Study On EM Hybrid Methods For Analyzing Antennas Around Electrically Large Target

The study of electromagnetic scattering and electromagnetic compatibility ofsome wire antennas mounted on electrically large complicated platform is a mostlychallenging research topic both in theory and engineering. The technology ofComputer Aided Design (CAD) is widely adopted in the design process of industriesproducts. The emergence of computational electromagnetics reduces a large amountof cost for the electromagnetic analysis. Today, the most popular method forgeometrical design is the Nonuniform Rational B-spline (NURBS) surface.Numerical method based on NURBS surface is an urgent and important issue. Beinglinked with research projects, the NURBS based hybrid Method of Moments andPhysical Optics (MOM-PO) and higher order MOM with Uniform geometricalTheory of Diffraction (UTD) are thoroughly studied. The author’s major work andcontributions are as follows:1. The Physics Optics (PO) method is used to calculate the disturbed patternof an antenna around the electrically large platform modeled with NURBS surface.The improved Ludwig integral method is applied to compute the integration ofinduced current over the surface. Compared with the existing NURBS-PO method,the new technique overcomes the limitation that the antenna must be located faraway from the scatterer. Since the true scattering field of antenna is replaced byspherical wave in conventional method. The present method also avoids thecomplicated theory and the possible invalidation in searching stationary points ofstationary phase method (SPM) method. Numerical results obtained from proposedapproach show great agreement with that obtained from pure Method of Moment(MOM). The feasibility of the proposed method is obviously demonstrated. And theapproach presented in this paper has the advantage over the MOM method in speedwhen they are used to solve high frequency problems.2. The MOM-PO hybrid method based on Ludwig integral and modifiedimpedance matrix is used to analyze the disturbed pattern of an antenna around theelectrically large scatterer modeled with NURBS surfaces. Compared with existingiterative MOM-PO method based on SPM, the new method avoids possible problemof iterative convergence. Ludwig integral is utilized to calculate the integral of POcurrents on Bezier surfaces. It overcomes the invalidity of SPM in the traditionalmethod. The comparison of the results obtained by this method with those publishedin literatures and from pure MOM shows the feasibility and accuracy of the present method.3. The conventional MOM-PO method based on NURBS modeling isimplemented by modifying the voltage matrix. However, the problem of iterativedivergence may occur. To solve this problem, a method of modifying the impedancematrix is presented to avoid the potential divergence. Ludwig integral combiningwith SPM is utilized to calculate the integral of PO currents on Bezier surfaces. Itovercomes the invalidity in the conventional method while keeping its efficiency.The comparison of the results obtained by this method with those published in theliterature and from pure MOM shows feasibility and accuracy of the present method.4. An efficient and stable MOM-PO hybrid method is proposed to calculatedisturbed pattern of antenna around electrically large platform modeled with NURBSsurfaces. Modified impendence matrix is used to implement the interaction of MOMregion and PO region for avoiding the convergence issue. SPM is used to calculatethe integral of induced current. Both grouping of MOM region and interpolation ofstationary points, are used to accelerate the present approach. Results obtained bythis method agree well with that from traditional Triangle MOM-PO. Comparison ofcomputing time of each method demonstrates the present method is accuracy enoughand much more efficient.5. The radiation pattern of the complex antenna around electrically largeplatform is analyzed by means of iterative vector fields and UTD method in thispaper. The complex field vectors produced by antenna without platform can beobtained by higher order MOM. The initial currents over the closed surface enclosingthe antenna are computed according to the equivalence theorem. Scattering fieldfrom platform is calculated by UTD method to iterate the currents over the enclosedsurface. After the electromagnetic currents are stable, the disturbed pattern can beobtained by UTD. Based on this method, an approximate method is also presented toimprove the efficiency. The results of the suggested methods show more accurate andhigher efficiency than that of MOM.