Fund Project:Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0204600) and the National Natural Science Foundation of China (Grant No. 51802352).
Received Date:03 November 2018
Accepted Date:05 March 2019
Available Online:01 May 2019
Published Online:05 May 2019
Abstract:High-performance absorbing material can play an important role in electromagnetic compatibility, electromagnetic radiation protection, and anti-detection of special equipment. Combining traditional absorbing material with metamaterial is an important direction for developing absorbing material. The composite absorbing body based on the development of metamaterial has advantages of thin thickness, light weight, strong absorption, and adjustable absorption band, but the super material absorption body composed of single-sized metal pattern elements possesses generally strong absorption only for electromagnetic waves at a certain frequency. It is difficult to meet the requirement for wide frequency absorption in practical applications. In order to broaden the absorption bandwidth of metamatial, metal spiral-ring metasurface coated short carbon fiber absorber with enhanced microwave absorbing performance is proposed. The absorber is a two-dimensional structure formed by periodically arranging a large number of individual absorber units in the horizontal and vertical direction. In the HFSS simulation software, a " master-slave boundary condition” consisting of " master boundary” and " slave boundary” is provided. Under this boundary condition, the electric field between adjacent boundaries has a phase difference, which can be used to simulate an infinite array. The research results show that the obvious enhancement of both the absorption peak and bandwidth can be observed by embedding the double-layer spiral-ring metasurfaces. The increase of initial length of spiral-rings and thickness of absorber are beneficial to further enhancing the microwave absorption. The reflection loss from 9.2 GHz to 18.0 GHz are under –10 dB (the bandwidth reaches 8.8 GHz), and the peak of S11 is –14.4 dB. Besides, we find that the effective electromagnetic parameters and impedance of spiral-ring metasurface embedded microwave absorber present obvious resonant phenomenon at multi-frequencies by calculating S parameters. Furthermore, an equivalent circuit model regarding double-layer spiral-ring embedded absorber is established to reveal the attenuation mechanism of microwave energy. The resonant frequencies derived from this model are well accord with the simulated results. Thereby, the multi-electromagnetic resonant frequencies make the composite microwave absorber combined with double-layer metal spiral-ring and carbon fiber have microwave reflection loss in a wide bandwidth. Keywords:spiral-ring metasurface/ electromagnetic resonant/ equivalent circuit model/ microwave loss mechanism
图 2 碳纤维损耗层复相对介电常数频谱 Figure2. Relative complex permittivity of carbon fiber.
由于吸波体实物是由大量的单个吸波体单元在横、纵两个方向周期性排列形成的一个二维结构. 在HFSS仿真软件中, 选择由“主边界”和“从边界”构成的“主从边界条件”, 在该边界条件下, 相邻的边界之间的电场存在相位差, 可用于对无限阵列的仿真模拟. 如图3所示, 在平行于xoy平面的端口处均设置Floquet端口激励, 电磁波垂直入射, 其余四个平面设置为两对主从边界条件, 以此模拟二维周期性结构. 扫描频率范围设置为8.0—18.0 GHz. 图 3 仿真模型边界条件与激励设置 Figure3. Settings of boundary conditions and incentive settings for the simulation model.
设计的双层金属螺旋环与碳纤维吸波材料复合结构、双层碳纤维吸波材料及介质基板FR-4上的双层金属螺旋环复合结构(总厚度均为3.1 mm)的吸波性能如图4所示. 可见, 双层碳纤维吸波体在18 GHz频点处的峰值反射损耗仅为–7.5 dB; FR-4上的双层金属螺旋环复合结构呈现谐振型多峰值的吸波特性; 双层碳纤维吸波层插入螺旋环后, 呈现明显的宽频吸波特性, 9.2—18.0 GHz频段的反射损耗均优于–10 dB(带宽达8.8 GHz), 吸收峰值达–14.4 dB. 图 4 双层金属螺旋环-碳纤维复合吸波体的反射损耗 Figure4. Reflection loss of double layers metal spiral-ring with carbon fiber composite microwave absorber.
螺旋环初始线长a、线宽b和上层吸波材料厚度h对复合吸波体反射率的影响规律见图5. 初始线长a对复合吸波体反射率的影响显著, 随a逐渐增加, 吸收峰值有所增加, 优于–10 dB的吸收带宽显著增加; 且随a增加, 吸收峰对应的频率向低频移动, 即在不增加吸波层厚度的前提下能增强低频段微波的吸收. 线宽b对复合吸波体微波反射率的影响较小; 而增加吸收层厚度h能有效增强低频段电磁波的吸收. 经优化设计发现, 当螺旋结构尺寸为a = 3.0 mm, b = 0.3 mm, h = 2.1 mm时的螺旋形超材料与短切碳纤维吸波材料组成的复合吸波体性能最佳, 反射损耗优于–10 dB的频段为9.20—18.00 GHz, 吸收峰值达–14.36 dB. 图 5 螺旋环结构参数对双层螺旋环-碳纤维复合吸波体反射损耗的影响 (a)初始线长; (b)线宽; (c)损耗层厚度 Figure5. Effects of structure parameters of spiral-ring on the reflection loss of double layers metal spiral-ring with carbon fiber composite microwave absorber: (a) Initial length of line; (b) width of line; (c) thickness of upper dielectric layer