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Antennas and Propagation, IEEE Transactions on

Issue 12 • Date Dec. 1991

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Displaying Results 1 - 19 of 19
  • Log-periodic array of dual-feed microstrip patch antennas

    Page(s): 1659 - 1664
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    The dual-feed microstrip patch antenna is presented. This new radiator has broadband match and unidirectional azimuth pattern, desirable characteristics for use in broadband endfire arrays. Design, construction, and experimental results (realized gain, input impedance and far-field patterns) are given for a log-periodic array of dual-feed patches. An array with only ten elements has been demonstrated to operate at nearly constant gain over a 2:1 frequency band with a voltage standing-wave ratio (VSWR) of less than 2:1.<> View full abstract»

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  • Simple analytical solution to electromagnetic scattering by two-dimensional conducting object with edges and corners. I. TM polarization

    Page(s): 1665 - 1671
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    A simple and approximate analytical solution is presented by invoking on-surface radiation condition theory for the analysis of electromagnetic scattering by a perfectly conducting object. The scattering object is assumed to be placed in a free space medium and is excited by a time harmonic plane wave having transverse magnetic (TM) polarization. The closed form analytical result for the monostatic as well as bistatic radar cross section is approximate. It is applicable only for the case of a convex conducting object having arbitrary two dimensional cross section with arbitrary edges and corners. Canonical scattering objects, such as a triangular scatterer and a thin strip scatterer, are analyzed for the transverse magnetic excitation to evaluate the usefulness of the analytical results. Numerical data for the monostatic and bistatic radar cross section are presented by comparing them with respect to the numerical solution obtained by solving an electric field integral equation based on the method of moments technique.<> View full abstract»

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  • On the asymptotic frequency behavior of uniform GTD in the shadow region of a smooth convex surface

    Page(s): 1672 - 1680
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    A number of canonical plane-wave and line-source solutions involving circular cylinders have led to a widely accepted uniform geometrical theory of diffraction (GTD) formulation for the scattered electromagnetic field in the shadow region of a smooth convex surface. The current solution does not constitute a properly formulated asymptotic high-frequency theory in the sense that it becomes increasingly inaccurate with increasing frequency. This inaccuracy results from transition-function dominance in the deep shadow region over the Pekeris caret function that is used. An improved formulation that circumvents this difficulty by avoiding use of a transition function is derived via a straightforward extension of the canonical line-source solution given by Jones (1963). This new solution takes the form of Keller-type modes with modified diffraction coefficients that result in convergence at the shadow boundary provided that the source and observation point are not both located at asymptotic distances from the scatterer.<> View full abstract»

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  • Statistics of low grazing angle radar sea scatter for moderate and fully developed ocean waves

    Page(s): 1681 - 1690
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    Results are reported of two deep ocean radar sea scatter experiments using calibrated marine navigation radars. The first experiment featured a continuous wind direction over a four-day period, satisfying time and fetch requirements for a fully developed sea. The second experiment experienced changing winds, which was expected to produce random seas. Radar cross section samples from a 60 degrees sector centered about the maximum clutter direction were assembled into cumulative distributions for several range bins, corresponding to different grazing angles between 1 degrees and 8 degrees . These distributions are bimodal for moderate to strong winds and were fitted by two Weibull distributions. The Weibull fit to the highest amplitudes was associated with discrete scatters, while that fitting the lower amplitudes was identified with distributed roughness. Models for scattering mechanisms are inferred from the results.<> View full abstract»

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  • Evaluation of input impedance and radar cross section of probe-fed microstrip patch elements using an accurate feed model

    Page(s): 1691 - 1696
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    Input impedance and radar cross section results for probe-fed single rectangular and circular microstrip patch elements obtained using an accurate and efficient numerical model are presented. The model uses a full-wave Green's function/Galerkin solution in which the connection of a vertical probe feed to the patch is rigorously included in the analysis by use of a special basis function called the attachment mode, which is derived from the corresponding cavity model solution. Comparisons with measured input impedance and monostatic radar cross section data demonstrate the efficacy of the theory. This model accurately predicts the performance of probe-fed patches printed on thick and/or high dielectric constant substrates and patch elements with more than one probe feed, cases where other less rigorous models fail.<> View full abstract»

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  • Statistical behavior of a simulated microwave multipath channel

