Showing papers in "IEEE Transactions on Antennas and Propagation in 1980"
TL;DR: In this paper, a simple and efficient numerical method is developed for treating electromagnetic problems of scattering and radiation from surfaces, where special consideration is given to the treatment of edges so that rather arbitrary geometrical configurations may be handled.
Abstract: Simple and efficient numerical methods are developed for treating electromagnetic problems of scattering and radiation from surfaces. Special consideration is given to the treatment of edges so that rather arbitrary geometrical configurations may be handled. For the conducting body problems considered, an electric field integral formulation is used, and the method of moments is applied using pulse expansions to represent both the current and the charge. It is demonstrated that proper placement of the current and charge subdomains relative to edges not only is important in treating edges but also yields a convenient numerical procedure. A simple testing scheme is used which is almost as efficient as point-matching. Numerical results indicate that the approach is free of anomalies in the behavior of current near edges and of other previously observed numerical instabilities. Problems considered include conducting strips (both TM and TE), a bent rectangular plate, and both material and conducting bodies of revolution.
533 citations
TL;DR: In this article, a uniform geometrical theory of diffraction (GTD) solution for the diffraction of an arbitrary ray optical electromagnetic field by a smooth perfectly conducting convex surface is proposed.
Abstract: 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.
270 citations
TL;DR: In this article, the mean backscattered return power for a short-pulse radar and near-normal-incidence scattering from a rough ocean surface is given by the convolution of several terms.
Abstract: Under assumptions common in radar altimetry, the mean backscattered return power for a short-pulse radar and near-normal-incidence scattering from a rough ocean surface is given by the convolution of several terms. For a nearly Gaussian transmitted pulse shape scattered from a nearly Gaussian distributed sea surface, a small-argument series expansion of one of the terms within the convolution leads to a several-term power series expansion for the mean return waveform. Specific expressions are given for the first four terms. These results, which require much less computer time than would the otherwise necessary numerical convolution, are useful for data analysis from current or past radar altimeters and for design studies of future systems. Several representative results are presented for an idealized SEASAT radar altimeter.
183 citations
TL;DR: In this article, the second derivatives of the free space scalar Green's function are treated as generalized functions in the manner described by Gel'fand and Shilov, and a new formula is derived that regularizes a divergent convolution integral involving the second derivative of g.
Abstract: The free-space scalar Green's function g has an R^{-1} singularity, where R is the distance between the source and observation points. The second derivatives of g have R^{-3} singularities, which are not generally integrable over a volume. The derivatives of g are treated as generalized functions in the manner described by Gel'fand and Shilov, and a new formula is derived that regularizes a divergent convolution integral involving the second derivatives of g . When the formula is used in the dyadic Green's function formulation for calculating the E field, all previous results are recovered as special cases. Furthermore, it is demonstrated that the formula is particularly suitable for the numerical evaluation of the field at a source point, because it allows the exclusion of an arbitrary finite region around the singular point from the integration volume. This feature is not shared by any of the previous results on the dyadic Green's function.
120 citations
TL;DR: The impulse response, frequency response, and response to extended random fields are formally derived, and the problem is addressed of estimating SAR impulse response from measures of spectra Corresponding to random distributed scenes.
Abstract: The general problem is considered of a partially coherent synthetic aperture radar (SAR) imaging a scene that is subjected to random fade. The impulse response, frequency response, and response to extended random fields are formally derived, and the problem is addressed of estimating SAR impulse response from measures of spectra Corresponding to random distributed scenes. Explicit results are obtained using Gaussian functions to describe the SAR, its coherence, and the scene coherence.
113 citations
TL;DR: In this paper, the measured voltage standing wave ratio (VSWR), gain, pattern, and axial ratio characteristics of a uniformly wound helical antenna are presented, and empirical expressions for the antenna peak gain and bandwidth as a function of the helix parameters are derived.
Abstract: The measured voltage standing wave ratio (VSWR), gain, pattern, and axial ratio characteristics of a uniformly wound helical antenna are presented. Experimental parametric studies were made of 1) fixed length helices with variable diameter and pitch angle (8.6 to 10 turns), and 2) variable length helices with constant diameter and pitch angle (5 to 35 turns). Parametric curves are presented to show the gain and half-power beamwidth (HPBW) of a constant pitch helix as a function of axial length L/\lambda and circumference C/\lambda . Empirical expressions are derived for the antenna peak gain and bandwidth as a function of the helix parameters. In addition, the antenna gain-beamwidth products are examined.
106 citations
TL;DR: In this paper, the authors derived the radiation integral for a doubly curved offset reflector antenna illuminated by an arbitrary source using the Jacobi-Bessel series to evaluate the Fourier transforms.
