Showing papers in "Electromagnetics in 1984"

Transmission Line Model Analysis of Rectangular Patch Antennas

Abstract: The transmission line model has been utilized to determine the input performance of a rectangular patch antenna excited by a coaxial probe. Approximate expressions have been developed for the resonant propagation constant (or the resonant frequency), quality factor and input resistance of such antennas which clearly indicate the effects of the various antenna parameters including the feed on these quantities under dynamic conditions. The results of computations based on the expressions developed are compared with available measured values. The present work establishes the transmisson line model as a valid representation of rectangular patch antennas, and would facilitate the design of such antennas without elaborate numerical computation. The model has also been applied to determine the resonant parameters for a similar antenna tuned by passive metallic posts. Validity of an approximate expression derived for the resonant frequencies of such tunable antennas has also been established by compariso...

Wave Approach to Brightness Temperature from a Bounded Layer of Random Discrete Scatterers

Abstract: By using the fluctuation-dissipation theorem, the strong fluctuation theory is applied to the calculation of brightness temperatures from a bounded layer of random discrete scatterers with the zeroth- and first-order approximations. Various functional dependences on wavelength, polarization, observation angle, medium depth, scatterer constituents, and other physical parameters are discussed. Theoretical results are favorably matched with experimental data for passive remote sensing of snowpacks.

Radar Glint-A Survey

Abstract: Radar glint is a target scattering phenomenon that causes an error in the measured target position. Thus the miss distance of radar guided homing missiles may be adversely affected, A brief history of research and development in the interpretation, understanding and modeling of glint is presented. Emphasis is placed on understanding the phenomenology and modeling techniques. A comprehensive bibliography of papers on glint and related subjects available in open literature is included.

Amplitude Distributions for Seaset SAR and Skylab Data with Statistics for Classification and Correlation

Abstract: Seasat SAR and Skylab scenes of Nantucket Island, Mass., have been mapped to the same scale and orientation for a comparison of their reflectance distributions and for a classification of terrain. Rayleigh and Poisson statistics are employed in random Monte Carlo computer simulations to model the intensity distributions of the reflectance data. Two-dimensional autocorrelation functions for SAR and optical data and an amplitude normalized cross correlation of the multisensor scenes are computed with fast Fourier transform methods. Both positive and negative correlations are observed in the cross correlation, depending on the tilt modulation and texture of scene features. Six classes of terrain are identified using the Skylab green optical bandwidth, which is particularly sensitive to subsurface topography, and the Seasat L-band synthetic aperture radar, which is particularly Insensitive to smooth horizontal surfaces.

Estimation of Random Surface Parameters

Abstract: Based on known scattering properties of randomly rough surfaces, methods for estimating surface parameters from average power or cross-section measurements are described. These include surface height standard deviation, surface roughness spectrum, surface rms slope and surface permittivity. Examples of recovering these parameters from measurements are given. To select methods for inversion, polarization and frequency measurements are important.

Numerical Methods of Noise Reduction for Frequency Domain SEM

Abstract: This paper addresses the numerical problem of performing a pole residue expansion on noisy data. A method of combining dissimilar data sets to achieve an effective increase in signal-to-noise ratio is tested on computer generated data simulating scattering from a sphere and from a thin wire. The data is corrupted with white Gaussian noise and the algorithm is tested for signal-to-noise ratios ranging from -10 dB to 60 dB. An iterative version of the algorithm is also tested. Finally, the problem of filtering noise from relatively quiet data is discussed and a novel filtering algorithm is presented.

A Differential Geometric Approach to Electromagnetic Lens Design

Abstract: A lens design technique developed by Baum for transitioning TEM waves, ideally with no reflection or distortion, between cylindrical and conical transmission lines is investigated. This method uses a differential geometric approach combined with Maxwell's equations and the constitutive parameters i and µ in an orthogonal curvilinear coordinate system. Isotropic but inhomogeneous media are considered. It is shown that rotational coordinate systems obtained from complex analytic transformations in the plane may be utilized in the design, and that a class of solutions to the design problem exists. This class of solutions is based on a Riccati type of differential equation.

