Research On Near-Field Electromagnetic Wave Control Techniques Based On Time Reversal

Electromagnetic wave near-field control technique has a wide range of applications in the fields of wireless power transmission,microwave non-destructive detection and microwave-assisted medical treatment.However,due to the complexity of the near-field characteristics,its regulation often faces the following problems :1.Traditional aperture synthesis techniques are usually based on classical models,such as ideal point source and ideal short dipole.These models can provide more accurate prediction and analysis results under far-field conditions,but under near-field conditions,it is difficult to accurately predict the results of classical ideal models due to the complexity of the analytical solution.2.The channel response of near-field electromagnetic wave propagation is difficult to express quantitatively.In the process of near-field electromagnetic wave modulation,it is necessary to consider not only the complex multipath environment,but also the special case of superposition of non-radiating fields in the induction and resonance regions.Therefore,it is difficult to accurately quantify the channel response of near-field electromagnetic wave propagation.Based on the theory of time reversal electromagnetics,this dissertation aims to study the integrated time reversal aperture technique for near-field beam shaping,near-field electromagnetic wave focusing and super-resolution focusing,as well as the realisation of beam-regulated metasurface based on the complexity and diversity of near-field electromagnetic wave regulation,and the controllability of near-field electromagnetic wave propagation and electric field distribution in near-field space.Through the above research,a time reversal mirror aperture to target response mapping model is established to support the design of time reversal mirrors and metasurface for different targets and missions.The main research contents of this dissertation are as follows:1.Based on the spatial retrospective characteristics of time reversal electromagnetic waves,the spatial similarity transformation,rotation transformation and superposition transformation methods of time reversal mirror aperture are proposed,which are used to regulate the depth of field,outgoing angle,number of beams and path morphology of the Bessel beam.Meanwhile,a dual-polarised artificial electromagnetic metasurface is designed and prepared to verify the effectiveness of the proposed method.2.Based on the spatial focusing characteristics of time reversal,the near-field focusing spot regulation method is improved.By combining the traditional time reversal technique with the information compensation mechanism,a method is proposed for the precise regulation of the near-field focusing region.The method combines time reversal and vector fitting algorithms,and by introducing a scale function,it incorporates multiple information losslessly into the iterative algorithm process,and effectively eliminates the cumulative error caused by the iteration.The method successfully solves the problem of focusing on large angles and complex regions,and it also has the advantages of fast response,global optimisation and low cumulative error.Finally,the effectiveness of the method is verified by constructing an artificial electromagnetic metasurface.3.Based on the adaptive spatio-temporal synchronous focusing characteristics of time reversal electromagnetic waves,a channel response reconstruction method based on time reversal is proposed to solve the problem of super-resolution wireless energy placement in complex multipath environments.The method adopts a 1-bit channel reconstruction method combined with the time reversal technique,which effectively improves the accuracy of electromagnetic energy placement for subwavelength arrays in multipath environments by reducing the correlation between receiving antennas.Finally,a metal cavity is used to construct the multipath environment,and full-wave simulation is employed to prove the effectiveness of the proposed method.4.The application method of time reversal near-field electromagnetic wave modulation technique in secure communication is explored.Based on the time reversal channel response function reconstruction technique and the near-field multipath effect,a wireless signal encryption and demodulation method is proposed.The method makes use of the principles of superposition,translation and weighted transformation of the channel to study the model of electromagnetic signal physical layer encryption as well as demodulation.Finally,the corresponding signal propagation model is constructed and the effectiveness of this encryption and demodulation method is verified using numerical methods.