Abstract: The physical optics approximation is employed to derive the radiation integral for a doubly curved offset reflector antenna illuminated by an arbitrary source. A novel procedure is presented for expressing the radiation integral in terms of a summation of Fourier transforms of an "effective" aperture distribution which includes the effect of the curvature of the surface. The Jacobi-Bessel series is used to evaluate the Fourier transforms. The vector nature of the far-field pattern is studied by evaluating its three Cartesian components in a unified fashion. The rapid numerical evaluations of the expressions obtained are demonstrated via extensive test cases. In particular, the scattering characteristics of symmetric and offset parabolic, spherical, and shaped reflectors are studied in detail, and comparisons are made with other available data.
104 citations
TL;DR: In this paper, a simple equivalent circuit for the narrow slot was developed, and the effect of loading the slot with a lossy dielectric was also considered. But it is not shown that the slot becomes resonant for certain screen thicknesses, and exceptionally large transmission of energy may occur.
Abstract: The general formulas for electromagnetic transmission through an infinitely long slot in a perfectly conducting screen of finite thickness are specialized to the case of a narrow slot. The slot may be filled with a homogeneous isotropic material. A simple equivalent circuit for the narrow slot is developed. It is found that for certain screen thicknesses the slot becomes resonant, and exceptionally large transmission of energy may occur. In fact, as the slot width approaches zero, the transmission width at resonance becomes 1/\pi wavelengths regardless of the actual slot width. The effect of loading the slot with a lossy dielectric is also considered. Computations are given to illustrate the validity of the equivalent circuit model and the accuracy of transmission characteristics computed from it.
103 citations
TL;DR: In this article, a set of radiometer measurements were recorded during a 1000-h day and night monitoring of irrigated fields from fully saturated to completely dry, recorded at 2.8 cm and 21.4 cm wavelengths for a range of incident angles from nadir to 50 degrees.
Abstract: A unique set of radiometer measurements is presented, recorded during a 1000-h day and night monitoring of irrigated fields from fully saturated to completely dry. Radiometer measurements were recorded at 2.8-cm ( X -band) and 21.4-cm ( L -band) wavelengths for a range of incident angles from nadir to 50\deg . Soil moisture and soil temperature profile measurements were recorded to a depth of 15 cm. The test site was located in east-central Texas on a clay soil (Miller clay). Three bare-surface plots were used, each having a distinctly different surface roughness. Vegetated plots were also measured. The data quantify the sensitivity of microwave radiometer measurements to soil moisture variations, the effect of surface roughness on the measurement, and the influence of surface vegetation.
96 citations
TL;DR: In this article, the general problems of TE and TM-excited slots in screens and conducting strips whose widths are narrow relative to the wavelength are investigated, and solutions for both polarizations as products of series of Chebyshev polynomials and appropriate singular functions exhibiting the known edge condition are presented.
Abstract: The general problems of TE- and TM-excited slots in screens and conducting strips whose widths are narrow relative to the wavelength are investigated. The narrow width approximation is imposed upon the slotted-screen and strip equations from which result approximate equations that are solved exactly. For general excitation, solutions are presented for both polarizations as products of series of Chebyshev polynomials and appropriate singular functions exhibiting the known edge condition. Special attention is afforded the problem of the slot/strip excited by illumination which can be represented by two terms of a Taylor series due to the practical importance of this situation.
95 citations
TL;DR: The experimental data indicate that a limited number of ray-theoretical wave components play an important role in propagation in urban mobile radio environments.
Abstract: An analysis is made of the physical structure of multipath propagation in an urban area. The incident angle, path length, and field strength are measured for each principal component of the received multipath signal. The results of the analysis reveal that the observed principal waves compare fairly well with those propagating via geometrical optics. The experimental data indicate that a limited number of ray-theoretical wave components play an important role in propagation in urban mobile radio environments.
TL;DR: In this article, a uniform quasi-stationary approximation to the exact solution for both periodic and aperiodic motion is presented, which has an error on the order of Ω(n) independent of the space variable, where n is the maximum acceleration of the boundary.
Abstract: The scattered field is studied that results when a plane wave is normally incident on a perfectly conducting flat plate in motion. The exact solution is analyzed for both periodic and aperiodic motion. The quasi-stationary approximation is compared with the exact solution, and the error is found to be on the order of \beta = \upsilon_{M}c^{-1} where \upsilon_{M} is the maximum speed of the moving boundary and c is the speed of light. This error estimate includes a factor which increases as the distance from the plate increases. A uniform quasi-stationary approximation is developed which has an error on the order of \beta independent of the space variable. By taking into account the Doppler shift, it is possible to construct a uniform approximation to the exact solution on the order of a_{M}c^{-2} where a_{M} is the maximum acceleration of the boundary.
