The problem of the diffraction of an arbitrary ray optical electromagnetic field by a smooth perfectly conducting convex surface is investigated. A pure ray optical solution to this problem has been developed by Keller within the framework of his geometrical theory of diffraction (GTD). However, the original GTD solution fails in the transition region adjacent to the shadow boundary where the diffracted field plays a significant role. A uniform GTD solution is developed which remains valid within the shadow boundary transition region, and which reduces to the GTD solution outside this transition region where the latter solution is valid. The construction of this uniform solution is based on an asymptotic solution obtained previously for a simpler canonical problem. The present uniform GTD solution can be conveniently and efficiently applied to many practical problems. Numerical results based on this uniform GTD solution are shown to agree very well with experiments.

A uniform GTD analysis of the diffraction of electromagnetic waves by a smooth convex surface

Asymptotic and hybrid methods are widely used to compute the Radar Cross Section (RCS) of objects that are large compared to the wavelength of the incident wave, and the objective of this paper is to present an overview of a number of these methods. The cornerstone of the asymptotic methods is the Geometrical Theory of Diffraction (GTD), which was originally introduced by J. B. Keller, and which represents a generalization of the classical Geometrical Optics (GO) by virtue of the inclusion of diffraction phenomena. After a presentation of the physical principles of GTD, we provide a description of its mathematical foundations. In the process of doing this we point out that GTD gives inaccurate results at caustics and light-shadow boundaries, and subsequently present a number of alternate approaches to dealing with these problems, viz., Uniform theories; Methods for caustics curves; Physical Theory of Diffraction; and Spectral Theory of Diffraction. The effect of coating perfectly conducting bodies with dielectric materials is discussed and hybrid methods, that combine the Method of Moments (MoM) with asymptotic techniques, are briefly reviewed. Finally, the application of GTD and related techniques is illustrated by considering some representative radar targets of practical interest. >

Asymptotic and hybrid techniques for electromagnetic scattering

Numerical calculations have been carried out and the beam squint for circular polarized excitation has been measured using a 100 m telescope. The telescope was operated in the Gregorian mode, where the equivalent focal length equals 387.5 m. The feed horn was laterally displaced by 1.364 m from the optical axis at the system focus. Good agreement was obtained between the numerical calculations and the experimental results. The authors found a shift of the two radiation patterns of approximately=2 arcsec. The orientation of the beam squint in the configuration with a laterally displaced feed is different from the orientation in offset reflector antennas. >

Beam squint in axially symmetric reflector antennas with laterally displaced feeds

5 List of abbreviations 7 List of major symbols 8 1. Introduetion 11 1.1. Background 11 1.2. Terminology 14 1.3. Organizations and responsibilities 15 1.4. Scope of the thesis and survey of the contents 19 1.5. References 21 2. Radio-inteiference problems and mutual coordination 23 2.1. Introduetion 23 2.2. Review of practical interference problems 23 2.2.1. General remarks 23 2.2.2. Interference between terrestrial stations 24 2.2.3. lnterference between satellite-earth links 26 2.2.4. Interference between terrestrial stations and earth stations 28 2.2.5. System studies 30 2.3. Interference from digital radio-relay stations into satellite TV receivers 32 2.4. Coordination areas 34 2.5. Relerences 40 3. Transhorizon propagation mechanisms -a review 43 3.1. General introduetion 43 3.2. Introduetion to transhorizon-propagation research 43 3.2.1. Time variability 43 3.2.2. General concepts and terminology 44 3.2.3. Presentation of results 50 3.3. Review of transhorizon-propagation mechanisms 50 3.3.1. General remarks 50 3.3.2. Permanent clear-air mechanisms 52 3.3.3. Occasional clear-air mechanisms 55 3.3.4. Permanent scatter mechanisms 58 3.3.5. Occasional scatter mechanisms 58 3.4. Relative importance of the mechanisms 61 3.5. References 61 4. Clear-air transhorizon propagation 65 4.1. Introduetion 65 4.2. Review of clear-air interference-predietion models 65 4.2.1. General remarks 65 4.2.2. CCIR interference-prediction model 66 4.2.3. Other clear-air interference--prt:>diction models 68 4.3. Transhorizon-propagation experiments 70 4.3.1. Measurement programme and equipment 70 4.3.2. Experimental results obtained at EUT 72 4.3.3. Discussion of the experimental results 73

Reduction of transhorizon radio interference in satellite earth stations

A novel technique for the analysis of the principal plane near-field patterns of conical and corrugated conical horns excited in the dominant mode based on the uniform geometrical theory of diffraction (UGTD) [1], [2] is presented. Numerical computations of the near-field amplitude patterns of a few typical conical and corrugated conical horns based on the GTD technique detailed here correlate well with the measured patterns, justifying the validity of the analysis presented.

GTD analysis of the near-field patterns of conical and corrugated conical horns

The edge diffraction coefficients of Kouyoumjian and Pathak, when applied to edges in curved screens produce discontinuities in the diffracted fields which do not coincide with those of the specular field on the concave side of the screen. In regions where coincidence occurs the discontinuities add, rather than cancel. The field calculated from the diffraction coefficient violates the principle of reciprocity.

P. Pathak

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