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Antennas and Propagation, IEEE Transactions on

Issue 3 • Date March 1996

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Displaying Results 1 - 22 of 22
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  • A comparison of theoretical and empirical reflection coefficients for typical exterior wall surfaces in a mobile radio environment

    Page(s): 341 - 351
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    This paper presents microwave reflection coefficient measurements at 1.9 GHz and 4.0 GHz for a variety of typical smooth and rough exterior building surfaces. The measured test surfaces include walls composed of limestone blocks, glass, and brick. Reflection coefficients were measured by resolving individual reflected signal components temporally and spatially, using a spread-spectrum sliding correlation system with directional antennas. Measured reflection coefficients are compared to theoretical Fresnel reflection coefficients, applying Gaussian rough surface scattering corrections where applicable. Comparisons of theoretical calculations and measured test cases reveal that Fresnel reflection coefficients adequately predict the reflective properties of the glass and brick wall surfaces. The rough limestone block wall reflection measurements are shown to be bounded by the predictions using the Fresnel reflection coefficients for a smooth surface and the modified reflection coefficients using the Gaussian rough surface correction factors. A simple, but effective, reflection model for rough surfaces is proposed, which is in good agreement with propagation measurements at 1.9 GHz and 4 GHz for both vertical and horizontal antenna polarizations. These reflection coefficient models can be directly applied to the estimation of multipath signal strength in ray tracing algorithms for propagation prediction View full abstract»

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  • Reduction of variances of source-tracking response zeros of power inversion arrays in the presence of random amplitude and phase errors

    Page(s): 413 - 419
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (604 KB)  

    A new technique to reduce the variances of the source-tracking response zeros of power inversion arrays in the presence of random amplitude and phase errors is investigated. Essentially, the zero variances are reduced by dividing the original system into several spatially shifted power inversion arrays, averaging the weight vectors of these arrays, and determining the zeros of the spatially averaged weight vector. Equations for the optimal averaging weights and the ensuing variance performance are derived and verified by simulation results. It is found that the new technique may reduce the zero variance substantially when compared with taking the entire system as a single array View full abstract»

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  • Electromagnetic scattering from simple jet engine models

    Page(s): 420 - 421
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    The mode-matching (MM) technique is employed for the evaluation of the radar cross section (RCS) of structures that simulate a jet engine inlet. The geometry consists of a perfectly conducting cylindrical inlet terminated by an array of blades mounted on a cylindrical hub. Comparisons of numerical results with actual measurements are presented for the first time View full abstract»

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  • A hybrid finite-element method for axisymmetric waveguide-fed horns

    Page(s): 280 - 285
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    A new method for finding radiation patterns and the reflection coefficients associated with an axisymmetric waveguide fed horn is presented. The approach is based on a hybrid finite element method (FEM) wherein the electromagnetic fields in the FEM region are coupled to the fields outside by two surface integral equations. Because of the local nature of the FEM, this formalism allows for the presence of inhomogeneities to be included in the problem domain. The matrix equation which results from the application of this method is shown to be complex-symmetric. Comparisons of calculated and measured data for two different horns show good agreement View full abstract»

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  • A method for the optimal pattern synthesis of linear arrays with prescribed nulls

    Page(s): 286 - 294
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    A new L optimal method for the synthesis of equispaced linear array functions with asymmetrical far-field pattern functions is proposed. This iterative method provides for the exact specification of the beam width, while at the same time allowing for the specification of the relative levels of individual sidelobes by index or as a function of bearing (e.g., angularly-extended nulls), as well as the realization of specified narrowband nulls. The resulting array factors are optimal in the weighted L sense and, in general, have complex coefficients. This new Remez-type method employs multiple objective functions to provide the degrees of freedom that are required for exact null placement. Examples which demonstrate the design flexibility offered by the method are included for various sum and difference patterns, including superdirective and shaped-beam arrays View full abstract»

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  • Electromagnetic scattering from eccentric multilayered dielectric bodies of revolution-numerical solution

