Ultra-wideband thin frequency-selective surface absorber against sheet resistance fluctuation

Abstract

The design of thin frequency selective surface (FSS) absorbers based on resistive films that meet the requirements of broadband, polarization independence, incident angle stability, and strong absorption is a challenging task. Fabrication tolerance of resistive film can result in fluctuations in sheet resistance, which negatively impact the absorber performance. To tackle these problems, this paper firstly investigates how sheet resistance fluctuations affect the absorbing performance of resistive film FSS absorbers. Through analysis of simulated current density distribution and impedance, it is revealed that the diversity of current paths provides an effective way to mitigate the influence of sheet resistance fluctuation. This is achieved by enabling flexible variation of surface current in response to sheet resistance fluctuates. Consequently, the variation of input impedance of the FSS absorber due to the fluctuation of sheet resistance is suppressed within a small range. Then, a method for bandwidth extension is proposed by employing the complementary variation of FSS impedance with frequency at different layers. By combining this approach with a miniaturization design, a thin and light FSS absorber is developed that exhibits ultra-wide bandwidth, polarization independence and angle stability while mitigating the effects of sheet resistance perturbation. The proposed FSS absorber achieves a 90% absorption bandwidth from 1.50 GHz to 20.50 GHz, covering Ku, X, C, S bands as well as part of the L and K bands, with a relative bandwidth of up to 173%. The absorber has a thickness of 0.093 λ<sub>L</sub> for both transverse electric (TE) and transverse magnetic (TM) polarizations, yielding a figure of merit (FoM, the ratio of the theoretical minimum thickness to the actual thickness) of 0.95, indicating that the thickness is close to the theoretical limit. The absorber retains over 90% absorption rate for TM polarization at an incidence up to 70° and 80% absorption for TE polarization at an angle of 45°. Furthermore, the 90% absorbance bandwidth of the absorber remains at 167.0% when the sheet resistance of any FSS layer fluctuates within a range from 12 Ω/sq to 30 Ω/sq. A prototype of the proposed FSS absorber is fabricated and measured, yielding experimental results that are in good agreement with the simulation results, thus validating the effectiveness of the proposed method.

Switchable and optically transparent ultrawide stopband frequency selective surface for electromagnetic shielding

“…入射到其上的电磁波呈现出独特的空间滤波特性 [1] ，在吸波材料 [2,3] 、天线罩 [4,5] 、 电磁干扰(EMI)抑制 [6,7] 等多个领域内具有广泛应用。与金属网屏蔽相比，FSS 电磁屏蔽结构更轻便且具有频率选择性，可应用于特定场所以满足其电磁安全或 健康需求，如重症监护室(ICU) 、数据中心、安全装置、军事设施和关键基础设 施等 [8][9][10][11][12][13][14] 。由于外部高频电磁波穿透混凝土墙壁时衰减严重，主要通过玻璃门窗 进入建筑物内部，因此在提供电磁屏蔽的同时，需要考虑光学透明性和美观。随 着无线技术和雷达技术的不断发展，对超宽带电磁屏蔽结构的需求日益增加。因 此，设计一种透明的超宽带电磁屏蔽体具有重要应用价值。 近年来，已有许多研究致力于电磁干扰(EMI)抑制的光学透明频率选择表 面(FSS)结构设计 [15][16][17][18][19][20] 。一种常用策略是应用透明导电材料，例如通过银纳米 线(Ag NW)或氧化铟锡(ITO)来实现透明屏蔽。Yang 等人使用银纳米线透明 油墨设计了具有 81.6%透明度的 FSS 结构，在 0.71 ~ 1.25 GHz 和 1.73 ~ 2.16 GHz 的测量范围内产生良好反射，实现了 GSM 频段 EMI 的有效屏蔽 [15] 。Zhang 等人 利用 ITO 制作 FSS 吸收体，通过吸收电磁波实现从 7.8 GHz 到 18.0 GHz 的透明 宽带屏蔽 [18] 。 另一种方式则是将 FSS 结构设计中常用的介质基板更换为透明基板， 通过设计图案未覆盖区域实现光学透明。例如，Dewani 等人利用丝网印刷技术 将基于电阻性油墨的 FSS 结构印刷在玻璃上，在 1.5 至 2.5 GHz 频段实现理想的 屏蔽 [20] 。因此，当前光学透明 FSS 设计可以有效抑制特定频段或双频甚至三频段 的 EMI，但尚缺乏能够抑制超宽带范围(包含 P、L、S、C、X 以及 Ku 波段) EMI 的电磁屏蔽设计。 随着无线新技术的不断发展，对多功能电磁屏蔽结构提出了新要求，超宽带 电磁屏蔽、电磁性能动态可调等成为电磁屏蔽设计的重要功能特点。对于宽带电 磁屏蔽的研究，目前主要基于小型化、多层技术实现宽带电磁特性 [21][22][23][24] 。 Habib S 等人通过多层及卷曲 FSS 方形和圆形环路元件实现了 S、C、X 频段的宽带带 阻 [21] ， Xu, S J 等人则通过三层级联 FSS 结构实现了小型化及 7.34 GHz ~ 15.0 GHz 的宽阻带抑制 [22] 。电磁性能动态可调通常分为可开关和可调谐两种 [25][26][27][28][29][30][31][32] ，可开关 [25] ；也可以利用液态 金属注入与否实现开关功能，切换 FSS 结构的滤波功能，例如 Ghosh S 等人将 EGaIn 注入 FSS 结构微通道，实现全通到带通(TE 极化)/带阻(TE、TM 极化) 的切换 [32] FSS ...…”