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Antennas and Propagation, IEEE Transactions on

Issue 5 • Date September 1980

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Displaying Results 1 - 24 of 24
  • [Front cover and table of contents]

    Page(s): 0
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    Freely Available from IEEE
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  • Strong coma lobes from small gravitational deformations

    Page(s): 652 - 657
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    Beam mapping at 1.3-cm wavelength has shown very strong sidelobes at the 140-ft telescope, up to four lobes in a row when pointing far west. Different types of observations and the theory of coma lobes lead in good agreement to the conclusion that the telescope suffers a large lateral defocusing, varying by 7.2 cm east-west (EW) and 2.8 cm north-south (NS) for pointing changes of 90\deg . Since the lateral deformation of the feed legs is only 0.5 cm, it must be mainly the optical axis which moves. This is achieved by a gliding rotation of the best-fit paraboloid, gliding along a slightly deformed surface, while rotating about the center of the average surface curvature. It is shown that the resulting lateral focal offset can be much larger than the surface deformation which causes it. The gliding rotation is also confirmed by a structural analysis. The observational and analytical methods described can also be applied to other telescopes. Polar mounts such as the 140-ft may show the effect in two directions: east-west and north-south, while alt-azimuth mounts can show it in only one direction: up-down. The effect of lateral defocusing is to be expected at other telescopes as well whenever the rim is more flexible than the center; and the resulting degradation of efficiency and beam shape will be significant whenever gravity is important in the error budget. In these cases the telescopes should be supplied with a variable lateral shift of the mount at the prime focus, computer-controlled as a function of the pointing, following the axial movements. This additional degree of freedom may improve the short-wavelength performance considerable for telescopes whose surface panels are more accurate than the gravitationally deformed backup structure. View full abstract»

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  • Tests of impedance theories for a transmitting dipole in an ionospheric plasma

    Page(s): 623 - 630
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    The voltage at the output of the sounder transmitter in both of the International Satellites for Ionospheric Studies (ISIS) ionospheric spacecraft is routinely recorded over the entire frequency range of 0.1-20 MHz. The observed dependence of the output voltage on the sounder frequency is determined by the values of the local plasma parameters, of which there is a considerable range in the ISIS data. Impedance theories for an ionospheric dipole have been tested by comparing observed curves of voltage versus sounder frequency with computed curves based on those theories. In cases where the plasma frequency is low, around 0.1 MHz, a realistic curve is obtained using standard vacuum dipole theory. For higher plasma frequencies, the observed response departs significantly from the low-density case at frequencies close to the electron characteristic frequencies. For fixed working frequency and plasma parameters, the voltage exhibits a slight dependence on antenna orientation. Attempts to explain these results for higher plasma frequencies using a transmission-line theory have qualified success when a vacuum sheath of 1-m radius is assumed. The need for a large sheath is just one kind of evidence that the sounder rf fields profoundly change the nature of the plasma near the dipole. View full abstract»

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  • Suppression of parasitic currents on feed lines of colinear dual antenna systems

    Page(s): 658 - 662
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    Investigations are reported of an antenna system consisting of a pair of colinear dipoles with a feed line "isolator section" which provides independent operation of the dipoles over an octave frequency range. The isolator section acts as a band elimination filter for the parasitic currents induced on the outer surface of the transmission line of the upper dipole. Suppression of these currents eliminates distortion of the antenna radiation patterns and significantly reduces coupling between the dipoles. Over the 200-400-MHz range, an interdipole isolation greater than 35 dB has been achieved between the colinear dipoles separated by 1.5 m. View full abstract»

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  • Characteristic modes of a symmetric wire cross

    Page(s): 712 - 715
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    The theory of characteristic modes is invoked to illuminate some features found in the radar scattering by the symmetric wire cross. Results discussed here may help to clarify conflicting literature on this subject. View full abstract»

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  • GTD analysis of the E-plane patterns of conical horns

    Page(s): 715 - 717
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    An analytical procedure for predicting accurately the E -plane patterns of conical horns with moderate aperture widths ( ka < 10 ), based on uniform geometrical theory of diffraction (UGTD) and supported by measured data, is presented. This analysis predicts the E -plane patterns more accurately (over the main beam) than the one presented earlier. View full abstract»

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  • Analysis of a wire antenna in the presence of a body of revolution

    Page(s): 604 - 609
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    The problem of an arbitrarily oriented thin-wire antenna located near a body of revolution is analyzed. The usual integrodifferential equation for a thin wire in unbounded space is generalized to account for scattering from the nearby body. The presence of the body is accounted for by a numerical dyadic Green's function. The modified wire equation is solved by standard numerical techniques to obtain the current distribution on the wire. The effects of various bodies on input admittance are compared with results for an isolated antenna. Measured and theoretical input admittance data for a monopole near several different bodies of revolution are found to be in good agreement. View full abstract»

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  • Scattering by a rotating dielectric sphere

    Page(s): 643 - 651
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    The scattering of a plane wave by a rotating dielectric sphere of arbitrary radius, \epsilon_{r} and \mu_{r} is investigated. A relativistically correct solution is found for the scattered fields and the fields inside the sphere. This solution is valid in the limit of "small" angular velocities. Attention is focused on the fields at large distances. The depolarization of the backscattered signal and the scattering in the low frequency limit are treated in some detail. View full abstract»

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  • Maximum entropy angular response patterns of microwave transhorizon signals

