Design And Applications Of Digital Coding Metamaterials

Coding metamaterial is a new branch of artificial electromagnetic metamaterials.It integrates the idea of digital coding into the design of metamaterials.The phase value is discretized and coded,and then the purpose of regulating electromagnetic wave can be achieved by designing the coding sequence.Therefore,how to design the appropriate cell structure and coding sequence is significant to the research of coding metamaterials.In this thesis,the applications of coding metamaterials in RCS reduction and beam control are studied.The main contents are as follows:1.In order to reduce the radar cross section of coding metasurface,particle swarm optimization(PSO)algorithm is used to optimize the coding sequence.Firstly,the algorithm is designed and run in Matlab to optimize the coding sequence.The fitness function to be optimized is the far-field function of the coding metasurface.The optimized coding sequence will be set to an array of 8×8.Each array unit is composed of 0 or 1.A 1 bit coding unit cell structure is designed.Based on the optimal coding sequence,the cell structure is arranged into a coding metasurface.The simulation results show that the coding metasurface can achieve more than 10 dB RCS reduction in the range from 25.5 GHz to 41.5 GHz.2.To solve the problem of RCS reduction in broadband,a coding metacell structure with polarization insensitivity,broadband and high-efficiency polarization conversion is designed based on polarized rotating materials.The optimal coding sequence is obtained by particle swarm optimization.In order to prevent the coupling between the cells,a 4×4 sub-array cell is used to construct the whole coding metasurface.The simulation results show that the RCS reduction of the metasurface in the range from 15 GHz to 35 GHz is over 10 dB,and the maximum reduction is 36 dB.Compared with the previous coding metasurface,the bandwidth has been improved.The metasurface is machined and tested in darkroom.The results are basically consistent with the simulation ones.3.In order to improve the design and analysis efficiency of coding metasurface,a method of calculating far-field characteristics based on Fast Fourier Transform(FFT)is studied.The far-field patterns corresponding to three coding sequences are calculated by this method in a few seconds,which is consistent with the results calculated by simulation software.Then a 2bit coding metamaterial is designed.The coding sequence and far-field pattern are analogized to time-domain signal and frequency-domain signal respectively.Three groups of different 2bit coding sequences are merged by using convolution theorem.The far-field characteristics of the merged new sequences are analyzed with FFT algorithm,and their far-field patterns from of different perspectives are drawn.The results show that arbitrary beam control angle can be achieved.