到媒质突变的平面边界作为基本模型,推广到垂直入射和导体、磁体边界等多种情况。本文将电磁波传播和网络理论有机结合,创新地提出多层媒质的[C]网络理论,从而把困难的多层媒质传播转化为[C]矩阵级联,概念简单,结果统一。文中还给出了具体实例。     

Scattering Characteristics of Dielectric Periodic Structure Composed of Left-handed Materials with Arbitrary Oblique Incidence

Scattering characteristics of monolayer and multilayer dielectric periodic structure composed of left-handed materials (LH-DPS) with plane wave arbitrary oblique incidence are carefully analyzed using a method which combines multimode network theory with the rigorous mode matching method. Our analysis results reveal that the arbitrary oblique incident angles and relative position between different LH-DPS have great effects to the scattering characteristics of LH-DPS which different from the situation of dielectric periodic structure composed of right-handed materials (RH-DPS). The reasons why the reflection characteristics of the LH-DPS are totally different from those of the RH-DPS with arbitrary oblique incidence are also given. The present quantitive investigation provides guidelines for the design of the monolayer and multilayer dielectric frequency selective surfaces for millimeter wave applications.     

Novel planar electromagnetic absorber designs using genetic algorithms

A new type of absorber is proposed created by drilling holes in a single layer of lossy material. The new absorber is designed using genetic algorithms to achieve low reflectance over a wide frequency band. Results based on genetic algorithms demonstrate that the new absorber outperforms multilayer absorbers and absorbers embedding frequency selective surfaces between lossy materials. Numerical results are presented for different bandwidths, wave incidence angles, and polarizations.     

A new 2-D image reconstruction algorithm based on FDTD and design sensitivity analysis

This paper proposes a numerical algorithm that reconstructs the complex permittivity profile of unknown scatterers by the design sensitivity analysis (DSA) and topology optimization technique. By introducing the DSA and adjoint-variable method, the derivatives of the error function with respect to the complex permittivity variables can be calculated, and the material property in each cell can be changed simultaneously using sensitivity information. The steepest descent method is used as an optimization technique. The proposed method is validated by applying it to reconstructions of unknown two-dimensional scatterers that are illuminated by TM/sup z/ with a Gaussian-pulsed plane wave.     

Analytical method of a multilayered meander-line polarizer plate with normal and oblique plane-wave incidence

Simple empirical formulas for perpendicular and parallel polarization susceptances for a meander-line grating plate are given. The numerical results compared favorably with experimental data and published data. Simple transmission-line model in terms ofE-type mode andH-type mode for multilayered meander-line polarizer plate is presented for plane wave incidence at normal and oblique angles. Numerical results for design examples are given for practical application.     

Two-dimensional beam diffraction by a half-plane and wide slit

The far field of a two-dimensional beam resulting from an electric line source at a complex position is described, its half-power beamwidth determined, and its validity as an antenna beam indicated. Farfield diffraction by a half-plane is then determined from an exact uniform solution for an isotropic line source by making the source position complex. The same basic solution and technique are used for beam diffraction by a wide slit, with first-order interaction between the slit edges included. Numerical results for normal incidence illustrate the evolution of the diffraction patterns from those for an omnidirectional source to those for a highly directive beam. Results for plane wave incidence by a slit also come out of this solution. The remarkable simplicity and convenience of this method relative to alternative asymptotic procedures is discussed.     

