机载天线辐射方向图的MM—UTD混合法分析

采用矩量法（MM）结合二致性几何绕射理论（UTD）的混合方法计算了机载单极天线的辐射方向图。该混合方法建立的机载天线辐射模型更符合实际，并且它同时具有MM的精确性和UTD的高效率，克服了单一方法的不足。计算结果对于预测天线间的电磁兼容性和优化天线布局具有一定的指导意义。     

Current-based hybrid analysis for surface-wave effects on largescatterers

Current-based hybrid analyses combine the method of moments (MM) with Ansatz currents derived from high-frequency methods such as physical optics, physical theory of diffraction (PTD), geometric theory of diffraction (GTD), and the Fock theory. The author introduces an analysis that incorporates a surface-wave basis set into the hybrid formulation. This approach substantially improves the modeling of nonspecular effects caused by surface waves. The discussion emphasizes the accurate representation of surface wave currents and the reduction of computational efforts in comparison with the conventional MM techniques. Scattering calculations for bodies of revolution (BORs) and two dimensional surfaces demonstrate the effectiveness of the analysis presented for large scatterers     

Hybrid UTD-MM analysis of the scattering by a perfectly conducting semicircular cylinder

A hybrid UTD-MM technique which combines the uniform geometrical theory of diffraction (UTD) and the method of moments (MM) is employed to analyze efficiently the problem of electromagnetic diffraction of transverse electric (TE) and transverse magnetic (TM) waves by a perfectly conducting semicircular cylinder. An analysis of this problem is useful for understanding the coupling between the mechanisms of edge and convex surface diffraction. The accuracy of the numerical results for the far-zone fields based on this solution is established by comparison with an independent formally exact MM solution.     

A hybrid technique for analyzing wire antennas in the presence of a plane interface

A hybrid technique which combines the method of moments (MM) with ray methods is employed to analyze the radiation of wires in the presence of a plane interface. In this technique, which is an extension of that proposed for combining the MM with the uniform geometrical theory of diffraction (GTD), a piecewise sinusoid (PWS) Galerkin method formulation is used. In this paper the basic assumption is made that a PWS dipole can be replaced by three sources of transverse, spherical waves, so that their fields can be treated separately by standard ray methods. Via this procedure the MM matrix can be easily augmented to account for the wire-interface interactions. Calculations of the field both radiated in the antenna half-space and transmitted through a plane interface are found in very good agreement with those performed by using the rigorous Sommerfeld integral representation. These results show that this technique provides an accuracy which is widely acceptable in most engineering applications, even when the wire is placed very close to the interface. This approach, which employs ray methods to calculate reflected and transmitted field contributions, appears promising to treat the case of curved interfaces.     

A diffraction coefficient for a cylindrically truncated planar surface

A numerically derived solution of the diffraction coefficient for a source mounted on a perfectly conducting planar surface which is smoothly terminated by a circular cylinder is obtained using the hybrid approach which combines the moment method (MM) with the geometrical theory of diffraction (GTD). This solution is valid not only in the region away from, but also in the region near, the shadow boundary. The accuracy and usefulness of this solution is demonstrated as various structures are treated using it.     

A hybrid technique for combining the moment method treatment of wire antennas with the GTD for curved surfaces

This hybrid technique is a method for solving electromagnetic problems in which an antenna is located near a conducting body. The technique accomplishes this by casting the antenna structure in a moment method (MM) format, then modifying that format to account for the effects of the conducting body via the geometrical theory of diffraction (GTD). The technique extends the moment method to handle problems that cannot be solved by GTD or the moment method alone. Wire antennas are analyzed to find their input impedance when they are located near perfectly conducting circular cylinders, although the methods used are not restricted to circular cylinders. Three orthogonal orientations are identified, and antennas to match them are analyzed. For each case, the hybrid solution is checked with one of three independent solutions: an MM-eigenfunction solution, image theory, or experimental measurement. In almost all cases, excellent agreement is obtained due in large part to the fact that the moment method near fields are, for the first time, cast into a ray optical form.     

A study of a monopole arbitrarily located on a disk using hybrid MM/GTD techniques

The input impedance of a monopole located off-axis on a disk and oriented in an arbitrary direction is investigated using hybrid method of moments/geometrical theory of diffraction (MM/GTD) techniques. The equivalent currents method (ECM) and the uniform geometrical theory of diffraction (UTD) are used to ensure a proper treatment of each situation. A criterion for switching from UTD to ECM in the vicinity of the axial caustic is discussed. Measurements of impedance have been made in order to check the numerical results and are presented here, showing good agreement with theory.     