TL;DR: In this paper, the mean, standard deviation, and confidence limits for quotients of random-variable sums are obtained for the extraction of sea-state parameters from high-frequency (HF) radar Doppler spectra.
Abstract: The mean, standard deviation, and confidence limits are obtained for quotients of random-variable sums. Such quotients are formed in the extraction of sea-state parameters from high-frequency (HF) radar Doppler spectra. An equivalent chi-squared process is shown to represent a spectral integration having a varying mean based on central-limit theorem arguments. The error of estimating the centroid of a spectral peak is derived and shown to depend only on the total length of the time series, not on how it is segmented into coherent spectra and incoherent averages. Finally, error in estimation of waveheight using a power-law relationship is analyzed and is seen to depend on how one does the averaging. When individual spectra contain varying path losses and/or system gains, averages of quotients for each spectrum remove such unknown factors; in this case spectra with greater frequency resolution lead to lower waveheight estimation errors.
TL;DR: In this article, an electrically short dipole with a nonlinear dipole load is analyzed theoretically using both analytical and numerical techniques, and the analytical solution is given in terms of the Anger function of imaginary order and imaginary argument.
Abstract: An electrically short dipole with a nonlinear dipole load is analyzed theoretically using both analytical and numerical techniques. The analytical solution is given in terms of the Anger function of imaginary order and imaginary argument and is derived from the nonlinear differential equation for the Thevenin's equivalent circuit of a dipole with a diode. The numerical technique is to solve the nodal equation using a time-stepping finite difference equation method. The nonlinear resistance of the diode is treated using the Newton-Raphson iteration technique. A comparison between the analytical and numerical solutions is given.
TL;DR: In this article, an efficient numerical evaluation of the Sommerfeld integrals, arising from the problem of a current element radiating over a lossy half-space, is considered.
Abstract: Efficient numerical evaluation of the Sommerfeld integrals, arising from the problem of a current element radiating over a lossy half-space, is considered. An integration technique based on the method of steepest descent is investigated. The details for computing possible pole and branch cut contributions resulting from the deformation of the path of integration are presented. A computer program for evaluating Sommerfeld integrals is developed on the analysis presented. Numerical results and estimates of computational efficiency of the method are given and a comparison with other available works in the literature is included.
TL;DR: In this article, a rectangular waveguide is terminated by an infinite conducting flange and radiates into half-space, and the power radiated from the aperture is expressed in terms of correlation functions of the aperture electric fields.
Abstract: A rectangular waveguide is terminated by an infinite conducting flange and radiates into half-space. Modal expansions of the TE and TM waveguide fields lead to Hermitian forms for the incident and reflected waveguide power ( P_{wg} = P_{wg}^{+} - P_{wg}^{-} ) in terms of the corresponding electric field mode amplitude vectors and a diagonal field admittance matrix [ Y ]. The power radiated from the aperture is expressed in terms of correlation functions of the aperture electric fields, and yields a Hermitian form for the radiated power P_{r} in terms of the electric mode amplitude vectors and a matrix derived from the field correlation functions-the correlation matrix. Applying the principle of conservation of complex power ( P_{wg} = P_{r} ) leads directly to a nonvariational expression for the scattering matrix [S] for the flanged termination. From [S] the effective TE_{10} load admittance Y_{L, 10} can be deduced. Numerical results for the latter are compared graphically with previous variational results. Also given are the farfield power patterns for two aperture sizes which are then compared with the traditional patterns due to the TE_{10} mode only.
TL;DR: In this article, a plane wave incident on a perfectly conducting strip (or strip grating), and let the incident electric vector be parallel with the edges of the strip, was considered, and the edge mode was included among the basis functions, which greatly improved the convergence of the moment-method solution.
Abstract: Consider a plane wave incident on a perfectly conducting strip (or strip grating), and let the incident electric vector be parallel with the edges of the strip. If the edge mode is included among the basis functions, it is found this greatly improves the convergence of the moment-method solution. Numerical data are included for the reflection coefficient of the strip grating. To correct an error in the previous literature, the rigorous solution is tabulated for broadside backscatter from a single strip.
TL;DR: In this article, the receiving and transmitting transient responses of the resistively loaded linear antenna, the TEM horn, and the conical antenna were investigated theoretically and experimentally using the fast Fourier transform (FFT) technique.
