| Metamaterials are artificial composites composed of sub-wavelength size structural units in a special arrangement,which is an important carrier to explore the interaction between electromagnetic wave and matter.Compared with the traditional threedimensional metamaterials,the two-dimensional metamaterials have the characteristics of ultra-thin and easy to fabricate,which has been widely concerned and studied.However,there are still some technical challenges:(1)The metasurface only uses phase modulation in most applications,so how to make the metasurface achieve joint amplitude and phase modulation and have stronger electromagnetic field regulation ability is an important challenge;(2)Reflection metasurface may cause occlusion in the feed direction,Therefore,designing transmissive metasurfaces to solve occlusion is the other technical challenge.This thesis presents a design method for electromagnetic metasurface,which can be used to convert an incident electromagnetic field into a transmitted field with different amplitude and phase to produce the required far-field radiation pattern,which provides a field regulation effect that cannot be achieved with conventional materials.The relationship between tangential electric and magnetic fields on both sides of the metasurface is defined by generalized sheet transitino conditions and the surface susceptibilities is introduced.The surface susceptibilities equivalent homogenization model is used to analyze the metasurface characteristics.At present,most macroscopic design methods are limited to the ideal case where the incident field and transmitted field of the metasurface are completely known.In fact,the specific distribution of transmitted field cannot be obtained a priori for the far-field radiation pattern that needs to be realized.In order to solve this limitation,the transmission field distribution in macroscopic metasurface design is regarded as an electromagnetic inversed problem,and the conjugate gradient method is used to solve the problem.The results show that the transmission field obtained by the conjugate gradient method can generate the desired radiation pattern.Local power conservation conditions are further forced into the inversion process to improve the efficiency of metasurface.The inversion results show that the obtained equivalent current is sufficient to meet the specified far-field radiation pattern and the power loss is improved.In order to illustrate the practicability of the method described in this thesis,the macroscopic surface susceptibilities are expressed as a three-layer impedance sheet model which can be established in the simulation software,and the design process of the three-layer impedance sheet model is given.Through the simulation of the antenna combined impedance sheet model,it is proved that the impedance sheet has the ability to convert the antenna radiation field into the specified far-field radiation pattern.Finally,in order to extend it to the physical application level,a metasurface composed of three layers of dogbone type metal patches is designed and its interlayer characteristics are analyzed to achieve the same effect as the impedance sheet model.The simulation and experimental results of the antenna-impedance sheet model and the corresponding antenna-metasurface model are compared.The field conversion ability of the metasurface model to change the antenna radiation pattern is the same,indicating that the metasurface also has the ability to convert the antenna radiation field into the specified far-field radiation pattern.It is concluded that it is feasible to design metasurface structures based on impedance sheet model. |
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