A hybrid MM-GTD technique for the optimization of the power gain of arrays of wire antennas near an elliptic cylinder

The power gain of arrays of wire antennas near an elliptic cylinder is maximized. The hybrid MM-GTD technique is used where the method of moments (MM) and the geometrical theory of diffraction (GTD) are formally combined. Results are presented for some special cases.     

Improved PO-MM hybrid formulation for scattering fromthree-dimensional perfectly conducting bodies of arbitrary shape

The method of moments (MM) represents a suitable procedure for dealing with electromagnetic scattering problems of arbitrary geometrical shape in the lower frequency range. However, with increasing frequency both computation time and memory requirement often exceed available computer capacities. Therefore a current based hybrid method combining the MM with the physical optics (PO) approximation suitable for three-dimensional perfectly conducting bodies is proposed in this paper. The hybrid formulation allows a substantial reduction of computation time and memory requirement, while the results are in reasonable agreement with those based on an application of the MM alone. Further improvement can be achieved for flat polygonal parts of the scattering body by a heuristic modification of the PO current density taking into account the effects of edges. As opposed to the physical theory of diffraction (PTD), no additional electric and magnetic line currents along the edges are necessary     

Hybrid methods for analysis of complex scatterers

Depending on the angle of illumination, electrically large scatterers can support a variety of electromagnetic (EM) phenomena, such as traveling waves, creeping waves, and edge/surface diffraction effects. The electrical size of a body limits the tractability of numerical methods such as the method of moments (MM), and the geometric complexity of an object circumscribes the applicability of optics-derived methods. Hybrid methods incorporating both numerical and high-frequency asymptotic techniques have the potential to substantially enlarge the class of EM scattering problems that can be treated. In this discussion, the current-based hybrid formulation is summarized for classes of two- and three-dimensional scatterers. The use of Ansatz solutions derived from physical optics, the physical theory of diffraction, and the Fock theory is illustrated for perfectly conducting, partially penetrable, and totally coated bodies. For the latter, a generalization rooted in the impedance boundary (Leontovich) condition is used. Complementing these Ansatz solutions, the Galerkin representation is used for regions where the foregoing are computationally or physically intractable. The above cases are illustrated by representative solutions explicating the approach     

A hybrid method for the solution of scattering from inhomogeneousdielectric cylinders of arbitrary shape

A hybrid moment method/edge-element method (MM/EEM) is presented. The formulation is quite general; however, the method is applied to two-dimensional scattering problems. Such a hybrid formulation unites the advantages of finite and integral-equation methods and is able to handle unbounded problems in which complex inhomogeneities are present. The edge-element method is easily coupled to the moment method, and it doesn't introduce spurious modes. The equivalence principle is used to divide the original problem into two separate problems: an unbounded homogeneous one in which the moment method is used and a bounded inhomogeneous one in which the edge-element method is used. Several examples involving two-dimensional scattering with TE and TM plane wave excitation are presented. The RCS is computed and compared to results obtained by other numerical techniques     

单翼圆柱电磁散射的混合解

本文利用混合法分析和计算了单翼理想导电圆柱远场散射。这种混合法应用了物理光学近似（ＰＯ）、ＦＯＣＫ理论、物理绕射理论（ＰＴＤ）、几何绕射理论（ＧＴＤ）和矩量法。计算结果与矩量法和Ｍｉｃｈｅａｌｉ的计算和测量结果一致。与Ｍｉｃｈｅａｌｉ的方法相比，混合法对于任意尺寸的圆柱半径与翼宽和任意的入射角都适用。     

Low-grazing scattering from breaking water waves using an impedanceboundary MM/GTD approach

The radar backscattering from water waves of various degrees of breaking Is numerically examined. A hybrid moment method geometrical theory of diffraction (MM/GTD) technique previously used for small-grazing scattering from perfectly conducting surfaces is reformulated using impedance boundary conditions, allowing the treatment of large (but finite) conductivity scattering media such as sea water. This hybrid MM/GTD approach avoids the artificial edge effects that limit the standard moment method when applied to rough surfaces, allowing the calculation of the scattering at arbitrarily small grazing angles. Sample surfaces are obtained through the edge-detection of video stills of breaking waves generated in a wave tank. The numerical calculations show that the strength of the backscatter is closely associated with the size of the plume on the breaking wave. Strong interference appears in the both horizontal (HH) and vertical (VV) backscatter when the surfaces are treated as perfectly conducting. The VV interference is dramatically reduced when a sea water surface is used, but the HH interference is unaffected. The interference leads to HH/VV ratios of up to 10 dB. The behavior of the scattering is consistent with the multipath theory of sea-spike scattering     

