Study On The Equivalence Of Electromagnetic Scattering

The study of electromagnetic scattering characteristics of targets has important theoretical significance and wide application prospects in microwave remote sensing,radar guidance and interception,target stealth and recognition,and target feature extraction.However,there are some scattering problems that been used conventional solutions to solve will encounter obstacle.For this type of problem,this thesis uses the idea of electromagnetic equivalence to achieve the transformation of the problem and then solve them.The main research content is divided into three sub-issue.In order to protect targets that are smooth,flat,and electrically big against military attack,this paper proposes a method for designing the electromagnetic convergence of the target and the environment,by changing the scattering characteristics of the target to achieve a similarity to the surrounding environment,thereby reducing electromagnetic recognition for protecting targets against military attack.Because the electromagnetic scattering characteristics for smooth,flat,and electrically big targets are mainly related to their reflection coefficients,the electromagnetic convergence design of the target and the surrounding environment(smooth,flat,and electrically big)can be converted to a design with a uniform reflection coefficient.Taking aircraft carrier decks and roads as examples,the change of reflectance can be achieved by coating dielectric materials on their surfaces.Genetic algorithms are used to search for the optimal coating material by using the reflection coefficient as an objective function of optimization.Under different angles,polarizations,and bandwidths,the convergence effect of the reflection coefficient and the surrounding environment reflection coefficient after being coated on the target was verified,and better results were obtained.Solving the scattering of a target in a half space using a half-space Green's function involving the calculation of Sommerfeld integral,which is computationally complex,tedious,and time consuming.In order to avoid the calculation of the Sommerfeld integral,this paper transforms the half space problem into the free space problem by the equivalent modeling.First,it is proved theoretically that the coated Pec plate with the same reflection coefficient as half space is equivalent in the upper half space.Using this conclusion,a new equivalent model is constructed by applying coated Pec plate instead of medium under half space.Then,through the reasonable and effective truncation of the interface,the infinity problem of the half space can be transformed into the calculation of the limited target in free space.Thus,the free space Green's function can be used to establish the integral equation to solve the problem and avoid the calculation of the Sommerfeld integral.Finally,numerical examples are used to verify the correctness of the model mentioned in this paper and to solve the half-space problem more efficiently than the general use of the half-space Green's function.Due to the lossy electromagnetic system will encounter the electromagnetic similar second incompatibility(because of the lossy medium dispersion),the theory of scale model in the lossy system can not be applied in engineering.In this thesis,aiming at the coated PEC target satisfying impedance boundary conditions,we put forward the scheme of using the existing coating material to construct equivalent model that the same surface impedance as the scale model.At the same time,It is Proved that the two targets with the same surface impedance and geometric model are the same as in the the scattering characteristics.In this way,an alternative model of the required scale model is constructed.The consistency of the alternative model with the surface impedance of the prototype can be achieved by using the existing material.In this way,an alternative model is constructed that can be realized in engineering,and there is scale relationship between the alternative model and prototype.In other words,we use the consistency of surface impedance equivalent to overcome the second electromagnetic incompatibility in the case of PEC target coating,so that the scale relations can be used in this case.For the case of lossy medium in the lower half space,in this thesis,a coated PEC plate is used to replace the lower half of the space and its equivalent impedance consistent with the lower half space.This not only solves the problem of the second incompatibility,but also greatly simplifies the difficulty of simulating the external environment in the laboratory.And the scattering problem of target is studied effectively and conveniently by using the general scaling relation in lossy half space.