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.     

基于集总电阻加载吸波器的紧耦合超宽带天线

基于集总型电阻频率选择表面吸波器,设计一款超宽带紧耦合天线阵列。在天线阵列与地板之间加入两层结构集总型电阻频率选择表面吸波器,改变天线与地板之间的传输特性,有效抑制天线短路零点出现,扩展天线带宽;同时利用天线间强耦合效应,减小天线单元结构尺寸。使用集总电阻和金属环构成的吸波器代替常规阻抗型频率选择表面结构,降低天线阵列设计与加工难度,同时可有效改善天线阻抗匹配。仿真实验表明,通过调节集总频率选择表面吸波器物理结构、加载电阻阻值和天线间耦合电容值等参数,当天线单元驻波比小于3 时,可实现带宽范围达12.6:1 (1.5～19 GHz)的超宽带性能;并在2.2～18.3 GHz 范围内具有驻波比小于2 的良好匹配性能。     

Varactor tunable frequency selective absorber

A three-layer frequency selective surface (FSS) tunable absorbing filter is presented. Tunability is achieved through the use, in every FSS layer, of commercially available, low-cost, surface-mount, radio-frequency, varactor diodes. The absorber is experimentally tested in a waveguide over the 1.8-2.4 GHz frequency band. It has the potential to form the basis of a smart microwave absorber.     

基于AFSS的吸波性能可调的微波吸收体

从传统的Salisbury屏结构出发,设计了一种由半圆和三角图形组合而成的频率选择表面(FSS)单元,随后将PIN二极管与FSS图形相结合,制得了一种AFSS吸波结构体.利用电抗加载方式对该结构体进行了仿真,并在微波暗室下对实物进行了测试.结果表明:测试结果和仿真结果变化趋势相似.在2～8 GHz内,结构体吸收峰从-4...     

Radio absorbers based on magnetic polymer composites and frequency-selective surfaces

Polymer composites filled with carbonyl iron are considered. A method is proposed for extending the operating frequency band of such radio absorbers with the help of band-reflection frequency-selective surfaces (FSSs). A functional relationship between the reflection characteristics of a radio absorber and the generalized characteristics of an FSS—the resonance frequency and Q factor—is established. On the basis of the rigorous theory of integral equations, programs are developed for computation of the reflection coefficient of a radio absorber with an FSS implemented as a biperiodic lattice with ring elements. The reflection coefficients of a series of samples are measured in the frequency band 1–10 GHz.     

频率选择表面结构吸波体的电磁特性研究

采用时域有限差分法(FDTD)计算了蝶形和方环形FSS单元图形的反射与传输特性以及反射信号与入射信号的相位关系,并对计算结果进行了分析.结果表明电磁波的干涉相消是FSS吸波体吸波的主要原因,并且当反射电磁波满足2nπ的相位关系时才能达到最佳吸波效果;FSS吸波体的中心频率随介质层厚度的增加而下降;介质层厚度为5 mm时...     

覆盖L 波段的宽带隐身雷达天线罩设计

设计了一款极化和角度不敏感的宽带频选吸波体,在X 波段实现了宽带透射窗口以及包含L波段在内的宽频带吸收。该频选吸波体采用频率选择表面与电磁超材料吸波体相结合的方式,通过级联加载多层耦合型频率选择表面和双层高阻表面完成总体结构设计。理论上,利用等效电路法对多层耦合型频率选择表面及整体结构展开分析,论证频选结构和频选吸波体的谐振机理。数值仿真结果显示,该频选吸波体可以实现8.1～11.7GHz频段内的宽频带透射,以及1.18～4 GHz、15～18 GHz 频段内的宽频带吸收,其中透射窗口的插入损耗不大于3 dB。将该频选吸波体作为平面雷达天线罩与微带天线相结合,分析天线的辐射性能和散射特性。研究结果显示,天线在工作频带内保持了良好的辐射性能,而在带外实现了RCS 的有效缩减,达到了天线系统的隐身目的。     

双层方环可电控FSS吸波屏设计和实验研究

提出了基于双层方环FSS结构的可电控吸波屏,通过加载PIN二极管在X波段实现了对FSS吸波屏吸波性能的电流控制.加工制作了可电控吸波屏的实验样品板并进行了实验测试,分别测量了垂直入射与斜入射情况下、电场极化同PIN管极性所趋方向相同时吸波屏的反射率频响曲线,曲线中保证有效吸收的频带区域的动态迁移范围覆盖了整个X波段.实验结果表明:该吸波屏能够有效吸收入射电磁波,其吸波性能经电流的控制可满足不同的带宽要求,且带宽的可电控特性在垂直入射与斜入射时均达到了预期的目标,体现了双层方环型FSS吸波屏对多角度入射波的适应性.     

