Ronald J. Marhefka

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.

Ohmic loss in frequency-selective surfaces

The present study was undertaken in order to quantify absorption effects due to ohmic loss in frequency-selective surfaces (FSS) at infrared frequencies. The structures considered in this work act as electromagnetic filters, and as such, are of interest for use as thermophotovoltaic spectral control devices. For this application, absorption is of primary concern since it leads to reduced filter efficiency. This work focuses on the behavior of single-layer, free-standing FSS arrays comprised of circular apertures (holes) and circular loop apertures (rings). Numerical calculations of the transmission, reflection, and absorption characteristics of various arrays were carried out for wavelengths between 1 and 15 μm using a commercial finite-element software package. Absorption effects were included using measured optical properties as input parameters to a surface impedance boundary condition. Analytical techniques were then employed to determine the absorption behavior in the static limit. An interesting res...

Antennas on complex platforms

The modeling of antennas on complex platforms is reviewed. The objective is to analyze the modification of the basic antenna performance due to its environment. This includes pattern distortion, antenna to antenna spacial coupling, and radiation hazard predictions. High frequency methods are used to develop computer codes to accomplish these tasks. Several types of generally available codes are discussed. >

High-Frequency EM Characterization of Through-Wall Building Imaging

A high-frequency asymptotic technique based on the Uniform Geometric Theory of Diffraction (UTD) is employed for building interior imaging. The analysis is implemented using a ray-tracing technique to account for multiple scattering interactions in a building, along with a set of heuristic diffraction coefficients for dielectric wedges and corners. Imaging of the synthetic aperture radar data is carried out by the conventional fast Fourier transform method to transform to the downrange domain, and combined with a coherent near-zone imaging function for cross-range. Comparisons with experimental data for a scaled-down building model are given to demonstrate the suitability and efficacy of our analysis for through-wall building imaging. The UTD ray mechanisms account for the dominant scattering features observed in the image.

Double diffraction at a coplanar skewed edge configuration

The problem of double edge diffraction in the near-field region of a coplanar skewed edge geometry, illuminated by a plane wave, is studied asymptotically via an extended spectral theory of diffraction approach. The resulting uniform dyadic double edge diffraction coefficient is expressed in terms of a universal integral (the generalized Fresnel integral) and remains valid when any one of the edges is within the transition region of a singly diffracted wave, while it asymptotically reduces to the ordinary geometrical theory of diffraction double diffraction coefficient elsewhere. Comparisons with method of moments computations for radiation patterns of sources in the vicinity of flat plate structures demonstrate the validity of the asymptotic approximation.

Antennas on Aircraft, Ships, or Any Large, Complex Environment

The overall capability of an electromagnetic radiating system is dependent on its ability to operate effectively in a complex environment, in that its pattern performance can be adversely limited by pattern distortion effects, such as blockage and structural scattering. In many cases these detrimental effects can be minimized by judiciously locating the antennas. This task is complicated by the large number of systems that are competing for prime locations on, for example, a modern military ship. Without an efficient means to position such systems one normally attempts to use locations similar to previous designs, which may be inexpensive but are certainly not optimum. As a result there is a great need for electromagnetic tools that can efficiently evaluate the pattern performance of radiating systems in their proposed environment.

Scattering from surface waves on finite FSS

This paper discusses the presence of surface waves on periodic structures in general and on frequency selective surfaces (FSS) in particular. While certain types of surface waves can exist on infinite as well as finite periodic structures, this investigation centers around the types that can exist only on finite FSS. Radiation caused by these surface waves may lead to a significant scattering increase in the backscatter as well as the bistatic directions. Thus, this paper is of both theoretical and practical interest.

A uniform ray approximation of the scattering by polyhedral structures including higher order terms

A uniform ray representation of the far field scattered by flat plate structures is investigated by postulating an approximation of the surface current on each face of the object, which is subsequently integrated either in closed form or asymptotically in terms of the well-tabulated edge transition function. Specifically, the current on each plate is approximated, in addition to the usual physical optics (PO) component, by a primary nonuniform current, obtained from the canonical solution to the wedge problem and truncated at edges of the plate, as well as a secondary nonuniform current induced by doubly diffracted fields and expressed in terms of an equivalent edge source. The superimposed effect of the rays resulting from the primary and secondary nonuniform current integration improves the agreement of the calculated pattern as compared with method of moments computations. >

Propagation modeling of indoor wireless communications at 60 GHz

In this paper, based on a 3D ray-tracing analysis, statistical parameters (i.e. mean excess delay and rms delay spread) for site-specific indoor environments were extracted. The fading statistics of these indoor environments were shown to obey a Weibull distribution. As is well-known, fading statistics are necessary for predicting channel capacity limit and this were presented in the conference

A UTD based asymptotic solution for the surface magnetic field on a source excited circular cylinder with an impedance boundary condition

An asymptotic solution based on the uniform geometrical theory of diffraction (UTD) is proposed for the canonical problem of surface field excitation on a circular cylinder with an impedance boundary condition (IBC). The radius of the cylinder and the length of the geodesic path between source and field points, both of which are located on the surface of the cylinder, are assumed to be large compared to a wavelength. Unlike the UTD based solution pertaining to a perfect electrically conducting (PEC) circular cylinder, some higher order terms and derivatives of Fock type integrals are found to be significantly important and included in the proposed solution. The solution is of practical interest in the prediction of electromagnetic compatibility (EMC) and electromagnetic interference (EMI) between conformal slot antennas on a PEC cylindrical structure with a thin material coating on which boundary conditions can be approximated by an IBC. The cylindrical structure could locally model a portion of the fuselage of an aircraft or a spacecraft, or a missile. Validity and accuracy of the numerical results obtained by this solution are demonstrated in comparison with those of an exact eigenfunction solution.