超材料吸波体在电磁隐身以及电磁防护领域具有广泛的应用，为实现雷达散射截面(Radar Cross Section, RCS)的减缩提供了新的方法和思路。传统的超材料吸波体具有斜入射特性差以及只能吸收沿着单个方向传播的电磁波的特点，本文分别设计具有宽角度入射特性和双向吸收特性的宽带超材料吸波体来提升吸波体的吸收方向性。

       针对超材料吸波体的吸收率随着斜入射角度的增大而减小问题，基于传输线理论，将金属开缝圆环和扇形金属谐振器分别等效为电阻R、电感L、电容C构成的并联回路，将介质基板和空气层分别等效为一段传输线，构建等效电路模型。根据此等效电路模型，设计一种开缝金属圆环和扇形谐振器加载集总电阻的宽带超材料吸波体。在45°入射角范围内，TE波和TM波在1.61 GHz ~ 4.45 GHz的吸收率大于0.9，相对带宽为93.73%。当电磁波垂直入射时，相较于同尺寸的金属板，吸波体的RCS缩减值可达到52.2 dB，并且在[-45°,45°]入射角范围内，均表现出良好的RCS缩减特性。采用印刷电路板工艺进行样品加工，采用自由空间法对吸波体的电磁特性进行测试，实验结果与仿真结果吻合。

       针对超材料吸波体只能实现单个方向传播的电磁波的吸收问题，基于传输线理论，分别将“十”字开缝环和交叉偶极子等效为RLC并联和串联等效电路。基于此等效电路模型，设计出一种“ABA”组合结构的宽带双向超材料吸波体，通过将两个I型结构正交地附着在超薄的FR4衬底上，实现双向完美吸收。在1.43 GHz ~ 2.51 GHz，+z和-z方向上的TE波和TM波的吸收率都大于0.9。并且该吸波体具有良好的极化不敏感特性和宽角度入射特性。与同尺寸的金属板相较，吸波体的RCS缩减值可达到17.56 dB，并且在[-30°,30°]入射角范围内，都具有良好的RCS缩减特性。加工一个10×10阵列的样品，验证所设计的吸波体的双向吸收特性。

       本文基于传输线理论研究具有斜入射增强以及双向吸收特性的宽带超材料吸波体，有益于推动全向吸波体的技术发展及其在电磁兼容以及电磁隐身等领域的广泛应用。

Metamaterial absorbers have been widely used in electromagnetic stealth and electromagnetic protection, providing a new method and idea for Radar Cross Section (RCS) reduction. Traditional metamaterial absorbers have the disadvantages of poor oblique incidence and can only absorb electromagnetic waves propagating along a single direction. In this paper, broadband metamaterial absorbers with wide angle incidence and bidirectional absorption are designed to improve the directivity of the absorber.

The absorptance of the traditional metamaterial absorber decreases with the increase of the oblique incident angle. In this paper, an equivalent circuit model of the metal resonator structure is established based on the transmission line theory, the metal split ring and the fan-shaped metal resonator are equivalent to the parallel circuit composed of resistance R, inductance L and capacitance C, and the dielectric substrate and air layer are equivalent to a transmission line, respectively. A broadband metamaterial absorber composed of double loop split-rings and a set of metal sector is designed according to the equivalent circuit model, the lumped resistors are inserted into the gaps of the loop split-rings and sectors for expanding the absorption bandwidth. The absorptance is larger than 0.9 in 1.61 ~ 4.45 GHz with the fractional bandwidth is 93.73%. The most important advantage of the designed absorber is that the absorptance can stay the absorptance at normal incidence as the incidence angle increases to 45º for both TE wave and TM wave. When the electromagnetic wave is incident vertically, the RCS reduction value of the absorber can reach 52.2 dB compared with the same size metal plate, and in the range of incidence angle [-45°,45°], it exhibits good RCS reduction characteristics. A sample is fabricated by using printed circuit board technology, and the electromagnetic characteristics of the absorber are tested by using free space method. The experimental results are in good agreement with the simulation results.

The previous metamaterial absorber can only absorb electromagnetic waves propagating in a single direction. In this paper, a cross and hollow crosses and cross dipole are equivalent to RLC parallel and series equivalent circuit based on the transmission line theory, respectively, the ADS circuit simulation software is used to fit the equivalent circuit model of metal resonant structure, and a broadband bidirectional metamaterial absorber with a combined ABA structure is designed based on the equivalent circuit model, and two orthogonal I-shaped structures are pasted on both sides of the ultra-thin substrate (FR-4) instead of the metal plate for enhancing absorptance of the absorber. Simulated results show that absorptance of the designed absorber is larger than 0.9 in 1.43 GHz ~ 2.51 GHz along forward and backward directions both under TE and TM polarizations. And the absorber exhibits excellent polarization insensitivity and wide angle incidence characteristics. Compared with the same size metal plate, the RCS reduction value of the absorber can reach 17.56 dB, and in the range of incidence angle [-30°,30°], it exhibits good RCS reduction characteristics. A 10×10 array sample is processed to verify the bidirectional absorption characteristics.

In this thesis, a broadband metamaterial absorber with oblique incident enhancement and bidirectional absorption is studied based on transmission line theory, which is beneficial to promote the development of omnidirectional absorber technology and its wide application in electromagnetic compatibility and electromagnetic stealth.

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