UTD analysis of a shaped subreflector in a dual offset-reflectorantenna system

The geometrical theory of diffraction (GTD) is known as an efficient high-frequency method for the analysis of electrically large objects such as a reflector antenna. However it is difficult to obtain geometrical parameters in order to apply GTD to an arbitrary shaped reflector, especially a subreflector. The geometrical parameters of an arbitrary shaped subreflector for the uniform theory of diffraction (UTD) analysis are derived based on differential geometry. The radiation patterns of various subreflector types, including hyperboloidal and a shaped subreflector, are evaluated by UTD. The computed result for the hyperboloidal reflector agrees well with that obtained by uniform asymptotic theory (UAT)     

A uniform geometrical theory of diffraction for an imperfectly conducting half-plane

Diffraction tensors are presented in the context of the uniform geometrical theory of diffraction (UTD) for the high frequency scattering by an impedance half-plane at normal and oblique (skew) incidence. These are based on the exact Wiener-Hopf solution and were derived according to the UTD ansatz. In addition, unlike previous uniform diffraction coefficients, the ones given here reduce to the known UTD diffraction coefficients for the perfectly conducting case. The coefficients are explicit and therefore appropriate for practical applications. Several scattering patterns are also presented and compared to a previous heuristic solution.     

Radiation pattern of an offset hyperbolic reflector

The radiation pattern of a focus-fed offset hyperbolic reflector is determined by using the uniform geometrical theory of diffraction (UTD) and the uniform asymptotic theory of diffraction (UAT). The patterns predicted by these two theories are observed to differ considerably around incident and reflection boundaries. The effects of the slope diffraction as well as those of additional terms in the improved UTD solution are ignored     

Ray-optical prediction of radio-wave propagation characteristics intunnel environments. 1. Theory

A tunnel is modeled as congregates of walls, with the wall being approximated by a uniform impedance surface. The aim is to get a solution for a canonical problem of a wedge with uniform impedance surface. The diffraction by a right-angle wedge with different impedance boundary conditions at its two surfaces is first considered. A functional transformation is used to simplify the boundary conditions. The eigenfunction solutions for the transformed functions are replaced by integral representations, which are then evaluated asymptotically by the modified Pauli-Clemmow method of steepest descent. The asymptotic solution is interpreted ray optically to obtain the diffraction coefficient for the uniform geometrical theory of diffraction (UTD). The obtained diffraction coefficients are related directly to the Keller diffraction coefficients in the uniform version. The total field is continuous across the shadow of the geometrical optics fields     

理想导电凸曲面上振子电磁辐射UTD解的并矢格林函数方法

P.H.Pathak,Wang Nan等人在研究典型问题几何绕射理论之后,于1981年发表了任意导电凸曲面振子天线高频电磁辐射一致性几何绕射理论近似解。本文应用并矢格林函数方法,通过典型曲面高频电磁辐射一致性近似解的研究和推广,导出了理想导电凸曲面上电、磁振子电磁辐射场在高频近似下一致性几何绕射理论近似解。与P.H.Pathak,Wang Nan等人的结果相比,主项并矢转移函数除个别系数外完全相同,高阶并矢转移函数在几何光学区略有差异。     

A UGO/EUTD solution for the scattering and diffraction from cubicpolynomial strips

A combined uniform geometrical optics (UGO) and extended uniform geometrical theory of diffraction (EUTD) solution is developed for scattering and diffraction by perfectly conducting cubic polynomial strips. The new solution overcomes the difficulties of the classic GO/UTD solution near caustics and composite shadow boundaries. The approach for constructing the UGO/EUTD solution is based on a spatial domain physical optics (PO) radiation integral representation for the scattered field, which is then reduced using a uniform asymptotic procedure. New uniform reflection, zero-curvature diffraction, and edge diffraction coefficients are derived and involve the ordinary and incomplete Airy integrals as canonical functions. The UGO/EUTD solution is very efficient and provides useful physical insight into the various scattering and diffraction processes. It is also universal in nature and can be used to effectively describe the scattered fields from flat, strictly concave or convex, and concave-convex boundaries containing edges. Its accuracy is confirmed via comparison with some reference moment method (MM) results     

A uniform GTD formulation for the diffraction by a wedge with impedance faces

A uniform high-frequency solution is presented for the diffraction by a wedge with impedance faces illuminated by a plane wave perpendicularly incident on its edge. Arbitrary uniform isotropic impedance boundary conditions may be imposed on the faces of the wedge, and both the transverse electric (TE) and transverse magnetic (TM) cases are considered. This solution is formulated in terms of a diffraction coefficient which has the same structure as that of the uniform geometrical theory of diffraction (UTD) for a perfectly conducting wedge. Its extension to the present case is achieved by introducing suitable multiplying factors, which have been derived from an asymptotic evaluation of the exact solution given by Maliuzhinets. When the field point is located on the surface near the edge, a more accurate asymptotic evaluation is employed to obtain a high-frequency expression for the diffracted field, which is suitable for several specific applications. The formulation described in this paper may provide a useful, rigorous basis to search for a more numerically efficient but yet accurate approximation.     