Characterization of Clutter and its Use in Maintaining CFAR Operation

Abstract: A statistical summary of the standard clutter models is given - Gamma, Inverse Gaussian, Log-normal, Weibull - and it is shown how mixtures of these distributions can be used to represent measured clutter data. This is then related to the problem of detecting a target embedded in clutter and it is shown how this type of clutter characterization can be used to construct a general CFAR detection method. This involves an adaptive adjustment of the detection threshold based on measured characteristics of the clutter. The procedure will maintain a constant false alarm rate as the received clutter data varies over different mixtures of distribution types.

Higher Order Characteristics of Waves on a Sinusoidally Modulated Reactance Surface

Abstract: The propagation of two-dimensional waves guided along a reactance surface with a weak sinusoidal modulation is investigated by a perturbation procedure up to fourth-order in the modulation index n for the wave vector of the guided wave parallel to the grating vector The periodicity of the modulation is chosen such that only one fast leaky wave exists The correction to the propagation coefficient of the guided wave due to the reactance modulation is obtained up to fourth-order in n The correctness of the leakage coefficient up to fourth-order in n is verified by developing a power conservation relation valid up to that order in n The higher-order expression for the leakage coefficient indicates the presence of saturation for large values of n This higher-order treatment has yielded the correct exponential growth of the fast leaky wave perpendicular to the reactance surface and has revealed that some of the slow Floquet modes which do not carry power away from the reactance surface also contri

Parametric Modeling of Electromagnetic Waveforms

Abstract: The problem of modeling and approximating a waveform by a linear combination of basis functions containing a variable parameter is considered. It is shown that the Kalman equation error concept of linear system identification theory can, in a modified form, be applied to a large class of modeling problems, provided the chosen basis function is a solution of a linear.functional equation in Hilbert space. This class includes rational and Tauberian modeling problems, known to be of relevance for electromagnetic transient response and wide bandwidth radar return signature identification respectively.

Transient Response of Line Source Excitation in Cylindrical Geometry

Abstract: The transient electric field due to a step excited line source, located at the axis of a dielectric cylinder buried in another dielectric medium is evaluated by the singularity expansion method, and by an approximate, explicit inversion approach. The explicit inversion approach is facilitated with a technique that preserves the principle of causality. The singularity expansion method and the explicit inversion technique complement each other as the former provides accurate results for the smoothly varying parts of the time–domain response and the latter accurately reproduces abrupt changes in the response.

Detection Probabilities for Beta-Distributed Scattering Cross Sections

Abstract: The generalized beta probability distribution provides an important description of the fluctuation of radar scattering cross sections of realistic targets. Consequently, there is a need to determine the probabilities of detection using the generalized beta distribution target model. These probabilities of detection are derived using Swerling's method Involving the use of characteristic functions. A numerical procedure IS developed for calculating arbitrarily accurate detection probabilities. This procedure involves algorithms that evaluate an inverse Laplace transform and the confluent hypergeometric function. Detection probabilities are derived for either the fast or slow fluctuation case, for any number of pulses per scan, for any false alarm probability, for any average signal-to-noise ratio, and for any choice of the two shape parameters of the generalized beta distribution.

Propagation in waveguides of arbitrary cross section with anisotropic boundary

Abstract: The problem of electromagnetic wave propagation in waveguides of arbitrary cross section and with walls characterized by an anisotropic surface admittance is formulated in terms of an integro-differential equation for the equivalent surface currents on the walls. The solution is obtained by application of the Galerkin Moments Method technique, where triangle expansion functions are employed. This model is applied to the study of transversally corrugated waveguides. A procedure to deal with step discontinuities in the boundary conditions (e.g. partially corrugated waveguides) is presented. Comparisons with results obtained with other approaches, in the case of simple geometries, are also presented.