Abstract: The receiving and transmitting transient responses of the resistively loaded linear antenna, the TEM horn, and the conical antenna are investigated theoretically and experimentally using the fast Fourier transform (FFT) technique. The receiving transient response of the resistively loaded linear antenna indicates that the shape of a 70-ps impulse is well preserved.
TL;DR: It is demonstrated that for time multiplex perturbation sequences the penality incurred because of the use of perturbations for gradient estimation in adpative beamforming is an increase by a factor N in the system time constants.
Abstract: The application of orthogonal perturbation sequences to adaptive beamforming is considered. The perturbation approach is relevant when all the array element signals in an array are not accessible but the array weights are independently adjustable. The perturbation sequences can be used to provide an estimate of the gradient required for adaptive beamforming. A unified analysis of the performance of orthogonal perturbation sequences is presented, and the performance of some specific sequences is investigated. It is demonstrated that for time multiplex perturbation sequences the penality incurred because of the use of perturbations for gradient estimation in adpative beamforming is an increase by a factor N in the system time constants.
TL;DR: In this article, the relationship between rain and ice attenuation and depolarization for several incident polarizations was determined experimentally for the first time based on measurements made using 19 and 28 GHz beacon transmissions from a Comstar satellite.
Abstract: Relationships between rain and ice attenuation and depolarization for several incident polarizations have been determined experimentally for the first time. The relationships are based on measurements made using 19- and 28-GHz beacon transmissions from a Comstar satellite. Vertical and horizontal incident polarizations experience much less depolarization on the average than circular polarization or linear polarization incident at 45\deg . The measurements confirm that the usual orientations of the major symmetry axes of nonspherical raindrops and ice crystals are nearly horizontal. Joint cumulative distributions of attenuation and depolarization for one year of continuous measurement at 19 and 28 GHz are also presented. These distributions are useful for determining joint attenuation and depolarization margins required to meet outage objectives of specific satellite communication systems. For example, the outage of a 19-GHz dual-polarized system received with a polarization angle 21\deg from horizontal and having an attenuation margin of 20 dB would be depolarization-dominated unless the system could also tolerate at least -15-dB depolarization.
TL;DR: In this article, the effect of the sea bottom on radio wave propagation is investigated in a three-layered sea, where the multipath reflection and multilateral waves that cannot be generated in a two-layer media are obtained.
Abstract: The problem of communication in the sea has been considered by many writers. They regarded the sea as a two-layered media, and the bottom of the sea has not been included. However, the effect of the sea bottom cannot be neglected in some situations, for example in the case of a low frequency, a shallow sea, and the access of a transmitter or receiver to the sea bottom. The effect of the sea bottom on radio wave propagation is ascertained theoretically, and it will act as an important part in such cases. Considering a three-layered sea, the multipath reflection and multilateral waves that cannot be generated in a two-layered media are obtained.
TL;DR: In this article, a general integral equation for an arbitrarily shaped thin-wire antenna over a lossy half-space is derived, and the method of moments is employed to process this equation numerically.
Abstract: A recently developed technique for approximate but accurate evaluation of the various vector potential components associated with a current element radiating over a lossy ground is used to study the problem of antennas radiating over a lossy ground. A general integral equation for an arbitrarily shaped thin-wire antenna over a lossy half-space is derived, and the method of moments is employed to process this equation numerically. Illustrative numerical results are presented to demonstrate the effect of the lossy half-space on a number of antenna configurations.
TL;DR: In this paper, the authors derived full-wave solutions for the scattered radiation fields from rough surfaces with arbitrary slope and electromagnetic parameters, which are consistent with reciprocity, energy conservation, and duality relations in electromagnetic theory.
Abstract: Full-wave solutions are derived for the scattered radiation fields from rough surfaces with arbitrary slope and electromagnetic parameters. These solutions bridge the wide gap that exists between the perturbational solutions for rough surfaces with small slopes and the quasi-optics solutions. Thus it is shown, for example, that for good conducting boundaries the backscattered fields, which are dependent on the polarization of the incident and scattered fields at low frequencies, become independent of polarization at optical frequencies. These solutions are consistent with reciprocity, energy conservation, and duality relations in electromagnetic theory. Since the full-wave solutions account for upward and downward scattering, shadowing and multiple scatter are considered. Applications to periodic structures and random rough surfaces are also presented.
TL;DR: In this article, a new computational approach is presented which allows a fast analysis of radiation properties of large antennas by using conventional techniques, e.g., physical optics and geometrical theory of diffraction, in prescribed sampled space directions, roughly one direction per lobe.