    Page(s): 1697 - 1706
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    To predict the statistical behavior of microwave multipath channels, computer simulations of medium-bandwidth transfer functions obtained by causing a number of simulated rays to interfere are used, and the transfer functions obtained are statistically analyzed. The simulated rays are defined by input physical quantities such as the number of interfering rays and ray amplitudes, delays and phases, governed by input statistical distributions which are determined by simple physical assumptions. The transfer functions are then mathematically represented by the model whose three defining parameters (notch-width parameter, tau , notch-depth parameter, b, and notch position parameter, phi ) result in output distributions representative of the statistical behavior of the simulated hop.<> View full abstract»

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  • Surface wave and related effects on the RCS of microstrip dipoles printed on magnetodielectric substrates

    Page(s): 1707 - 1715
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    The scattering cross section of microstrip dipoles is examined. The primary emphasis is to demonstrate the effects of surface waves on the scattering cross section, and to provide a simple physical understanding of the surface wave mechanism. The electric field integral equation is solved in the spatial domain using the method of moments. Special techniques used to speed up the evaluation of the Sommerfeld integrals are explained. The radar cross section for several microstrip dipole configurations is presented. The mechanism for generation of surface waves plays a major role in the scattering behavior of the dipoles, and a physically intuitive interpretation of this mechanism is used to successfully explain the pattern structure. For generality, magnetodielectric substrates with and without loss are considered.<> View full abstract»

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  • Radiation pattern analysis of arbitrary wire antennas using spherical mode expansions with vector coefficients

    Page(s): 1716 - 1721
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    A spherical mode expansion with Cartesian vector coefficients is presented for the study of the radiation patterns of arbitrary wire antennas. Coefficients are obtained by using a spherical harmonic series for the free space Green's function in conjunction with the current distribution obtained with the method of moments. Transverse components of the far field are ultimately obtained yielding a compact pattern representation suitable for rapid retrieval and display. The length of the expansion is shown to be related to the precision specified for the pattern; i.e., more precision requires more terms. A useful truncation criterion is proposed and verified by comparing results obtained for several typical wire antennas.<> View full abstract»

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  • Analysis of infinite arrays of printed dipoles on dielectric sheets perpendicular to a ground plane

    Page(s): 1722 - 1732
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    The analysis of infinite arrays of printed dipoles on finite-height dielectric sheets is described. A full-wave solution is obtained by using a combination of the Green's function-moment method approach and mode matching techniques. Numerical calculations and experimental results validating the theory are shown for the air-dielectric case and for epsilon /sub r/ not=1. Scan blindnesses are identified in arrays with epsilon /sub r/ not=1. Pertinent numerical considerations and issues of convergence are discussed.<> View full abstract»

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  • Singly curved dual-reflector synthesis technique applied to a quasi-optical antenna for a gyrotron with a whispering-gallery mode output

    Page(s): 1733 - 1741
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    An improved-efficiency quasi-optical antenna for the transformation of the output of a gyrotron operating in a whispering-gallery mode has been reported by M. Iima et al. (1989). The improved antenna incorporates a shaped reflecting surface attached to the helically-cut waveguide launcher of the configuration proposed by S.N. Vlasov et al. (1975). This second reflector allows for the manipulation of the output-beam power distribution parallel to the direction of the polarization of the beam. A measured Gaussian-like near-field profile in which 93% of the beam power is contained in the main lobe has been obtained. The present authors establish, in a rigorous theoretical manner, a dual-reflector synthesis technique based upon geometrical optics principles to obtain the shapes of the reflectors. The design procedure may be extended to allow for manipulation of the power distribution across both dimensions of the output beam.<> View full abstract»

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  • Inverse scattering: an iterative numerical method for electromagnetic imaging

    Page(s): 1742 - 1753
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    The authors propose a spatial iterative algorithm for electromagnetic imaging based on a Newton-Kantorovich procedure for the reconstruction of the complex permittivity of inhomogeneous lossy dielectric objects with arbitrary shape. Starting from integral representation of the electric field and using the moment method, this technique has been developed for 2-D (for TM and TE polarization cases) objects as well as for 3-D objects. Its performance has been compared with spectral techniques of classical diffraction tomography, the modified Newton method, and the pseudo-inverse method.<> View full abstract»

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  • Polarization properties of offset dual-reflector antennas

    Page(s): 1753 - 1756
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    For an offset dual-reflector antenna design of zero geometrical optics cross polarization, the diffraction-induced cross polarizations calculated from the differential phase shift between the fundamental TEM/sub 00/ and higher order TEM/sub 01/ Gaussian beam modes, which represent principal and cross-polarization components, respectively, are in agreement with measured data. Since this residual cross polarization in the main reflector aperture is in approximate phase quadrature with the principal polarization, the far-field cross polarization is in phase with the principal polarization. Thus, both principal polarizations exhibit the same slight rotation for a given far-field direction. An important consequence is the preservation of orthogonality between two orthogonal linear polarizations from the feed.<> View full abstract»

