Research On Algorithms For Scattering Analysis Of Objects Within A Half Space And Applications

Electromagnetic scattering problem of objects in complex environments(such as ground/air and sea/air half-space environments)has great scientific and research significance in both civil and military fields.Traditional methods for half-space scattering problem are mostly based on half-space Green's function,which have the advantages of simple and easy to mesh the model,but there are also problems such as time-consuming calculation of half-space Green's function,lack of efficient fast algorithm for adjacent interface or straddling interface targets.Therefore,based on the physical background of the scattering problem of targets within a half-space environment,this dissertation proposes and deeply studies the half-space target scattering model based on the free space Green's function.Moreover,based on the existing work and the method proposed in this dissertation,we make a specific research and analysis on the typical problems existing in the practical engineering.In the beginning,the traditional method based on half-space Green's function has been reviewed in the dissertation.The problems in solving the half-space Green's function are introduced,and the characteristics of Sommerfeld integral are summarized and analyzed,the expression of half-space Green's function is derived;then based on the equivalent principle,the derivation process of the surface integral equation is given,and solution process of the moment method is introduced from the aspects of basis function selection,matrix element calculation and so on;next,from the perspective of reducing computational memory and time,two kinds of fast methods are introduced.One is the half-space multilayer fast multiple algorithm(MLFMA)which relies on the integral kernel(half-space Green's function),it uses the weighted real image method to process the reflection term and it's only applicable to the cases where the target is located on one side of the interface and away from the interface;the other is the algebraic method that does not depend on the integral kernel,Adaptive Cross Approximation(ACA).ACA approximates the impedance matrix by the characteristics of low-rank compression,when there are many unknowns,the calculation efficiency of ACA is much less than that of MLFMA.The research and discussion of these methods build a good foundation for the follow-up work.For the complex time-consuming calculation of half-space Green's function,the half-space surface integral equation lacks efficient and fast algorithm,in this dissertation,the physical model of the half-space problem is simplified,and the infinite large half-space interface is truncated at a finite distance,the lower region is equivalent to a finite large closed dielectric object,therefore,the original half-space target scattering problem is simplified to the scattering problem of the composite dielectric object in free space.The proposed method avoids the calculation problem of half-space Green's function,and can effectively solve the acceleration problem by using the traditional fast algorithm MLFMA.Taking the dielectric target as an example,the surface integral equation based on the free space Green's function model is deduced in detail,and the effectiveness of the proposed method is verified by numerical examples.Next,the dissertation analyzes and discusses the main problems and solutions existing in the model based on the free space Green's function.Aiming at the truncation problem of the half-space interface,the main factors affecting the selection of the cut-off radius and the influence of different cut-off radius on the calculation accuracy and efficiency are discussed.At the same time,for the approximate models commonly used in practical engineering problems,this dissertation makes a systematic discussion and verification,and gives the applicable scenarios of different models,which provides a certain reference for theoretical research and engineering applications.Then,the scattering problem of high contrast dielectric object is studied.In practical engineering applications,the seawater or soil in the lower half space tends to have higher relative dielectric parameters,the scattering problem of high contrast dielectric object is another difficulty for the model based on free space Green's function.For the problem of convergence and accuracy of the dielectric equation,this dissertation firstly analyzes the influence of identity operator on the convergence and accuracy from the view of characteristics of operator,and proposes an improved surface integral equation.By adjusting the combination coefficient of the internal and external equations to cancel the identity operator,the calculation accuracy is improved,and the magnetic current normalization method is used to improve the matrix impedance characteristics and accelerate the convergence speed.Moreover,based on the traditional PMCHWT equation,a method based on quasi-Helmholtz decomposition is proposed.By constructing a projection operator,the vector potential and scalar potential in the L operator are separated,and the normalization factor is used to balance the influence of dielectric parameters on different components of the L operator,improving the impedance matrix characteristics and iterative efficiency.Finally,the directional enhancement of all-dielectric optical nano-antenna is also investigated by using the proposed method.Finally,based on the scattering calculation problem and related research methods of half-space objects,this dissertation analyzes the related problems in practical engineering applications:(1)analyzing and verifying the Brewster's effect of object scattering in a half-space environment,which provides a new idea for the detection and tracking of stealth targets in complex environments;(2)since the characteristic mode of an object is related to the geometric structure and material properties and independent of the external excitation,a half-space object wide-angle calculation based on the characteristic mode theory is proposed;(3)by the full-wave numerical simulation,the influence of the half-space environment on the radiation performance of the antenna from the perspective of the dielectric parameters in the lower region and the location height of the antenna,and for the ship platform,the radiation performance of the antenna at different positions of the ship is discussed.