Walter D. Burnside

High frequency scattering by a thin lossless dielectric slab

A high frequency solution for scattering from a thin dielectric slab is developed, based on a modification of the uniform geometrical theory of diffraction solution for a haft-plane, with the intention of developing a model for a windshield of a small private aircraft. Results of the theory are compared with experimental measurements and moment method calculations showing good agreement. Application of the solution is also addressed.

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

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 technique to combine the geometrical theory of diffraction and the moment method

For many years, The Geometrical Theory of Difraction (GTD) has been applied to antenna and scattering problems for which the structure is large in terms of wavelength. GTD solutions have mainly been concerned with pattern computations with little information concerning antenna impedance and/or aperture distributions. This is especially true for wire-type antennas. On the other hand, The Method of Moment (MM) has been applied to such problems for which the structure is small in terms of wavelength. This paper presents a technique which can be used to combine these two solutions such that a whole host of new problems can be handled.

A uniform GTD solution for the radiation from sources on a convex surface

A compact approximate asymptotic solution is developed for the field radiated by an antenna on a perfectly conducting smooth convex surface. This high-frequency solution employs the ray coordinates of the geometrical theory of diffraction (GTD). In the shadow region the field radiated by the source propagates along Kellers surface diffracted ray path, whereas in the lit region the incident field propagates along the geometrical optics ray path directly from the source to the field point. These ray fields are expressed in terms of Fock functions which reduce to the geometrical optics field in the deep lit region and remain uniformly valid across the shadow boundary transition region into the deep shadow region. Surface ray torsion, which affects the radiated field in both the shadow and transition regions, appears explicitly in the solution as a torsion factor. The radiation patterns of slots and monopoles on cylinders, cones, and spheroids calculated from this solution agree very well with measured patterns and with patterns calculated from exact solutions.

A doubly periodic moment method solution for the analysis and design of an absorber covered wall

A periodic moment-method solution for scattering from a doubly periodic array of lossy dielectric bodies is developed. The purpose is to design electromagnetic wedge and pyramidal absorbers for low reflectivity so that one can improve the performance of anechoic chamber measurements. The spectral-domain formulation and the moment-method volume polarization current approach are used to obtain the expressions for determining the scattering from a doubly periodic array of lossy dielectric bodies. Some wedge and pyramidal absorber configurations that have been designed, fabricated, and tested in the OSU/ESL compact range measurement facility are presented. By taking into account the complexity of real-world material structures, good agreement between calculations and measurements has been obtained. >

A novel antenna for ultra-wide-band applications

An ultra-wideband antenna based on a slotline feed structure, a bowtie horn, and a rolled edge termination was developed, analyzed, and measured. Empirical data showed that its beamwidths and bandwidth are dependent on its physical dimensions which are easily controllable by an antenna designer. Measured patterns of models with various radiation properties are shown to substantiate these design rules. A flat plateau-like main beam, low voltage standing-wave ratio (VSWR), the ability to produce both wide (60 degrees ) and narrow (30 degrees ) half-power beamwidths, low sidelobes and backlobe (40-50 dB down), low cross-polarized levels (20-25 dB down), and independent control of E- and H-plane beamwidths over an ultra-wide bandwidth, say 2-18 GHz, are some of the strong points of this antenna type. >

Electromagnetic scattering by pyramidal and wedge absorber

Electromagnetic scattering from pyramidal and wedge absorbers used to line the walls of modern anechoic chambers is measured and compared with theoretically predicted values. The theoretical performance for various angles of incidence is studied. It is shown that a pyramidal absorber scatters electromagnetic energy more as a random rough surface does. The apparent reflection coefficient from an absorber wall illuminated by a plane wave can be much less than the normal absorber specifications quoted by the manufacturer. For angles near grazing incidence, pyramidal absorbers give a large backscattered field from the pyramid side-faces or edges. The wedge absorber was found to give small backscattered fields for nuclear-grazing incidence. Based on this study, some new guidelines for the design of anechoic chambers are advocated because the specular scattering models used at present do not appear valid for pyramids that are large compared to the wavelength. >

A periodic moment method solution for TM scattering from lossy dielectric bodies with application to wedge absorber

The periodic moment method (PMM) solution for the scattering from two-dimensional lossy dielectric bodies is developed. The purpose is to design a microwave wedge absorber for low reflectivity so that one can improve the performance of anechoic chamber measurements. With PMM, the reflection and transmission coefficients of periodically distributed bodies illuminated by a plane wave have been accurately calculated using a Cray Y-MP supercomputer. Through these studies, some wedge absorber configurations have been designed, fabricated, and then tested in the OSU/ESL compact range measurement facility. Two 8-in commercial wedges, a curved wedge, and a four-layer wedge, were studied. In all cases, good agreement between calculations and measurements was obtained. >

An aperture-matched horn design

A novel horn design is presented which provides significantly better performance in terms of the pattern, impedance, and frequency characteristics than normally obtainable. The modification necessary to accomplish these improvements is achieved with a moderate increase in the size and weight; yet, the overall cost of design and construction need not significantly increase. The basic concept utilizes an ordinary horn except that curved surface sections are attached to the outside of the aperture edges. Although all of the original studies associated with this new antenna were made using elliptic cylinder sections, they can be arbitrary smooth convex shapes which are attached to the horn such that the junction forms a smooth surface to the touch.

Performance trade-off between serrated edge and blended rolled edge compact range reflectors

The performance trade-off between serrated edge and blended rolled edge compact range reflectors is investigated. The edge diffracted fields which contribute to stray signals in the quiet zone have to be minimized. Consequently, an iterative approach has been developed to design serrated edges such that the edge diffracted rays from the serrations can be kept as far away as possible from the desired quiet zone area. Performance of various designs based on the number and size of serrations have been studied using a physical theory of diffraction solution. In addition, an optimized blended rolled edge design with the same size as the serrated edge reflector has also been evaluated for comparison. Finally, numerical results are presented in this paper to address the performance trade-off issue among the various designs. It is clearly shown from this study that the blended rolled edge reflector performs significantly better, which should offset the increased cost needed to construct a rolled edge reflector versus a serrated one.