    Page(s): 295 - 301
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    In this paper, a numerical method for the scattering problem of multilayered dielectric bodies of revolution is introduced. Since the scatterer may be eccentrically layered, and since the rotation axis of each layer may be different from each other, conventional methods for bodies of revolution are not applicable to the scattering problem of this type. Thus, an iterative method is introduced in this paper. The electromagnetic problem of each layer is solved separately to take advantage of the rotational symmetry of each layer. This boundary value problem is solved from the outermost layer to the innermost layer, and this procedure is repeated until the desired degree of accuracy is obtained. The validity of this method is shown from numerical results. A mode-matching solution for a multilayered dielectric sphere is given to provide data which verify the accuracy of the method introduced in this paper View full abstract»

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  • Electromagnetic interaction between two parallel circular cylinders on a planar interface

    Page(s): 321 - 325
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    Multiple interaction between two parallel and infinitely long circular cylinders on a planar interface separating two different media is analyzed theoretically. The scattering equations are derived from the so-called extinction theorem applied to this particular geometry. For simplicity, the surface is considered to be perfectly conducting although the method can be extended for any material. The equations, solved numerically by means of an appropriate discretization of the surface, provide the electric surface current density from which the scattered intensity can easily be calculated. Scattering of the transversal-magnetic and transversal-electric incident wave is studied as a function of the cylinder separation for cylinder diameters from 0.2λ to 4λ (λ being the incident wavelength). The effects of the interaction between cylinders are shown in the scattering cross section and in the surface current density of the planar substrate and of the cylinders View full abstract»

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  • A loop antenna in front of a resistive sheet

    Page(s): 405 - 412
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    Formulas for self inductance, mutual inductance, and image for a horizontal loop placed above a resistive sheet are presented. These results extend the magnetostatically-derived self- and mutual-inductance concepts to account for the finite speed of propagation of the fields. Classical network equivalents are introduced for low- and high-resistive sheet values. For capacitive sheets, the structure is capable of selective radiation (eminently below the plane of the sheet) View full abstract»

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  • Rayleigh-Mie approximation for line-of-sight propagation through rain at 5-90 GHz

    Page(s): 273 - 279
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    An earlier heuristic model of attenuation and phase changes through a layer of oblate spheroids is replaced by a new, simpler, model with much greater accuracy. The model is meant to cover propagation through rain at 5-90 GHz frequencies and at rain rates from 5-150 mm/hr. Accurate predictions of co and crosspolar attenuation, of co and crosspolar discrimination, and of the various phase changes associated with each polarization of the incident wave are now possible by means of calculations requiring no more than simple numerical extensions of Mie calculations superposed upon the Rayleigh forward-scattering cross sections. Some degree of canting-angle variations is included. Comparison of calculations by this approximation to a variety of empirical or simulated rain statistics available in the literature is presented View full abstract»

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  • The use of shorting posts to improve the scanning range of probe-fed microstrip patch phased arrays

    Page(s): 302 - 309
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    This paper presents a thorough investigation of the use of shorting posts to improve the scanning potential of microstrip probe-fed patch arrays. A rigorous spectral domain integral equation (SDIE) technique is used to analyze the scan impedance and related radiation characteristics of several proposed shorting post-patch array configurations. It is shown that by using shorting posts, potential scan blindnesses can be removed and the useful scanning range of the array can be significantly extended beyond 80° in the plane of interest. Comparisons of the scan performance of several microstrip patch arrays containing up to three shorting posts are presented. Novel hybrid shorting post configurations are also proposed and examined, which provide excellent scanning potential in the three principal planes (E-, H-, and D-planes). These modified microstrip patches incorporate switching diodes and provide minimal scan impedance variation for scan angles near 80° in all three planes, as well as very low cross-polarization levels View full abstract»

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  • Numerical dispersion of higher order nodal elements in the finite-element method

    Page(s): 317 - 320
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (348 KB)  

    The discretization inherent in the finite-element method results in numerical dispersion. This dispersion is investigated for a time-harmonic plane wave propagating through an infinite, two-dimensional, finite-element mesh composed of uniform quadrilateral and triangular elements. The effects on the dispersion due to the propagation direction of the wave, the order of the elements, the node density, and the mesh geometry are studied. Results are given which can serve as a guide in selecting the appropriate element order, node density, and mesh geometry when applying the finite-element method View full abstract»

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  • Evaluation and verification of the VHF/UHF propagation channel based on a 3-D-wave propagation model