A single-layer tuneable microwave absorber using an active FSS

An experimental single-layer active microwave absorber in described. The absorber is a planar structure based upon the topology of a Salisbury screen, but in which the conventional resistive layer is replaced by an active frequency selective surface (FSS) controlled by pin diodes. The resulting structure has superior reflectivity-bandwidth characteristics compared to conventional passive absorbers of corresponding thickness. Measured data are presented and show that the reflectivity response of the absorber can be controlled over the frequency band from 9 to 13 GHz.     

A 3-D hybrid finite element/boundary element method for the unifiedradiation and scattering analysis of general infinite periodic arrays

We present a rigorous frequency domain variational 3-D electromagnetic formulation for the general nonself-adjoint infinite periodic array problem. The hybrid method described combines the vector finite element and Floquet boundary element techniques. It is general in the sense that it is applicable to infinite periodic arrays of the open or aperture-types. It is thus effective for modeling both the scattering and radiation performance of diverse FSS, absorber, and phased-array structures. The technique accurately handles arbitrarily complicated 3-D geometries, lossy inhomogeneous media and internal as well as external excitations. These analyses can be applied to general skewed grids under arbitrary scan and polarization conditions     

频率选择表面的等效电路设计方法综述

本文回顾了频率选择表面(FSS)等效电路的理论发展,分析了将其应用于提高FSS设计效率的必要条件,重点介绍了利用具有准集总电感和准集总电容效应的元件,依据等效电路设计实现一系列高性能带通、带阻、单频和多频FSS 的方法。理论和实测结果表明:这类FSS普遍具有极小的尺寸,良好的入射角和极化稳定性,而且谐振频率、带宽和极化特性独立可控;同时,单元结构参数和等效电路参数之间的解析关系式为FSS的快速优化提供了条件,从而为FSS 设计提供了从构建到优化的高效解决途径。     

Fast and noninvasive fluorescence imaging of biological tissues in vivo using a flying-spot scanner

We have developed a flying-spot scanner (FSS), for fluorescence imaging of tissues in vivo. The FSS is based on the principles of single-pixel illumination and detection via a raster scanning technique. The principal components of the scanner are a laser light source, a pair of horizontal and vertical scanning mirrors to deflect the laser light in these respective directions on the tissue surface, and a photo multiplier tube (PMT) detector. This paper characterizes the performance of the FSS for fluorescence imaging of tissues in vivo. First, a signal-to-noise ratio (SNR) analysis is presented. This is followed by characterization of the experimental SNR, linearity and spatial resolution of the FSS. Finally, the feasibility of tissue fluorescence imaging is demonstrated using an animal model. In summary, the performance of the FSS is comparable to that of fluorescence-imaging systems based on multipixel illumination and detection. The primary advantage of the FSS is the order-of-magnitude reduction in the cost of the light source and detector. However, the primary disadvantage of the FSS its significantly slower frame rate (1 Hz). In applications where high frame rates are not critical, the FSS will represent a low-cost alternative to multichannel fluorescence imaging-systems.     

频率选择表面谐振特性在玻璃钢结构吸波材料中的应用研究

研究了频率选择表面(FSS)的谐振特性对玻璃钢(FRP)结构吸波材料吸波特性的补偿提高作用。为了提高FRP在8~11 GHz频段内的吸波特性,利用有限元法仿真设计了圆环孔径型以及双方环贴片型两种不同图形样式的FSS,并用自由空间法对其谐振特性进行了测试,最后利用FSS谐振特性对FRP在8~11 GHz频段内的吸波特性进行了补偿优化。结果表明,利用FSS在对应频段内的谐振特性,能够将FRP在8~11 GHz频段内的反射率全频段降至–10 dB以下,从而大幅度提高了FRP的雷达吸波特性。这为玻璃钢结构吸波材料的隐身化研究开辟了一条新的途径。     

利用频率选择表面改善微波吸收材料S波段的吸波性能

研究了如何利用金属周期性频率选择表面(FSS)的频率特性来改善微波吸收材料S波段的吸波性能。利用频率选择表面的等效电路和传输线理论分析了FSS和吸波材料涂层双层结构的微波反射特性。采用基于有限元方法的电磁波全波分析软件设计并仿真分析了FSS的结构和尺寸,实际制作了FSS和吸波材料涂层双层结构,测量了微波反射性能。理论分析和实验研究表明,利用FSS可以明显改善吸波材料涂层S波段的吸波性能,展宽涂层的吸波带宽,从而改善吸波材料的低频吸波性能。     

Small periodicity FSS screens with enhanced bandwidth performance

A novel approach for obtaining small periodicity frequency selective surface (FSS) screens with enhanced bandwidth performance is proposed. The issue of miniaturising FSS unit cells has gained extreme importance in recent metamaterials applications. To this end several procedures have been suggested in the literature. However, such procedures usually result in a bandwidth degradation as long as miniaturisation is improved. On the contrary, the bandwidth performance of the structures here presented is enhanced as the FSS periodicity gets smaller. Application of such structures to the synthesis of artificial magnetic conductors are also presented.     

