Research On The Eletromangnetic Scattering And Radiation Characteristics Of Arbitrary Wire Structure

The research of electromagnetic scattering from arbitrary wire has attracts much attentions for long time for strong engineering application requirement. The moment method has been used to solve the electromagnetic scattering from an arbitrary thin-wire. This thesis presents a comprehensive implementation of a moment method that is useful for the computation of electromagnetic scattering. By using the presented method, the electromagnetic scattering from an arbitrary conducting wire can be analyzed based on the combined field integral equations.The differential equation methods, integral equation method used for analysis of scattering are reviewed in this thesis. On solving this integral equation, the moment of method is used. The method of moments is an integral equation based numerical technique is to reduce the formal problem to a set of linear equations that can be solved by matrix inversion. It can be used for any kind of electromagnetic problems and guarantee convergence for a sufficiently dense discretization. It also fits for implementation on parallel computers since the elements of the matrix can be computed independently. The solution of the current on straight wire using the method of moment is extensively studied. Using this method, the integral equations are numerically solved. The first step in developing the solution for the currents is to driven the appropriate electric field integral equation for general structures and the basics of the method of moments, which were then applied to a straight wire antenna. Then method of moment will then be used to convert the integral equation into a system of linear equations which can be solved by various techniques of linear algebra.The electric field integral equation for straight wires was extended for use in arbitrary wire antenna structures. The helix geometry was presented as the primary example of this thesis. The general electric field integral equation presented can be applied to any wire antenna by defining the appropriate unit vector, ?l , and position vector, rr . Triangular basis functions and pulse weighting functions are used in the analysis of the helix antenna.In the investigation, first, an efficient analysis of a helix antenna is carried out by the moment method, and the electric-field integral equation is combined with the moment method to obtain the currents in the antenna, with the triangle basis and pulse testing functions exactly following the contour of the helix antenna. During the analyses, the thin-wire approximation is adopted and the helix is assumed to be oriented along z axis. The thin-wire approximation and the delta gap source model are also described. Numerical results demonstrate the validity and accuracy of the proposed method.Next, the sheath helix model and Huygens principle are developed for investigating the scattering of plane wave by conducting finite helix. Applying the solution for scattering by the conducting infinite helix, the scattering coefficients for the conducting finite helix are obtained. The analytical expresses of scattering field are obtained based on the boundary condition of a sheath helix model. The effects of the pitch angle and the electrical size of the helix on the resonant behavior are also investigated. It is shown that the resonant behavior of the forward/back scattering far field only depends on the pitch electrical size and the radial electrical size, and that the turn of the helix has no effect on the resonant behavior. We focus on the resonant behavior of the helix because near the resonance frequency, the effective Chirality parameter is strongly depended on the wave performance.Then, by combining the electric-field integral equation with the presented moment method, an efficient numerical analysis procedure is developed to solve the problems of wire antennas with arbitrary shapes, whose exact kernel representations of Green function are given in place of the thin-wire approximation kernel for the moment-method solution to arbitrary wire antennas and scatterers with moderately thick radius. As the kernel expressions are exactly expanded on the basis of series summations, the elements of the impedance matrix are accurately and fast evaluated. These exact expressions, on the basis of series summations, can efficient remove the singularity of integrands of Green functions and accelerate the computation of impedance matrices. In this thesis the exact kernel for the arbitrary wire structure is based on series expansion. But in evaluating the self term of impedance elements, the separation of singular exact kernel is acceptable due to slowly convergence of series representations. These exact expressions are independent of usual restrictions such as wire radius and field point distance. The numerical results demonstrate the efficiency and generality of the exact expressions when applied to the mixed potential integral equations for arbitrary wire structures.Finally, the time-domain integral equation is analyzed in detail and is deduced. The calculation of the current induced on the wire structures is analyzed and presented. The issues such as the geometry of the wire structure, the calculation of vector and scalar potential, and the solutions to the integral equation are discussed. The analysis based on the time-domain integral equation, in which a thin wire approximation is used. The time-domain electric-field integral equation is adopted with the proposed moment method to develop a numerical procedure for solving the problem of scattering from antennas with arbitrary shapes. Thus, an efficient numerical method for the calculation of the electromagnetic scattering from perfectly conducting structures with arbitrary shapes in time domain is proposed, with a comprehensive treatment of a single, straight thin wire. In the time domain electric field integral equation, a Gaussian impulse plane wave is used as the excitation source. An ideal excitation source would have an impulse function with frequency spectrum extends from zero to infinity and a constant amplitude. The method uses pulse expansion for the spatial variation and a linear interpolation in time. A time domain electric field integral equation is formulated for the problem of a thin wire of arbitrary shape. We used the pulse basis function to solve the electric field integral equation and compute the RCS. We also developed numerical procedures for calculating the transient electromagnetic scattering from thin-wire structures by solving the electric field time domain integral equation. For solving scattering from an arbitrary oriented thin-wire excited by an incident field, the application of the method of moment solution procedure to the thin wire integral equation is obtained by enforcing the boundary condition on the electric field integral equation. The numerical results are performed for validation of the efficiency of and accuracy of the proposed method.All the numerical results in this thesis agree well with the results in the literature. The proposed moment method is of high efficiency and accuracy, and is proved useful for the accurate modeling and efficient analysis of scattering from conducting wire structures the arbitrary shapes.