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  • The totally random versus the bin approach for random arrays

    Page(s): 1757 - 1762
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    The array factors and the statistical properties for two types of random arrays, namely, the totally random and the binned random arrays, are compared. In totally random arrays, the array elements are distributed independently across a common aperture according to some common probability distribution function. In binned arrays, the aperture is divided into nonoverlapping bins of equal length and array elements are distributed independently, one per bin, according to some probability distribution across each of the bins. Significant differences exist in the resulting array factors and underlying statistical properties for the two types of random arrays. These differences are delineated, with particular emphasis upon the depressed near-in sidelobe behavior that is observed for the binned arrays. Tables and plots that illustrate the differences are included, and several issues pertaining to implementation are noted.<> View full abstract»

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  • A frequency-selective surface using aperture-coupled microstrip patches

    Page(s): 1763 - 1769
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    A frequency-selective surface based on the aperture coupled microstrip is presented. This surface has a narrow bandpass response useful for building radomes with low out-of-band radar cross section (RCS). The analysis employs the spectral domain moment method. The design criteria and a simple lumped element model are derived and presented. Experimental data from a waveguide simulator are presented which confirm the theory.<> View full abstract»

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  • Millimeter-wave design of wide-band aperture-coupled stacked microstrip antennas

    Page(s): 1770 - 1776
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    K-band aperture-coupled stacked microstrip antennas are studied and numerical results based on the solution of integral equations solved in the spectral domain are presented. The effects of varying physical parameters of the structure are investigated with the goal of designing millimeter-wave wideband microstrip antennas. Antennas with different characteristics are then analyzed and compared with experimental data. Bandwidths in excess of 20% are obtained and applications to phased array antennas are discussed.<> View full abstract»

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  • Scattering cross sections for non-Gaussian rough surfaces: unified full wave approach

    Page(s): 1777 - 1781
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    The unified wave approach is used to compute both the like- and the cross-polarized scattering cross sections for composite rough surfaces characterized by a family of gamma functions of order K ranging from K=1 to K=25. This family of joint height probability density functions properly decorrelates to the product of the marginals as the surface height autocorrelation function vanishes. These results are compared with the like and cross-polarized cross sections for the surface with Gaussian characteristics (K to infinity ). It is shown how radar could be used most effectively to remotely sense the rough surface statistics of non-Gaussian rough surfaces.<> View full abstract»

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  • The equivalence principle for two-dimensional anisotropies

    Page(s): 1781 - 1784
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    The mathematical foundation of the equivalence principle for an electromagnetic field in anisotropic regions that characterize two-dimensional problems is presented. It is shown that in the absence of polarization coupling, the equivalence principle retains the same functional form as in the homogeneous isotropic case, with the equivalent currents being defined in the same manner as in the isotropic case. Even though such a formulation appears simple, the basic physical processes, such as lack of reciprocity, optical rotatory power, inhomogeneous character of elemental wave behavior, etc., are properly accounted for by the point source responses which are polarization dependent. Only the H-polarization case has been treated explicitly; the corresponding E-polarization results are obtainable via duality.<> View full abstract»

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  • Stability of scattering formulations with multiple elemental current excitations

    Page(s): 1784 - 1788
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    The incident magnetic field can serve as an alternative to the incident electric field as the exciting function in electrical field integral equation formulations. For planar problems, the incident magnetic field is scaled to produce the equivalent incident current as the useful excitation function. The incident current can be split into subdomain basis functions, giving rise to a number of current distributions, called the pulse responses, which can be combined to synthesize the response to any incident field. This synthesis process is investigated with respect to its stability in the presence of finite errors generated in the course of computing the individual pulse responses. The accumulation mechanism of these errors is established by physical reasoning and a numerical study which supports the suggested physical mechanism. The error is accumulated in an oscillatory manner as basis functions are added farther away from the planar scatterer. The total error can be kept to fairly low levels.<> View full abstract»

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  • Adaptive evaluation of the Sommerfeld-type integral using the chirp z-transform

    Page(s): 1788 - 1791
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    An adaptive integration algorithm for the computation of the Sommerfeld-type integral has been developed. The algorithm allows the user to specify the precise location of the desired range sample points and change the integration contour. In addition, the algorithm can be used to recompute the Sommerfeld-type integral with different range resolutions without recomputing the integrand. It is also shown how the algorithm can be used to adaptively increase the number of integration points required to evaluate the Sommerfeld-type integral. Numerical results show that the algorithm is useful in many interesting areas.<> 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