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Theoretical Extension of a Microwave EM Method for Predicting the Terahertz Scattering of Electrically Large Complex Target
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摘要: 微波方法到太赫兹散射特性建模的延拓面临两个关键的科学问题研究，其一是材料响应特性延拓，包括金属属性向合金属性过渡导致Drude模型无法准确描述，以及介质材料在太赫兹频段的响应模型研究；其二是表面随机粗糙结构、以及复杂细微精细结构在太赫兹频段下的散射行为建模方法的延拓研究。微波频段下可视同为光滑的金属表面在太赫兹频段可能呈现出表面微粗糙特性。此外，针对含介质涂覆或全介质表面太赫兹散射特性的建模，需要结合随机边界散射理论，建立多层描述模型，以涵盖其中的面散射和体散射现象。该文首先采用积分方程方法描述和分析了金属粗糙表面的太赫兹散射规律，与实测数据吻合较好。其次，对于含涂覆或介质材料的目标表面，除表面粗糙的影响外，材料内部的微小粒子成分(如碳粉、石墨、金属粉等)的电尺寸与太赫兹波长相比拟，实验显示其体散射贡献不可忽视。该文尝试用矢量辐射传输理论与积分方程方法结合的多层模型来描述含介质材料表面的散射特性，很好地解释了实测规律。最后，该文提出基于“半确定性”描述的射线追踪高频算法，实现了复杂目标表面相干和非相干散射特性的一体化快速建模，为超电大复杂目标太赫兹散射特性的建模分析提供有效手段。Abstract: Two important issues must be considered when modeling the terahertz (THz) wave scattering behavior using microwave EM methods. The first is the material response characteristics, including the metallic materials that may be beyond the scope of the Drude model’s description and the dielectric materials that may lack appropriate description models. The second is the modeling method for investigating the THz scattering behavior of the surface random roughness and the complex fine structures. Several theoretical endeavors are presented in this paper to elucidate the surface- and volume-scattering phenomenon observed in the experiment data. First, we employ the Integral Equation Method (IEM) to fit the measured data of an aluminum plate. Good agreements confirm the superior applicability of IEM to the metallic materials. Nevertheless, for dielectric materials, the volume-scattering contributions of the inner microstructures or particles whose sizes are comparable to the THz wavelength are required to be considered. Furthermore, our findings state that, for dielectric materials, it can be fit to the experimental data with the help of the Vector Radiative Transfer (VRT) theory. Finally, this research proposes a semi-deterministic description-based ray-tracing high-frequency algorithm to realize the rapid modeling of the coherent and incoherent scattering of electrically large complex targets at THz bands.
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图 1 单层随机粗糙边界散射示意图
Figure 1. The scattering geometry of one-layer randomly rough boundary
图 2 单层粗糙表面模型散射模拟与STL实测对比结果
Figure 2. Comparison between the simulated data by one-layer rough boundary scattering model and the experiment data by STL
图 3 多层随机粗糙边界散射示意图
Figure 3. The scattering geometry of multi-layer randomly rough boundary
图 4 多层模型和单层模型实验校验对比
Figure 4. Experimental validation of one-layer and multi-layer scattering model
图 5 采用多种内部粒子描述的多层模型实测校验(240 GHz，粒子直径分别为24 μm和100 μm)
Figure 5. Experimental validation of multi-layer scattering model with hybrid particles (240 GHz, the particle size is set by 24 μm and 100 μm respectively)
图 6 微波方法对复杂目标太赫兹散射特性建模思路
Figure 6. Theoretical extension of microwave EM method for the terahertz scattering prediction of extremely large electric size target
图 7 基于射线追踪散射几何描述示意图(大尺度外形由小面元表示；路径1为1次直接散射，路径2为多次散射)
Figure 7. The scattering geometry of ray tracing (Large scale profile is descript by myriads of small patches; Path 1 denotes the single scattering, path 2 denotes the multiple scattering)
图 8 基于面边界(2维粗糙纹理)电磁模型的复杂目标太赫兹成像仿真结果
Figure 8. Simulated THz image data of complex target under the description of surface scattering model (with two-dimensional texture)
图 9 基于体边界(3维粗糙纹理)电磁模型的复杂目标太赫兹成像仿真结果
Figure 9. Simulated THz image data of complex target under the description of volume scattering model (with three-dimensional texture)
图 11 光滑与粗糙表面弹头模型成像结果对比
Figure 11. Comparison on imaging data between rough and smooth missile warheads
图 12 不同粗糙程度表面立方体RCS结果对比
Figure 12. Comparison on RCS by cubes with different kinds of rough surface
图 13 光滑与粗糙表面模型RCS极化响应结果对比
Figure 13. Comparison on polarized RCS between rough and smooth cubes
图 14 某“全向角反射器”几何结构示意图
Figure 14. The geometry of kind of “Omni-directional corner reflector”
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