Near-field High-frequency Asymptotic Method

With the increase of working frequency of modern communication electronic devices,the requirement of analysing near-field EM characteristics of target with super large size become urgent.Comparing with traditional full wave methods,the high frequency asymptotic method saves the storage and possesses a higher efficiency.High frequency asymptotic method therefore become one of the most important tools in analysing EM characteristics of targets with large electric size.Near-field high frequency asymptotic method has been investigated in this thesis and the author's major work and contribution are as follows:· A novel line-integral representation of the physical-optics radiation integral from a perfectly conducting surface illuminated by a finite number of electric/magnetic Hertzian dipoles is presented.This novel rep-resentation guarantees the integrand free from singularities along the computational path.As such,it can be easily integrated for arbitrary positions of the source and observation points.The time reduction per-formance would be improved by this novel formula.The line-integral representation offers an alternative method to rapidly solve the scattering problem as it is usually more efficient than producing the same result in conventional surface-integral.· Contour-integral representations have been presented to evaluate the physical-optics backscattered electric and magnetic fields from a perfectly conducting object illuminated by a dipole source.The advantage of the proposed representations is that the integrands in the contour integrals along the rim of the scatterer are free from singularities for all the source/observer positions.In addition,the proposed algorithm is easily extensible to analyse the scattering from complex objects.Such a liberty is achieved by deriving the novel representations using vector-algebra theorems in global coordinates.· The field scattered by a scattering target can be described in terms of a double integral.Such integral on a triangular patch,with linear amplitude and quadratic phase variation approximation,can be exactly expressed in the format of uniform geometrical theory of diffraction which is convenient for numerical computations.This method can be applied to near-field scattering calculation scenario of arbitrary complex targets.The algorithm has some excellences like robustness and high efficiency.· A time domain line integral representation of the physical-optics radiation integral is developed for the scattering from a perfectly conducting surface illuminated by an electric Hertzian dipole.The proposed line integral representation,which is valid and singularity-free for all near-field observers,is derived di-rectly in the TD domain.This TD representation can be further expressed in terms of geometric-optics and boundary-wave components of the scattering mechanisms.Using the proposed line integral representation,the computational complexity is reduced by an order of the scatterer's linear electric size,compared with the straightforward numerical quadrature of the PO surface integral.· An efficient SBR method is developed to analyse the scattering responses from large perfectly conducting objects.By extending frequency/time domain line integral representation to the degenerate case of plane wave excitation,high computational efficiency can be achieved in SBR method under plane wave illumi-nation.In addition,through vector point/dipole source piecewise approximation of antenna pattern,line integral representation can be used in SBR method under arbitrary antenna illumination.The proposed S-BR method in conjunction with line integral technique is more efficient in near-field calculation of complex targets than conventional SBR method combining surface integral technique.· Numerical implementation of the method of truncated wedge equivalent edge current(TW-EEC)is pre-sented and be extended to the calculation of near-field diffraction.This method compensates the error in near-field calculation of only considering the physical optics contribution.The contributions of the TW-EEC component and the physical optics component have exactly the same line integral form,so they can be combined to generate a complete near-field line integral computing framework.· A new automatic target recognition system based on bistation radar echo signal is proposed using deep learning method.This method can avoid the complex process of matched filtering in SAR images gen-eration.The corresponding training and testing database is obtained by means of our proposed near-field line-integral method.The method of deep learning is adopted to avoid the complicated preposition and feature extraction processes of the echo signal which transform the raw input into a representation.Thus the recognition process can be greatly simplified.