    Page(s): 393 - 404
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    This paper is intended to increase and improve the knowledge concerning the characterization of digital VHF/UHF communication channels. This characterization of the propagation channel is based on the field strength-delay-spectrum (FDS) of the three-dimensional (3-D) multipath wave propagation. A statistical superposition of the predicted multipath signals yields the probability density function of the narrowband signal and the averaged channel impulse response of the wideband propagation channel. Thereof, the relevant parameters, e.g., standard deviations, mean values, delay spread, and correlation coefficients, are derived both in time and frequency domain. The angles-of-arrival of the multipath signals are used to calculate the Doppler spectra of moving receivers for the characterization the time-variant properties of the propagation channel. In total, a rather complete summary of derivations, algorithms and characteristics of the VHF/UHF propagation channel is presented. Narrowband and wideband measurements are included for verification View full abstract»

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  • Nonplanar microstrip arrays

    Page(s): 389 - 392
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    Pattern synthesis using planar arrays was done extensively in the past, mainly in the context of radar antennas where a cosec2 beam is required in the elevation plane. The synthesis leads to a set of coefficients for the array elements which may be difficult to realize and which, if realizable, may lead to a very narrow-band performance. In this paper, a new type of array is proposed. The shaped-beam nonplanar array is an array of microstrip elements, where the phasing is not generated through the feeding network, but by shaping the substrate. Microstrip arrays lend themselves well to a configuration similar to the one proposed, and as it is shown, the final thickness of the array does not exceed a few substrate thicknesses View full abstract»

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  • Inverse scattering for conductors by the equivalent source method

    Page(s): 310 - 316
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    The inverse scattering for two-dimensional conductors is investigated. Assume that an incident wave is incident upon conducting cylinders of unknown shape and the scattered field is measured outside. By properly processing the scattering data, the position and shape of the conducting scatterers can then be reconstructed. The proposed theory of inverse scattering is a combination of the diffraction tomography and the equivalent source method. To overcome the ill-posedness, the condition of the measured scattered field and the boundary condition on the conductor surfaces are combined and recast into an optimization problem using the least-squares concept. By comparing the reconstructed and the prescribed results, the inversion algorithms are numerically examined. Note that if a priori information of the shape is sufficient and the equivalent sources are properly placed, the algorithms can be flexibly applied to the cases of multiple conductors. Also the effects of noise contamination and multiple scattering on the reconstruction result are numerically investigated View full abstract»

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  • Auroral-E mode oblique HF propagation and its dependence on auroral oval position

    Page(s): 383 - 388
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    The characteristics of 25.5 MHz one-hop mode propagation on a 950 km Alaskan path, over a one-year period shortly after sunspot maximum are presented as a function of the “planetary geomagnetic index-Kp”. The location of the E-region reflection point lies within the auroral oval for 3<Kp<5, and the specific behavior of the signal is related to auroral oval phenomena such as substorms, geomagnetic storms, and the “harang discontinuity”. The auroral electrojet location, with respect to the path midpoint, is also of considerable importance. The 75 watt CW beacon has been in continuous operation since mid-August 1991 on a frequency of 25.545 MHz. The signal is characterized by “bursty” behavior lasting from about one minute to over three hours, with an average duration of eleven minutes. The authors believe that this is the first quantitative demonstration of the “forward propagation characteristics” of a one- or two-hop E-propagation path inside and parallel to the auroral oval on a frequency at the high end of the HF band View full abstract»

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  • Ray theory for scattering by two-dimensional quasiperiodic plane finite arrays

    Page(s): 375 - 382
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    Many scattering configurations of interest include finite portions with periodic or quasiperiodic features. Several recent investigations have dealt with this problem for the planar two-dimensional case and have developed high-frequency asymptotic solutions that include multibeam reflections obeying the Bragg condition and Bragg-modulated edge diffractions. These constituents have been interpreted as wave objects in a generalized geometrical theory of diffraction (GTD). The present investigation adds to these previous results and formalizes them into a ray theory. This allows the scattered fields due to a finite quasiperiodic array of obstacles, excited by an arbitrary incident field, to be constructed entirely by ray tracing. Scattered ray plots and caustics for various shapings of incident fields and array parameters illustrate the variety of phenomena associated with this class of scattering environments View full abstract»

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  • The effect of perturbating a frequency-selective surface and its relation to the design of a dual-band surface

