小型化频率选择表面研究现状及其应用进展

Research Status and Application Progress of Miniaturized Element Frequency Selective Surface

频率选择表面是一种在平面内排布的二维周期阵列,由金属贴片单元或孔径单元构成,其对不同频率电磁波呈现不同的滤波响应,被广泛应用于雷达隐身、卫星通讯和电磁兼容等领域。随着应用对技术的要求不断提高,传统频率选择表面难以满足实际应用。原因有:(1)传统频率选择表面较大的单元尺寸会导致单元谐振时栅瓣过早出现,对传输特性产生不良影响;(2)传统频率选择表面较大的单元尺寸导致其很难在有限空间的区域内以及不规则天线罩上使用,且大尺寸使得现有工艺很难完成贴装布阵。而单元尺寸远小于工作波长的小型化频率选择表面MEFSS的出现为解决上述问题提供了新的思路,其优势日益凸显了自身的价值,逐渐发展成为新的研究方向。最初的小型化频率选择表面通过结构上的交叉、旋绕、连接等模式来达到缩小单元尺寸的目的,最典型的方法就是单元的卷曲、交指设计。为了突破"单元尺寸与谐振波长一致"这一限制,实现小尺寸结构可以控制长波长谐振,通过金属贴片和网格经介质级联,产生了基于容性表面与感性表面耦合技术的小型化频率选择表面。该频率选择表面通过减小单元的有效电尺寸来实现小型化。耦合型双屏小型化频率选择表面主要分为:网栅及其互补结构、网栅和方环贴片阵列结构、方环贴片及其互补结构。这三种典型的耦合双屏结构是研究复杂耦合型小型化频率选择表面结构的基础。由于前两类频率选择表面在实现小型化特性时都无法摆脱周期单元尺寸的制约,若需要更高的小型化需求时则难以实现。而加载无源集总元件(电容器、电感器)的频率选择表面可以增大等效电感L以及等效电容C的值,能够更大程度地缩小单元的有效电尺寸,使得频率选择表面小型化程度更高。为了拓展小型化频率选择表面的功能,在普通小型化频率选择表面单元或介质衬底上加入有源器件(变容二极管和PIN二极管等)或采用特征媒质作为介质衬底等组成有源小型化频率选择表面。该频率选择表面在实现小型化特性的同时,通过对有源器件的调节来控制该频率选择表面的通带开关和变频特性等。本文从单元卷曲和交指设计、基于容性表面与感性表面耦合技术设计、加载无源/有源集总元件技术设计等方面综述了小型化频率选择表面的研究状况。随后,对小型化频率选择表面在实际中的应用进行了介绍并对其未来发展方向进行了展望。

Frequency selective surface(FSS)can be thought of as a kind of spatial filter.It is a two-dimensional(2D)periodic array of metallic patches or slots in a thin metallic sheet etched on a substrate,correspondingly with band-pass or band-stop behavior for incoming electromagnetic wave operating in different frequency ranges.FSSs are widely used for different applications,including radomes stealth,satellite communications,wireless security and electromagnetic compatibility.With increasing requirement of FSS technology,the traditional FSS can no longer well satisfy the practical application.The reasons lie in the following two aspects.Firstly,the large unit size of the traditional FSS will lead to the premature appearance of the grating lobe when the cell resonates,which will exert an adverse effect on the transmission characteristics.Secondly,due to the large unit size of the traditional FSS,it is extremely difficult to be applied in a limited space area or on an irregular radome,and the mounting layout can be hardly completed by the existing technology.Fortunately,the appearance of a miniaturized element frequency selection surface(MEFSS)whose unit size is much smaller than the working wavelength has provided a new idea for solving the above problems.The MEFSS has gradually shown its prominent advantages and developed into a new research direction.Originally,cross,convolution and connection modes in structure are employed by MEFSS to achieve the purpose of unit size reduction.The most typical method is the unit curling and interdigitating design.Aiming at breaking through the restriction that“the unit size should be consistent with the resonance wavelength”and realizing the control of long wavelength resonance by small size structure,a MEFSS based on the capacitive surface and inductive surface coupling technology is produced by cascading metal patches and grids through the medium.This FSS achieves miniaturization by reducing the effective electrical size of the unit.The MEFSS surface of the coupling type double screen mainly includes mesh grid and its complementary structure,mesh grid and square ring patch array structure,square ring patch and its complementary structure,which lay the research foundation of complex coupled MEFSS structures.Since the first two types of FSSs cannot get rid of the limitation of the periodic unit size when realizing miniaturization,it is hard to achieve hig-her miniaturization requirements.However,FSS loaded with passive lumped elements(capacitors,inductors)can increase the values of the equivalent inductance L and the equivalent capacitance C,and reduce the effective electrical size of the unit to a greater extent,making FSS more miniaturized.For the sake of expanding the function of MEFSS,active devices(varactors,pin diodes,etc.)are introduced to ordinary MEFSS units or dielectric substrates to form active MEFSS,or featured media is used as dielectric substrates to form active MEFSS.When this FSS realizes miniaturization characteristics,it controls the passband switch and frequency conversion characteristics of the FSS by adjusting the active devices.This article summarizes the research status of MEFSS from the aspects of unit convolution design,interdigital design,capacitive surface and inductive surface coupling technology based design,and technical design of loading passive/active lumped elements.Subsequently,the application of MEFSS in practice is introduced and its future development direction is proposed.