材料电磁屏蔽效能测试的屏蔽室法的尺寸效应、位置效应及本征特性

By and large, there are two primary methods for the shielding effectiveness measurement of a material, that is, the coaxial line method and the shielding enclosure method. The former is only feasible for frequency below 1.5GHz due to the minimum aviable dimension of a coxial line. The latter is feasible for frequencyies up to 40GHz, but its results are dependent on the enclosure's dimension, the window's dimensions and the position of the observation point. So, a matetial will display different shielding effectivenesses for different configurations,which leads to the problem that results of a given configuration will not be useful for a different configuration and hence lots of measurements will have to be carried out repeatedly for different configurations. In order to solve the problem, two kinds of approached are presented in this project advice: one is to master the characterestics of the shielding effectivenes varing with the dimensions and the position, and the other one is to bridge a clear mathematical relasionship between the shielding effectiveness and the material's condcutivty by introducing a novel definition of shielding effectiveness of a material. To this goal, a FDTD numerical algorithm for the electromagnetic field computaion of shielding enclosure method will be developed in this study and then is used to find out the ralationship, and to validate the effectivenes of the novel definition. Where, three types of approaches are used to modeling the material layer: the transfer coefficient matrix, the subcell gird tchnique, and the nonuniform grid technique. Also, comparisons with experimental and other numerial resluts will be conducted to validate and improve the effectivenes of the FDTD algorithm. Then, a lot of computations will be carried out and the corresponding results will be analyzed in detail to find the relationship, the effectivenes of the novel definition, and transfromation method of the shielding effectiveness between different configurations.The study is very helpful for improving the miniature of enclosure's dimensions, the transportablity of measurement results, and the felxiblity of measurement configuration.

材料电磁屏蔽效能测试方法主要有同轴法和屏蔽室法。受尺寸限制，前者适用频率1.5GHz以下。后者频率可达40GHz，但结果受屏蔽室尺寸、窗口尺寸及观测点位置的影响，使得同一材料在不同配置下的测试结果不同。针对屏蔽室法的上述问题，本项目提出两条思路来探索测试结果在不同配置之间的转换方法：一是掌握尺寸和位置等因素对屏蔽效能的影响规律；二是提出新的屏蔽效能定义，该定义能独立于尺寸和位置，并表征出测试结果的本征特性，即它与材料电磁参数之间有明确的数学关系。为此，本项目将发展屏蔽室法的有限差分计算模型。在该计算模型中，将分别对材料层采取转移系数、子网格和不均匀网格技术的处理方式。同时，通过与实验结果和其它计算方法的比较，检验和改进计算模型。然后，基于发展出的计算模型分析相关影响规律，检验新定义的有效性，总结不同配置下屏蔽效能的转换方法。研究成果有助于改善屏蔽室法的尺寸小型化、结果可移植性及操作灵活性。

屏蔽室法是材料电磁效能测试的主要方法之一，具有测试动态范围大、频率范围宽等特点。但该方法也存在测试结果受屏蔽室尺寸、谐振效应、场强观测点位置等因素影响的缺点，使得测试结果不能直接反应材料本身在电磁屏蔽方面的作用，无法在不同的实际应用中移植。本项目的研究试图对屏蔽室法进行改进，以消除上述因素的影响，从而提出一种能直接反应材料在电磁屏蔽中的本征作用的测试方法。为此，项目开展了以下5项研究工作：1）基于矩量法+模式展开法，发展了一种计算屏蔽室法中参考场强（屏蔽室测试窗保持开放状态时屏蔽室内部的电磁场）的半解析方法；2）基于互易原理+模式展开+转移阻抗，提出了一种计算屏蔽室法中屏蔽场强（屏蔽室测试窗被待测材料覆盖时屏蔽室内部的电磁场）的半解析方法；3）通过和全波计算和实验结果的比对，检验了计算方法的有效性；4）通过计算分析，定量展示了屏蔽室法中尺寸效应、位置效应和谐振效应的影响；5）提出了一种基于参考材料比对的材料屏蔽效能的新定义和测试方法，能有效消除上述效应的影响，直接反应材料的本征特性（该定义下材料屏蔽效能可与无限大材料对垂直入射平面电磁波的屏蔽效能等效）。