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 UTD type analysis of the plane wave scattering by a fullyilluminated perfectly conducting cone

A uniform high-frequency asymptotic solution, based on the physical optics (PO) approximation, is obtained in the format of the uniform geometrical theory of diffraction (UTD) to describe the fields diffracted by the tip of a semi-infinite, perfectly conducting cone when it is fully illuminated by an electromagnetic plane wave. The solution is expressed in terms of an integral, over finite limits which can be integrated numerically without difficulty. The results computed from the uniform asymptotic PO solution compare well with previously published results given for narrow-angle semi-infinite cones. In addition, they compare well with measurement and with an independent moment method (MM) solution for the scattering by a finite flat-backed cone in which several higher order wave interactions are found to be significant; one such interaction is between the tip and the base of the cone. Expressions are provided which are useful for calculating this tip-base interaction and confirm its relative importance. These expressions also provide tip diffraction effects which are important within the forward paraxial zone for the radiation by antennas on cones     

MONOSTATIC RADAR CROSS SECTION FOR REFLECTOR ANTENNAS

The calculation formulas of monostatic radar cross-section (RCS) of arbitrary re-flectors with arbitrarily polarized plane-wave incidence are derived, where the spicular field isobtained by geometrical optics (GO) and the edge-diffracted field is calculated by the method ofequivalent currents (MEC). Some typical calculated results are given by means of RCS spatialgraphs. For both horizontal and vertical polarizations, the theoretical results obtained in thispaper agree very well with the experimental results as well as the results from uniform theory ofdiffraction.     

High-frequency scattering from the edges of impedance discontinuities on a flat plane

A high-frequency solution is presented for the scattering of a plane wave at the edges of surface impedance discontinuities on a fiat ground plane. Arbitrary uniform isotropic boundary conditions and a direction of incidence perpendicular to the edges of the discontinuities are considered for both the transverse electric (TE) and transverse magnetic (TM) cases. An asymptotic approximation of the exact solution given by Maliuzhinets and a spectral extension of the geometrical theory of diffraction (GTD) are used. Uniform expressions for the scattered field received at a point on the surface are given, including surface wave contributions. Numerical results are shown and in some examples they are compared with those obtained from a moment method (MM) solution.     

反射器天线的单站雷达截面积

本文用几何光学法计算反射器天线的镜面场,用等效电磁流法(根据物理绕射论与与电流线积分公式导出)计算边缘的绕射场,得到了任意旋转反射器天线在任意极化平面波入射下的单站雷达截面积(RCS)的计算公式,并给出了一些典型的数值计算结果及相应的立体RCS图。在水平和垂直极化入射下,本文理论值与已有的实验结果以及与一致性绕射理论的结果吻合较好。     

Time domain version of the uniform GTD

The uniform geometrical theory of diffraction (UTD) solutions can be inversely transformed analytically to obtain a time-domain version of the UTD. The time-domain solutions are valid in the early time period where an observation time t is close to the time after the arrival of the first diffracted wavefront. Comparisons with GTD (geometrical theory of diffraction) and also with available rigorous results (J.B. Keller and A. Blank, 1951) reveal that the UTD solutions are accurate for substantial early time periods while the GTD (Keller and Blank) results are valid for very early time periods     

Calculation of the high-frequency diffraction by two nearby edges illuminated at grazing incidence

The uniform geometrical theory of diffraction (UTD) together with a generalized spectral extension arc applied to calculate the high-frequency scattering by two nearby edges illuminated at grazing incidence. Several examples arc considered which involve the diffraction by a pair of parallel edges where one edge is illuminated by the shadow boundary field of the other. Expressions for the diffracted field have been obtained for plane, cylindrical, and spherical wave illumination with either the electric or magnetic field perpendicular to the edges. Extensive numerical results are given for a pair of staggered parallel half-planes, a thick screen, and a rectangular cylinder. Comparisons with the results calculated by other techniques are also presented to demonstrate the accuracy of the method.     