二维fBm分形分层介质粗糙面电磁波透射特征研究

该文运用微扰法研究了平面波入射分层介质粗糙面的电磁波透射问题,推导出不同极化状态的透射波散射系数公式。采用二维fBm分形粗糙面来模拟实际的分层介质粗糙面,结合其功率谱导出了平面波入射时的透射系数计算公式。通过数值计算得到了HH极化状态下二维fBm分形分层介质粗糙面透射系数随透射波的散射角变化的曲线,分形特征、基本特征、随频率变化的特征。结果表明分维、底层介质介电常数、中间介质介电常数和厚度、粗糙面参数及入射波频率对透射系数的影响是非常复杂的。     

Electromagnetic wave absorber with wide-band frequencycharacteristics using exponentially tapered ferrite

As a countermeasure of EMI or EMC, various types of electromagnetic wave absorbers are used. A wide-band design method of an electromagnetic wave absorber with using exponentially tapered ferrite, which has very wide-band frequency characteristics, is proposed and discussed. The wide-band electromagnetic wave absorber can be designed under some approximations by the theoretical model using the equivalent material constants (equivalent complex permittivity and permeability) method for the regions varying spatially in the shape of ferrite. Based on the model, wide-band electromagnetic wave absorbers with taper, which have not only excellent reflectivity frequency characteristics but also a bandwidth of 30 MHz to 2150 MHz or 2430 MHz under the tolerance limits of -20 dB reflectivity, were designed     

Electromagnetic wave absorbing characteristics of Ni-Zn ferritegrid absorber

The reflection loss of Ni-Zn ferrite grid absorber is studied in the low frequency limit which the period of the array is small compared with wavelength. The relative equivalent complex permeability and permittivity with air volume fraction is obtained using the Hashin-Shtrikman (1962) upper-bound formula. The impedance matching conditions for maximum electromagnetic wave absorption are obtained by the “contour map method.” Two impedance matching thicknesses and frequencies are compared with theoretical matching relationships derived under the limiting case of impedance matching formula. As the air volume fraction increases, the first matching frequency, f_m1 remains nearly constant, while the second one, f_m2 decreases and approaches f_m1. Therefore, the absorption band of a ferrite grid absorber is broader than that of the single layered ferrite plate absorber     

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。     

基于集总电阻的S波段小型化超材料吸波器

提出了一种基于集总电阻的S波段小型化超材料吸波器(Metamaterial Absorber,简称MMA)结构, 并进行了理论分析和仿真验证。设计的MMA 基本单元结构由加载了集总元件的电谐振器、中间隔离层和金属背板组成。利用S 参数反演算法求出等效阻抗,结果表明集总电阻的加入使该MMA 在较宽频带范围内有较好的阻抗匹配特性。根据等效电路理论,分析了MMA谐振吸收频率与其单元结构几何参数之间的关系。通过MMA 单元结构表面电流和能流分布分析了其吸波机制。不同模式下的极化角和斜入射角吸收率表明:该MMA具有极化不敏感和宽入射角特性。进一步研究MMA 结构-参数对其吸波性能的影响,通过单元结构几何参数和集总参数的调节获得最优的MMA模型。最终,通过MMA基本单元结构的进一步优化设计,使其在1.9～6.0GHz 范围内吸收率达到90%以上,对应的相对带宽达到101.9%。该设计有望在电磁能量捕获及隐身领域拥有潜在应用价值。     

Frequency Selective Characteristics of Dielectric Periodic Structures for Millimeter Wave Application

In this paper, the frequency selective reflection characteristics of dielectric periodic structures for the oblique incidence of a plane wave are analyzed by a method which combines the multimode network theory with the rigorous mode matching method. The variations of the total reflection characteristics with the geometric dimensions of the dielectric periodic structures are systematically investigated to develop useful guidelines for the design of the dielectric frequency selective surface. Moreover, the relationship between the related wave phenomenon and the dispersion characteristics of multilayer plane dielectric structure is explained with the theory of plane dielectric waveguide.     