    Page(s): 368 - 374
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    A single-layer dual-band frequency selective surface (FSS) created by the perturbation of a single-band dipole array is studied. A simple algebraic formulation is derived to analyze both element and spacing perturbations. Guidelines are then proposed to allow dual-band FSSs to be designed with no anomalous behavior occurring in the two desired bandstop responses. Examples of unsuccessful and successful dual-band FSSs are calculated View full abstract»

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  • Constrained Hung-Turner adaptive beam-forming algorithm with additional robustness to wideband and moving jammers

    Page(s): 361 - 367
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    We present a new modification of the Hung-Turner (HT) adaptive beam-forming algorithm, providing additional robustness of a narrowband adaptive array in wideband and moving-jammer scenarios. The robustness is achieved by involving the derivative constraints toward the jammer directions in the conventional Hung-Turner (1983) algorithm. The important advantage of the constraints used is that they do not require any a priori information about jammer directions. The computer simulations with wideband and moving jammers show that the proposed algorithm provides the significant improvement of the adaptive array performance as compared with the conventional HT algorithm. At the same time, for a moderate order of derivative constraints, the new algorithm has a computational efficiency, comparable with the conventional HT algorithm View full abstract»

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  • Wave-oriented processing of scattered field data from a plane-wave-excited finite array of filaments on an infinite dielectric slab

    Page(s): 352 - 360
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    In a companion paper, we presented the formulation and solution for time-harmonic plane wave fields scattered by truncated periodic and aperiodic arrays of infinitely long filaments on an infinite dielectric slab. The solution was constructed so as to highlight the coupled phenomenologies associated with slab loading of the Floquet mode beams characteristic of the truncated arrays and with the slab-guided leaky modes excited by these arrays. Asymptotic reduction of the solution yielded a parameterization of the scattered fields and a numerically accurate algorithm, based on physically observable ray- and mode-field constituents. This study is concerned with the inverse procedure of extracting from the scattered field data the phenomenological footprints of the scattering mechanisms found by forward problem asymptotics; this information is relevant to target classification and identification. The data processing tools involve Gaussian windowed transforms and superresolution algorithms that yield projections onto appropriate subdomains of the (space)-(spectral wave number) phase space. A variety of examples demonstrate physically and quantitatively the interplay of Floquet-mode and leaky-mode phenomena, and how these are affected by changes in the problem parameters. Of special interest is the performance of superresolution algorithms for “cleaning up” the diffuse Gaussian windowed phase space distributions, in the absence and presence of system noise View full abstract»

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  • Wind-speed dependence of small-grazing-angle microwave backscatter from sea surfaces

    Page(s): 333 - 340
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    Results of X-band microwave backscatter from sea surfaces at a small-grazing angle for a range of wind speeds (3.8-10.3 m/s) are presented. The data indicate that the speed of faster-than-Bragg scatterers and their time-averaged polarization ratio both increase with wind speed. At a friction velocity above ~21 cm/s, super events (i.e., backscattering events where the horizontal-polarization power exceeds the vertical-polarization power) begin to appear. The fraction of fast scatterers which produce super events also increases with wind speed. The presence of super events indicates that in addition to scattering mechanisms, such as Bragg-resonant scattering, composite surface theory, wedge scattering, and specular reflection, other mechanisms which favor the horizontal-polarization returns must be considered View full abstract»

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  • Propagation constant and the velocity of the coherent wave in a dense strongly scattering medium

    Page(s): 326 - 332
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    Frequency- and time-domain experiments are conducted to study the effective propagation constant of the coherent wave in a dense strongly scattering medium. A wide-band microwave signal (10-40 GHz) is propagated through randomly distributed glass spheres with a 5.73 mm average diameter and separated into incoherent and coherent fields. The real and imaginary parts of the propagation constant are obtained from the coherent field. The narrow size distribution of the particles enables the authors to study scattering from the Rayleigh region through the Mie resonance scattering region. The results of the experiments are compared to independent scattering, effective-field approximation (Foldy's), and the higher order quasi-crystalline approximation (QCA) using Mie scattering coefficients and the Percus-Yevick approximation for the pair-distribution function. The phase and group velocities of the coherent wave are obtained from the effective propagation constant and compared with theory. In addition, the velocity of the coherent wave in random media is measured using the time-domain technique. It is shown that the velocity of the coherent wave in random media is neither phase nor group velocity 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