Overview of selected hybrid methods in radiating system analysis

In many situations it is not sufficient to know the performance of an antenna in isolation, but instead it is necessary to know the performance characteristics of the antenna on, or in proximity to, another body or platform. Hybrid techniques which use the method of moments (MM) to treat part of the problem and some other method to account for the (usually larger) reminder are examined. In the MM/Green's function techniques, to the usual MM matrix is added another matrix, derived typically by some other method, that accounts for the normally larger part of the problem without increasing the order of the MM matrix. These techniques utilize electric fields in their formulation and are referred to as field-based hybrid techniques. The last two hybrid techniques considered in this review paper are current-based techniques. Both use the MM for a part of the platform and use approximate current forms for the remainder of the platform. All these methods are reviewed, and their utility is illustrated by examples or by comparison to other methods     

Numerical computation of diffraction coefficients

Direct numerical methods are explored for the computation of diffraction coefficients for a wide class of canonical structures. Although these canonical structures are infinite or semi-infinite, the initial computations are performed on finite bodies. Extraction of the desired coefficient therefore necessitates the elimination of radiation from extraneous diffraction centers. This is accomplished by a variety of techniques including current windowing, tapered resistivity, and screening by conducting shields. Singularities at shadow boundaries are derived via physical optics (PO). The procedure is elucidated through a number of examples for which analytic results are available for comparison. These include the perfectly conducting half-plane, wedge, rounded wedge, and grooves in a ground plane. Computed diffraction coefficients for lossy dielectric wedges are applied within the context of geometrical theory of diffraction (GTD) to the computation of diffraction from a triangular prism and the results are in satisfactory agreement with those of the method of moments (MM).     

The mutual coupling and diffraction effects on the performance of aCMA adaptive array

The performance of the constant modulus algorithm (CMA) with the steepest descent method used in an adaptive array of monopole antennas mounted on a rectangular conducting plate was investigated. The mutual coupling (MC) effect among the array elements and the diffraction effect caused by the conducting plate were taken into account in the calculation by a hybrid method of moment method (MM) and geometrical theory of diffraction (GTD). Simulations showed that the CMA adaptive array performs differently when the MC and the diffraction effects are taken into account. In some cases, the speed of convergence is slower with MC, and in other cases it is faster. Also, in multipath scenarios the array sometimes converges on a weaker delayed ray rather than the direct ray when MC is included. The capture property is explained by the fact that the CMA algorithm is sensitive to initial conditions and the initial array pattern is directional due to MC-not omnidirectional as in the ideal case. The performance of the array on a finite ground plane is different from that on an infinite ground plane due to diffraction effects     

Analysis of a monopole mounted near an edge or a vertex

The problem of a monopole mounted near the edge of a wedge or a vertex is considered. Three types of solutions-surface patch modeling, moment method/geometrical theory of diffraction (MM/GTD), and MM/eigenfunction-are presented, discussed, and compared with measurements. Results are in the form of input impedance and radiation patterns.     

Hybrid solutions for large-impedance coated bodies of revolution

Electromagnetic scattering solutions are developed for coated perfectly conducting bodies of revolution (BOR) that satisfy the impedance boundary condition. The integral equation arising from the impedance (Leontovich) boundary condition is solved by use of the method of moments (MM) technique along with an Ansatz for the surface currents that is derived from physical optics (PO) and the Fock theory that is modified for imperfectly conducting surfaces. The MM solution is expressed in terms of two integral (Galerkin) operators. The form of the Galerkin expansion used results in a symmetric MM system matrix. The hybrid solution is specialized for BOR's although the approach is applicable to a broader class of scatterers as well. The results are compared with the Mie solution for penetrable spherical scatterers, which satisfy the impedance boundary condition, and with recently published MM solutions for nonspherical scatterers.     

High-frequency backscatter from a finned cylinder at grazing incidence--Comparison of two theoretical methods

A closed-form, high-frequency approximation is obtained for the diffracted-reflected-diffracted field contribution to the backscattered field resulting from the grazing illumination of a perfectly conducting, infinitely long, finned cylinder by a normally incident cylindrical transverse magnetic (TM) wave. The result, which is valid for any relative magnitudes of the cylinder radius and the fin width provided both of them are electrically large, is derived by two different methods. One of them is the radiation integral method related to the physical theory of diffraction. The other is the complex ray tracing method related to the spectral theory of diffraction (STD). Relative merits and disadvantages of the two methods are pointed out.     

Analysis of time domain scattering by a flat plate

A numerical modeling technique is developed to calculate time-domain responses for a range which radiates short pulse signals. The theoretical time domain response for the case of bistatic scattering by a finite flat plate is calculated by combining frequency data from uniform geometrical theory of diffraction (UTD) and moment method (MM) solutions. The theoretical responses are compared to measured time waveforms showing good agreement.