Abstract: A new computational approach is presented which allows a fast analysis of radiation properties of large antennas. The radiated field is first computed using conventional techniques, e.g., physical optics and geometrical theory of diffraction, in prescribed sampled space directions, roughly one direction per lobe. Then sampling theory is used to reconstruct the complete radiation diagram. Numerical experiments are presented in the last part of the paper, showing the excellent performance of the method.
TL;DR: In this article, the most significant aspects of a moment method surface patch/wire formulation are speed, accuracy, convergence, and versatility, and techniques for improving these parameters are discussed and applied to a solution based on the piecewise sinusoidal reaction formulation.
Abstract: The most significant aspects of a moment method surface patch/wire formulation are speed, accuracy, convergence, and versatility. Techniques for improving these parameters are discussed and applied to a solution based on the piecewise sinusoidal reaction formulation.
TL;DR: Results of a newly formulated analysis for computing patterns of an aperture or monopole antenna mounted on the fuselage of an aircraft are presented and the accuracy of the numerical solutions obtainable with the analysis is demonstrated by comparison with model measurements.
Abstract: Results of a newly formulated analysis for computing patterns of an aperture or monopole antenna mounted on the fuselage of an aircraft are presented. Approximate models of the aircraft structure are employed in conjunction with the geometrical theory of diffraction (GTD) to obtain the computed fields. A major feature of the analysis is that it can accommodate receiver range specifications varying from as close as a wavelength to the aircraft surface to the true far field. This feature is especially useful in that computed on-aircraft pattern performance can be compared with measurements taken at any convenient range, including the near field. Further, after such crucial checks between computations and measurements have been made, the numerical solution can be employed to predict accurately the far-field performance of the on-aircraft antenna system. The accuracy of the numerical solutions obtainable with the analysis is demonstrated by comparison with model measurements.
TL;DR: A noniterative method for approximating signals by a linear combination of exponentials is presented and natural insensitivity to noise in the data and a capability for approximately determining signal order are demonstrated.
Abstract: A noniterative method for approximating signals by a linear combination of exponentials is presented. Although the technique results in a suboptimal approximation, the continuous dependence of the suboptimal exponents \sim{s}_{i} on the integral square error \epsilon is such that lim (\epsilon = 0) \sim{s}_{i} \rightarrow {s}_{i} , the best least squares exponents. The method is also useful for system identification, where the system is modeled by a black box and one has access only to the input and output terminals. A technique is demonstrated for finding the multiple poles of a system along with the residues at the poles when the system output to a known input is given. Advantages of the method are natural insensitivity to noise in the data and a capability for approximately determining signal order. Representative computations are made of the poles from the transient response of a conducting pipe tested at the ATHAMAS-I EMP simulator.
TL;DR: In this article, the shadowing function of a Gaussian rough surface is determined by a single integration over the marginal probability density function for the surface slopes in the plane of incidence.
Abstract: The approach developed by Smith for estimating the shadowing properties of a Gaussian rough surface is applied to an arbitrarily distributed surface. Under the assumption that the surface height is statistically independent of the surface slopes, a very simple result is obtained. In particular, it is shown that the general bistatic shadowing function is completely determined by a single integration over the marginal probability density function for the surface slopes in the plane of incidence. From this general result it is immediately obvious that for backscatter the shadowing function is unity at normal incidence and zero at grazing incidence. The backscattering shadowing function is computed for an exponential slope density and compared to the Gaussian case.
TL;DR: In this article, the geometric characteristics of conic-section-generated offset reflectors are studied in a unified fashion and some unique geometrical features of reflector rim constructed from the intersection of the reflector surface and a cone or cylinder are explored in detail.
Abstract: Geometrical characteristics of conic-section-generated offset reflectors are studied in a unified fashion. Some unique geometrical features of the reflector rim constructed from the intersection of the reflector surface and a cone or cylinder are explored in detail. It is found that the intersection curve (rim) of the rotationally generated conic-section reflector surface and a circular cone with its tip at the focal point is always a planar curve and has a circular projection on the focal plane only for the offset parabolic reflector. Furthermore, in this case, the line going through the center of the circle, parallel to the focal axis, and the central axis of the cone do not intersect the reflector surface at the same point. Numerical results are presented to demonstrate some unique features of offset parabolic reflectors.
TL;DR: In this article, a wide range of input impedances can be obtained depending on the shape of the polygonal loop antenna and the current distribution on the loops obtained by the moment method.
Abstract: Several types of polygonal loop antennas are investigated and their fundamental properties demonstrated. For example, a wide range of input impedances can be obtained depending on the shape of the loop. Some loop configurations exhibit a more broad-band property than others. This property is explained by examining the current distributions on the loops obtained by the moment method.