一种旋绕型频率选择表面的传输特性分析

利用基于矩量法的电磁分析软件Ansoft Designer研究了一种新的旋绕型双方环频率选择表面(new convoluted double square loop frequency selective surface,简称NCDSL FSS)的传输性能.结果表明,该结构对入射电磁波的入射角、极化方式等不灵敏,比经典CDSL FSS受栅瓣影响小;通过调节其凹槽深度,两频带间隔可围绕单元尺寸、阵列周期均相同的传统同心双方环频率选择表面(double square loop frequency selective surface,简称DSL FSS)的频带间隔双向调节,这是CDSL FSS无法做到的;而且NCDSL FSS高频谐振点处的-0.5 dB相对反射带宽也总是比CDSL FSS和DSL FSS的大.因此该结构是一种简单且性能较优的频率选择器件.     

频选雷达罩电性能的基本问题及解决方法

频率选择表面(FSS)雷达罩技术是新一代战机实现隐身的重要途径。基于目前国内外FSS研究和实践的结果,从FSS的基本要素出发,对FSS雷达罩电性能设计涉及的谐振频率、带宽、栅瓣等基本问题进行了分析、研究、梳理和归结,针对雷达罩入射角和入射波极化变化的情况,对FSS如何增加频带宽度、实现谐振频率与带宽的稳定性、避免栅瓣对RCS的影响等提出了应对办法,指出FSS单元选择、设计小尺寸单元并将它们紧凑排列以及正确地进行介质加载是获取优良电性能的关键。     

频选膜拼接工艺误差对复材天线罩电性能的影响

加载频率选择表面(FSS)薄膜的复合材料天线罩在实际制作过程中,需将若干FSS薄膜结构进行拼接以构成一个整体,FSS膜的拼接工艺误差则对天线罩的电性能产生实质性的影响。文中设计了一款X波段的A夹层平板复合材料天线罩,并建立了由拼接工艺误差带来的FSS膜搭接和FSS膜缝隙的电磁仿真模型,研究了不同拼接工艺对复合材料天线罩的透波率、电磁波传输特性的影响以及对天线辐射特性的影响。研究结果表明：天线罩中的FSS薄膜结构的拼接错位对电磁波传输影响明显,会明显降低天线主瓣增益和天线罩透波率;当入射电磁波的极化方向与FSS薄膜结构的搭接缝方向相同时,FSS膜结构对天线罩透波率影响较大,当极化方向与FSS薄膜结构的搭接缝方向垂直时,则其对天线罩透波率影响较小;两块FSS膜结构边缘搭接的相对位置和搭接距离对天线方向图有明显影响;两块FSS薄膜结构边缘无搭接且存在缝隙则对天线罩透波率影响较小。该研究对复合材料天线罩的制造工艺具有工程指导意义。     

太赫兹波段频率选择表面的设计与测试

讨论了太赫兹波段频率选择表面(FSS)在天文探测、遥感卫星以及航天器辐射计系统中的应用。对太赫兹波段频率选择表面的设计、加工及测试方法进行了研究。介绍了太赫兹波段频率选择表面的设计、分析方法及电磁仿真软件。由仿真及实测结果可知,太赫兹频段的频率选择表面由于受限于物理尺寸,造成了设计、加工及测试上的困难,其性能与其他频段相比更易于受到加工精确度的影响。在实际加工过程中,应尽量采用微纳加工技术进行加工。而在设计频率选择表面时,应当充分考虑器件厚度以及单元形状对加工造成的影响。测试过程中应当尽量避免反射回路造成的多次回波。     

Frequency-selective surfaces to enhance performance of broad-band reconfigurable arrays

We present a novel frequency-selective surface (FSS) design aimed at enhancing the performance of broad-band reconfigurable antenna apertures. In particular, reconfigurable printed dipole arrays are examined in the presence of a multilayer FSS. Of particular interest is the design of FSS structures whose reflection coefficient has prespecified phase response over a broad set of frequencies. Previous FSSs primarily considered designs on the basis of the reflection coefficient amplitude and were intended for radome applications rather than substrates. Designing FSSs subject to phase requirements is seen to require some compromise in the magnitude. Broad-band requirements also present us with a need for noncommensurate FSS designs.