Design of multilayered cylindrical shields using a geneticalgorithm

The design of infinitely long multilayered cylindrical shields with circular cross-section are considered and a method based on the genetic approach is proposed. An analytical method for the calculation of the shielding effectiveness of a cylindrical shield, consisting of homogeneous layers is presented for the case of an obliquely incident plane wave. By making use of this method, a genetic algorithm is implemented for the design of multilayered cylindrical shields in order to achieve a prespecified shielding effectiveness for a given band of frequencies or a range of angles of incidence     

基于平面波斜入射的人体信道远场路径损耗建模

人体信道路径损耗计算对植入式通信链路预估具有重要意义.文章利用有耗媒质的电磁场边界条件、反射和透射定理并引入切向等效波阻抗定义,推导出平面波向人体斜入射时各人体组织分界面上的入射角、透射角、反射系数、透射系数、切向等效波阻抗以及各人体组织中的电磁合成波,提出了一种基于平面波向多层有耗媒质斜入射的人体信道远场路径损耗解析模型.然后以植入在肌肉为例,计算了TM波和TE波在5个常用工业通信频率以不同角度斜入射的人体信道电磁场分布与路径损耗,结果显示,电磁波在入射面的反射是影响人体信道路径损耗的关键因素,当频率在1.4 GHz附近时总路径损耗最小,TM波性能优于TE波,且当入射角小于等于30°时,总路径损耗基本保持不变.最后采用COMSOL Multiphysics建立了有限元仿真模型验证解析模型,二者结果高度吻合,最大误差仅为0.039,有力证明了解析模型的正确性和有效性.     

Diffraction by a wide double wedge with cylindrically capped edges

The diffraction by two conducting sharp wedges with cylindrically capped edges is investigated using a recent asymptotic solution proposed by the authors for the diffraction by a wide double wedge. AnE-polarized plane wave incident at any angle is considered and the cap is assumed to be either a conducting or dielectric cylinder whose axis coincides with the wedge edge and its radius is much less than the separation between the two virtual sharp edges. The effects of the cap radius, permittivity, and wedge angle on the diffraction pattern, transmission coefficient, and edge-edge interaction term are presented. The transmission coefficient of the aperture is increased over the uncapped wedge case for dielectric caps and decreased for conducting caps. Other effects of the caps on the diffraction pattern such as beamwidth, level, and position of the first sidelobe are also investigated.     

A Quasi-Optical Method for Measuring the Complex Permittivity of Materials

A quasi-optical method for measuring the complex permittivity of materials is described. The determination is derived from measurements of the transmission of a perpendicularly polarized wave through a dielectric slab at different angles of incidence. This relatively simple method is quite sensitive. Accurate estimates of the relative permittivity and the loss tangent can be obtained by accurately measuring the frequency of the signal and by the use of large, precisely machined Fabry-Perot plates. Estimates of the standard errors in the determinations are obtained by using a bootstrap resampling technique. The measurements here are made at a frequency of 93.788 GHz at ambient temperature.     

Radar absorbers based on metamaterials

Application of metamaterials for creation of new types of radar absorbers (RAs) is investigated. Typical frequency dependences of the permittivity and permeability of composites fabricated from various inclusions employed in designing RAs are considered. It is shown that the structures of RAs can contain metamaterials with both positive and negative values of the real parts of permittivity and permeability. An RA having a low reflection coefficient over a wide range of the incidence angles of an electromagnetic wave for both wave polarizations is developed. It is based on a metamaterial with negative values of the permittivity and permeability.     

Improvement in the Low Frequency Performance of Geometric Transition Radar Absorbers Using Square Loop Impedance Layers

A technique is described for improving the low frequency performance of geometric transition (GT) radar absorbers based on lossy foam pyramids. The technique makes use of the fact that at high frequencies, only the geometric transition region of the absorber is utilized whereas at low frequencies, the whole absorber thickness interacts with the incident wave. Hence the low frequency performance may be improved, without compromising that at high frequencies, by electrically loading the absorber base layer using one or more frequency selective surfaces (FSS) whose elements are typically in the form of single or nested loops. Other advantages of this technique include minimal increases in weight and manufacturing costs. The paper includes comparative predictions of unmodified and loaded GT absorber reflectivity at both normal and oblique incidence and discusses the effect on absorber performance of tolerance variations in the dimensions and location of the loading FSS elements. Finally, free-space reflectivity measurements on unmodified and loaded commercial absorber blocks are made over the frequency range 1-10 GHz and these confirm the validity of the technique.