Pulsed field diffraction by a perfectly conducting wedge: aspectral theory of transients analysis

The canonical problem of pulsed field diffraction by a perfectly conducting wedge is analyzed via the spectral theory of transients (STT). In this approach the field is expressed directly in the time domain as a spectral integral of pulsed plane waves. Closed-form expressions are obtained by analytic evaluation of this integral, thereby explaining explicitly in the time domain how spectral contributions add up to construct the field. For impulsive excitation the final results are identical with those obtained previously via time-harmonic spectral integral techniques. Via the STT, the authors also derive new solutions for a finite (i.e., nonimpulsive) incident pulse. Approximate uniform diffraction functions are derived to explain the field structure near the wavefront and in various transition zones. They are the time-domain counterparts of the diffraction coefficients of the geometrical theory of diffraction (GTD) and the uniform theory of diffraction (UTD). An important feature of the STT technique is that it can-be extended to solve the problem of wedge diffraction of pulsed beam fields (i.e., space-time wavepackets)     

An implementation of the UTD on facetized CAD platform models

The Aircraft inter-Antenna Propagation with Graphics (AAPG) 2000 computer code relies on realistic computer-aided design (CAD) platform geometrical modeling to support uniform geometrical theory of diffraction (UTD) predictions of antenna-to-antenna coupling for aircraft-mounted antennas. The code employs novel ray-tracing techniques that permit the computation of UTD propagation paths over facetized objects of general shape. Moreover, geometrical data, required by UTD formulations for wedge and smooth-surface diffraction and reflection, are computed entirely from path and facetized-object geometries. Since the entire object representation in AAPG 2000 is in terms of facets, division of path-object interactions into wedge and smooth-surface cases must be performed heuristically     

An aperture-matched compact range feed horn design

The moment method and the uniform geometrical theory of diffraction are used to obtain two separate solutions for theE-plane far field pattern of an aperture-matched horn antenna. This particular horn antenna consists of a standard pyramidal horn with the following modifications: a rolled edge section attached to the aperture edges and a curved throat section. The resulting geometry provides significantly better performance in terms of the pattern, impedance, and frequency characteristics than normally obtainable. The moment method is used to calculate theE-plane pattern and voltage standing-wave ratio (VSWR) of the antenna. However, at higher frequencies, the moment method requires large amounts of computation time. On the other hand, the uniform geometrical theory of diffraction provides a quick and efficient high frequency solution for theE-plane field pattern. In fact, the uniform geometrical theory of diffraction may be used to initially design the antenna; then the moment method may be applied to "fine tune" it. In both methods, a two-dimensionalE-plane model of the antenna is used, but these two-dimensional solutions yield excellent agreement with measured data of the actual three-dimensional antenna. This procedure has been successfully applied to design a compact range feed horn.     

A hybrid asymptotic solution for the scattering by a pair ofparallel perfectly conducting wedges

The overlapping transition regions of the double diffraction by a pair of parallel wedge edges are considered for the hybrid case where the gap between the edges is small compared to the distances from the source and the observation point (plane-wave-far-field limit) and the scatterer as a whole is large (or infinite). A closed-form asymptotic solution for the scattered field continuous at all angles of incidence and scattering is constructed for this case. The peculiar feature of this solution is a hybrid representation of the field singly diffracted by the first wedge: a part of it is described by a nonuniform, geometrical theory of diffraction (GTD) expression, while the other part is described in terms of the uniform theory of diffraction (UTD). The rest of the diffracted ray fields are described by nonuniform expressions, with singularities mutually canceling on summation. This solution is applied to the scattering by a perfectly conducting rectangular cylinder with appropriate geometrical parameters, and agreement with moment method calculation is demonstrated     

Simple high frequency solution for interior fields of open-ended parallel plate waveguide

The problem of electromagnetic (EM) plane wave scattering by an open-ended, perfectly-conducting, semi-infinite two-dimensional (2-D) parallel plate waveguide with a thin uniform layer of lossy material on its inner walls is analysed using a high frequency method. The fields coupled into the waveguide from the exterior are found via the uniform geometrical theory of diffraction (UTD) ray method.<>     

On the GTD scattering by a polygonal cylinder

Using Keller's geometrical theory of diffraction (GTD) the field diffracted by a wedge is infinite at the shadow and reflection boundaries. In general, uniform diffraction coefficients must be used to provide continuous fields at these boundaries. In this communication it is shown that by properly adding the singular contributions from a pair of adjacent edges, Keller's diffraction coefficients yield a continuous far-zone field at the reflection boundaries of a polygonal cylinder illuminated by a plane wave. Furthermore the procedure is justified by noting that the uniform diffraction coefficients reduce to the Keller diffraction coefficients for this case.