    Page(s): 722 - 724
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    The maximum entropy method (MEM) is applied in the computation of angular response patterns based on amplitude and phase measurements from a 12-element vertical array receiving transhorizon microwave signals. The patterns are compared with those obtained using conventional Fourier transform techniques (FTT). The MEM patterns exhibit higher resolution and lower sidelobes, revealing in some cases features not seen in the FTT patterns. View full abstract»

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  • Backfire antennas constructed with coaxial circular loops

    Page(s): 663 - 667
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    The radiation characteristics of backfire antennas constructed entirely of circular loops are studied. A comparison between this type of backfire antenna and the conventional one, made of a solid reflector, reveals that the optimum dimensions of the reflector and the peripheral rim are approximately the same for both kinds of antennas. In addition, it is found that as few as six loops are sufficient to construct the optimum reflector, and almost the same number of loops are required to form the optimum peripheral rim. The new structure reduces the weight, windage, and obstruction of view which accompany solid reflectors. In addition, the new antenna lends itself to exact analytical investigation. View full abstract»

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  • Axial cross polarization in reflector antennas with surface imperfections

    Page(s): 610 - 616
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    Expressions for the probability density function and mean of boresight cross polarization field intensity in terms of the rms surface error \epsilon are derived. It is shown that for small errors the mean on-axis cross polarization field intensity is directly proportional to the rms surface error. For relatively large errors ( \epsilon > 0.04 \lambda ) forward cross polarization is proportional to the square root of ( 1 - \exp (-2\sigma ^{2}) ) where \sigma is the rms phase error. It is also shown that forward cross polarization is proportional to the correlation diameter and is a function of the polarization efficiency. Experimental measurements are revealed and results are compared with theoretical findings. View full abstract»

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  • Microwave radiometer measurements of soil moisture content

    Page(s): 680 - 686
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    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. View full abstract»

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  • Simple and efficient numerical methods for problems of electromagnetic radiation and scattering from surfaces

    Page(s): 593 - 603
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    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. View full abstract»

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  • Simultaneous optimization of an index and determination of the polarization of arrays of nonparallel wire antennas by the eigenvalue method

    Page(s): 724 - 730
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    A study for maximizing an index of an array of nonparallel wire antennas under the simultaneous determination of the polarization is carried out. The eigenvalue method is used, and it is shown that the maximum obtainable index of an antenna which has not a predefined polarization is the largest of the only two nonzero eigenvalues of the regular pencil of its matrices. The maximization is applied for cases where there are no constraints and also in cases where constraints in the pattern nulls or/and sidelobe levels are posed. Some examples are presented. View full abstract»

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  • Tropospheric microwave propagation--An X band diagnostic system

    Page(s): 693 - 699
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    A microwave system for direct measurement of angles of arrival and multipath delay times is described. Based on the utilization of interferometer and very wide frequency sweeping (1- GHz range) techniques, high accuracy measurements on these parameters are possible. Some experimental results are presented which suggest that movements in the angle of arrival of a single ray path (as opposed to multipath) may be responsible for much of the fading experienced on light-of-sight microwave links in southwestern Ontario. View full abstract»

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  • Magnetic polarizability of some small apertures

    Page(s): 703 - 707
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    Curves are given for the dimensionless magnetic polarizabilities \nu_{mx} and \nu_{my} of a few characteristic apertures. The relevant integral equations are solved by the moment method. The subareas are triangular, and the basis functions for the triangles touching an edge take the edge singularity into account. Some data are included for a few typical ring-shaped apertures. View full abstract»

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  • Electromagnetic transmission through narrow slots in thick conducting screens

    Page(s): 616 - 622
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    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. View full abstract»

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  • On the accuracy of the transmission line model of the folded dipole

    Page(s): 700 - 703
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    A folded dipole is basically an unbalanced transmission line which radiates as a result of its unbalanced condition. It is known that the folded dipole may be analyzed by considering its current to be composed of two distinct modes, namely, a transmission line mode and an antenna mode. The transmission line model of the folded dipole is investigated with the objectives of determining a) the accuracy of the model and b) the importance of using an equivalent radius in the model. To check the transmission line model, the method of moments is used as a standard for comparison. View full abstract»

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  • Systems study of overlapped subarrayed scanning antennas

    Page(s): 668 - 679
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    The properties of an overlapped subarrayed scanning antenna are studied, with emphasis on obtaining low sidelobes and large bandwidths. View full abstract»

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

    Page(s): 631 - 642
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    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. View full abstract»

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  • Scattering by circular metallic disks

    Page(s): 707 - 712
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    The eigenfunction solution for the far-field scattering by a thin, circular metallic disk is summarized. This solution yields both components of the scattered electric field for an incident plane wave of arbitrary polarization and arbitrary incidence directions. Sample amplitude, phase, and cross section results are presented. View full abstract»

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  • Radar altimeter mean return waveforms from near-normal-incidence ocean surface scattering

    Page(s): 687 - 692
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    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. View full abstract»

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  • Effects of secondary diffraction in the radiation pattern of the paraboloid

    Page(s): 718 - 721
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    The radiation of a paraboloid, the scattering by the primary feed, and the scattering by the feed supports are compared with experimental results in the whole of the radiation pattern. View full abstract»

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IEEE Transactions on Antennas and Propagation includes theoretical and experimental advances in antennas.

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Editor-in-Chief                                                 Kwok W. Leung