Research On Basic Theory And Key Technologies Of Shaping And Manipulating The Spatial Electromagnetic Field

Spatial electromagnetic field is the steady state or quasi-steady state electromagnetic field distributed in space with ignoring the time factor.In general,the methods for manipulating the distribution of spatial electromagnetic field focus on the beam pattern shaping in the far field zone.However,with the development of electromagnetic theory and technologies,application scenarios represented by local microwave hyperthermia,near-field imaging,near-field wireless power transfer,etc.,no longer focus on the beam control in the far field angular domain.Instead,it is hoped to precisely manipulate the spatial distribution of electromagnetic field in the given target area,so as to achieve more accurate energy transfer or information acquisition.Although there are some special types of electromagnetic field shaping technologies such as near-field focusing and plane wave synthesis,there are few researches on general methods for shaping and manipulating spatial electromagnetic field.Especially,there are some shortcomings in general methods of analytical shaping field,shaping in complex environment and shaping vector electromagnetic field.In the face of the above problems of existing research,the basic theory and key technologies of shaping and manipulating spatial electromagnetic field are studied in this dissertation.The main work and contributions are summarized as follows.1.The basic theory of shaping and manipulating spatial electromagnetic field is deduced and summarized.Based on the basic electromagnetic principle,the target field information model and vector radiation source information model are established respectively.On this basis,a general solution model of electromagnetic field shaping problem is established,which provides theoretical support for field to source solution.At the same time,quantitative evaluation indexes are established for different types of target fields to facilitate the comparison and analysis between different shaping methods.2.A general analytical method of shaping electromagnetic field based on angular spectrum projection is proposed.The basic principle of angular spectrum projection is explained from two aspects of spatial field transformation and sampling strategy,and an array design and excitation calculation method based on the far field angular spectrum projection of elements are proposed.The semicircle array and umbrella-shaped array are designed respectively,and a variety of different types of shaped fields are successfully generated in one-dimensional and two-dimensional target area.Aiming at the problems of projection error and large array aperture in the element far field method,an electromagnetic field shaping method based on periodic array near-field angular spectrum reconstruction and projection is further proposed.This method overcomes the shortcomings of the element-based far field projection method to a certain extent,and has better shaping accuracy and more general applicability.The design criteria of periodic array in this method are analyzed in detail,and the analytical conditions of key parameters such as array element position,array element spacing,array element number,etc.are given to guide the construction of shaping-array.Based on this method,a variety of onedimensional and two-dimensional shaped fields,including the target fields with complex patterns,are successfully generated.3.The electromagnetic field shaping method based on intermediate objective optimization and the shaping method based on dimension reduction optimization idea are proposed to solve the efficient electromagnetic field shaping problem under complex environment conditions.First of all,the basic idea of the intermediate objective optimization method is to construct a group of standard orthogonal intermediate fields in the target field function space as the intermediate optimization objective,and take the optimized reconstruction field as the basic field,so that it can realize the generation of target shaped field through weighted superposition.Different target fields are shaped in free space,inhomogeneous medium and complex scattering environment respectively.On this basis,the idea of intermediate objective is further extended to the optimization variable domain of specific target,such as low sidelobe flat top target field,uniform plane wave field,etc.By constructing an intermediate function with a small number of parameters,the reduction of the dimension of optimized independent variable search space is realized.Then,taking flat-top uniform field and far field flat-top beam as examples,a smooth reference template with low sidelobe level and ripple is constructed as the intermediate objective function,and the electromagnetic field shaping with low sidelobe level and low ripple coefficient is realized under the conditions of uniform array and non-uniform array respectively.At the same time,this method is compared with the convex optimization method.The results show that the proposed dimension reduction optimization algorithm has higher computational efficiency and good computational time stability under different number of elements.In general,the two optimization methods proposed are essentially to decouple the process with large amount of computation from the iterative process by modifying the target domain,and then reduce the computational burden in the iterative optimization process through pre-optimization and pre-calculation.4.A method for shaping vector electromagnetic field based on the optimization of surface current distribution is proposed,which realizes the independent shaping of multivector electromagnetic field.The method directly solves the optimal surface current distribution of the radiation source,and achieve electromagnetic vector control at any position in space based on the field integral equation of the equivalent current.The current basis function is used to discretize the surface current distribution of any radiator,and a direct current optimization method based on convex optimization is derived.On this basis,a mode current optimization method based on the generalized characteristic modes theory is further proposed.This method can transform the original direct optimization problem into the optimization problem of the characteristic mode current coefficient,thus avoiding large-scale optimization calculation.Through the generalized characteristic modes optimization of the surface current of continuous structures such as two-dimensional perfect electric conductor(PEC)plate and three-dimensional spherical PEC surface,the optimal shaping of different vector target fields is achieved.In order to further reduce the number of mode currents,a surface current optimization method based on discrete structure is proposed,and a complex vector target electromagnetic field is shaped based on a periodic PEC plate array.