Detection Thresholds for Clutter Backgrounds

Abstract: When a point target is embedded in a clutter background, the probability of detection depends on the detection threshold as well as the clutter statistics. Detection thresholds are calculated for gamma, inverse-Gaussian, Weibull and log-normal clutter models with known clutter statistics. These thresholds are useful for calculating an approximate probability of detection and for comparing system performance with different clutter models.

Radiation of Ion Acoustic Waves from Finite Sources

Abstract: The radiation characteristics of a longitudinal ion acoustic wave in a plasma from several different sources are experimentally measured. Results are Interpreted in terms of the solution of an appropriate scalar-Helmholtz equation or where appropriate, in terms of the germane plasma physics phenomena.

Diffraction of Linear and Nonlinear Ion-Acoustic Waves by a Rectangular Aperture

Abstract: The diffraction of planar ion–acoustic waves, both linear and nonlinear by a single rectangular aperture has been investigated. The results of an experiment conducted in a double plasma device have been compared with the predictions of an application of the fluid equations and Poisson's equation to the antenna–in–a–plasma problem, solved in closed form for the far field. It is found that linear Ion–acoustic waves are diffracted by rectangular apertures qualitatively the same as any other longitudinal or transverse wave Nonlinear waves exhibit less propensity to diffract and have a tendency to localize.

Statistics of Radar Cross Section Scintillations

Abstract: The appropriateness of the two-parameter beta distribution as a characterization of RCS scintillation for a complex target is developed and discussed with particular attention to evidence afforded by experimental data. Distributions of measured data supplied to the authors by the Boeing Corporation for the 727-100C aircraft and by Dynalectron Corporation for the RATSCAT-measured drone are compared with Rayleigh, chi-square. log-normal, and beta distributions. A theoretical statistical analysis forms the basis for the observations and conclusions from the experimental data: although the distribution of RCS of a complex target is sometimes well represented by the Rayleigh distribution, it is very often best represented by the beta distribution.

Several Problems Involving Moving Isotropic Medium

Abstract: Several electromagnetic problems involving a moving isotropic medium are formulated and solved in this work. The potential functions of a stationary charge in an unbounded moving isotropic medium and in a circular waveguide are presented first. The relation between these functions and the Lie'nard-Weichert potentials is pointed out. The dyadic Green's functions in a circular waveguide with a moving isotropic medium have been reformulated. In contrast to the earlier works by Stubenrauch and this author the causality as demanded by the Cerenkov radiation condition is applied to construct the proper solution Tor this problem.

Transmission and Reflection Properties of a Random Medium

Abstract: The parabolic approximation for wave propagation in a random medium is extended to include the backscattering. Approximate analytic solutions for the mutual coherence functions of the transmitted and reflected waves due to a plane wave normally incident to a layer of random medium are obtained. The results are expressed in the form of a scattering matrix, which may be conveniently used in multi-layer problems. Some typical numerical results illustrating the effects of layer thickness and strength of permittivity fluctuation on the mutual coherence functions are presented.

Field Distribution in the Trapped Image Guide

Abstract: An efficient analytical method for accurate calculation of the field distribution in trapped image guide is presented. Based on the Weber-Schafheitlin integral, the method yields field distributions which are compared with measured data at Ku-band.

Probabilistic Modeling of Radar Images of Agricultural Scenes

Abstract: A probabilistic model for radar images containing a variety of homogeneous fields is derived and validated. The model predicts the probability density function of radar images of agricultural scenes taken as one population. It is demonstrated that the statistical properties of SEASAT-A SAR (Synthetic Aperture Radar) images of agricultural scenes are described by this model. The model is based on three assumptions. The first assumption is that each field has the same (mean)2 /variance. For a very large class of targets and systems (e.g., transmitted wavelengths and spatial resolutions) this ratio is determined by the sensor in the degree of noncoherent averaging and is target independent and known a priori. Second, it is assumed that all of the target classes cover approximately the same area. Finally it is assumed that the true reflectivity which characterizes each individual target class is a uniformly distributed random variable. Techniques to estimate the parameters of this new probabilistic m...