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Antennas and Propagation, IEEE Transactions on

Issue 5 • Date May 1991

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Displaying Results 1 - 25 of 28
  • Near-field imaging for conducting objects

    Publication Year: 1991 , Page(s): 600 - 605
    Cited by:  Papers (12)
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    A spherical back-projection algorithm is proposed to reconstruct near-field microwave images of conducting objects. In this imaging scheme, the object is mounted on a rotating pedestal and its backscattered near field is measured at discrete-frequency steps. Implementation of the method consists of first estimating the spherical projection (or range profile) at each aspect by inverse Fourier transforming the range-corrected scattered near field, and then coherently spherically back-projecting the range profiles into the image plane. The images are interpreted and predicted based on the procedure of the spherical back-projection method, and numerical and experimental results are presented to verify the effectiveness of the proposed algorithm View full abstract»

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  • An application of the boundary element method to the magnetic field integral equation

    Publication Year: 1991 , Page(s): 606 - 611
    Cited by:  Papers (12)
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    A boundary element method (BEM) for the solution of electromagnetic scattering problems using the magnetic field integral equation (MFIE) is discussed. The discretized form of the MFIE is written in indicial notation with no limitations placed on the order of either the geometric or functional approximation. By considering several different types of boundary elements, it is determined that geometric errors can be significant and degrade the accuracy of the numerical solution. It is shown that a higher-order approximation for the current could significantly improve the accuracy of the numerical solution. The superparametric boundary element in which the geometry was given quadratic approximation and the current was given linear approximation was more efficient than elements using lower-order approximations. The BEM results are compared to the results obtained using the dielectric bodies of revolution (DBR) code View full abstract»

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  • Beam squint determination in conic-section reflector antennas with circularly polarized feeds

    Publication Year: 1991 , Page(s): 612 - 619
    Cited by:  Papers (7)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (612 KB)  

    The beam squint phenomenon in circular-polarized-feed paraboloidal, Cassegrain, and Gregorian antennas is characterized. The geometrical optics/aperture field method is used to derive generalized squint-predicting formulas, the accuracy of which is verified by computer simulations based on diffraction analysis. From these formulas, the squint-free conditions are developed. An extension of these formulas is suggested to predict the squint angles for general multireflector antennas View full abstract»

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  • Characteristics of fading on low-elevation angle Earth-space paths with concurrent rain attenuation and scintillation

    Publication Year: 1991 , Page(s): 657 - 661
    Cited by:  Papers (17)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (376 KB)  

    Signal-level variations on Ku-band low-elevation Earth-space paths, where large signal fades due to rain and tropospheric scintillations sometimes occur simultaneously, are examined. The difference in the amplitude variation rate for rain attenuation and scintillation fading is used to extract the effects of the two phenomena individually from raw data with the aid of a filtering technique. Characteristics of signal fading during rain are discussed on the basis of the analysis. It is shown that the decrease of signal level due to scintillation is much smaller than that due to rain attenuation, particularly for time percentages below 0.1%. However, for time percentages above about 1%, the effect of scintillation becomes dominant and no longer negligible, particularly for low-margin systems operating at low elevation angles View full abstract»

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  • Scattering by two and three spheres computed by the generalized multipole technique

    Publication Year: 1991 , Page(s): 703 - 705
    Cited by:  Papers (2)
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    An analytic solution to the problem of scattering by multiple spheres was given by J.H. Bruning and Y.T. Lo (ibid., vol.AP-19, p.378-90 and 391-8, May 1971). Results from this analytic solution are compared to results obtained using the generalized multipole technique (GMT), a numerical method that uses vector spherical harmonics as basis functions. The boundary conditions are imposed at a discrete set of points and the solution is obtained by numerically inverting a system of linear equations. The problem is particularly well suited to the GMT because the surfaces are very smooth View full abstract»

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  • Scattering from a cavity-backed slit in a ground plane-TM case

    Publication Year: 1991 , Page(s): 661 - 663
    Cited by:  Papers (9)
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    Two-dimensional transverse-magnetic wave scattering from a cavity-backed slit in a ground plane is analyzed by the Harrington-Mautz generalized network formulation (1976). The admittance matrix of the cavity of arbitrary shape and medium is obtained by the finite-element method. The computed admittance matrix, added to the radiation admittance matrix of the equivalent magnetic current on a ground plane, is used to find a solution for the equivalent magnetic current on the slit. Numerical results for coated staggered cavities are included. Accurate results for the magnetic surface currents and radiation patterns have been obtained View full abstract»

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  • Calculation of transfer functions of three-dimensional indented objects by the physical optics approximation combined with the method of stationary space

    Publication Year: 1991 , Page(s): 585 - 590
    Cited by:  Papers (3)
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    Transient scattering by variously shaped objects is obtained from the transfer functions and the spectrum of the incident pulse by using the Fourier synthesis technique. In order to evaluate the transfer functions of indented three-dimensional smooth objects, an effective method based on the physical optics approximation combined with the method of stationary phase is presented. It is shown that for indented three-dimensional objects, there exist several stationary phase points, including the complex ones. It is confirmed from the numerical results that an accurate solution can be obtained by taking into account the complex stationary points. Also, for a complete solution, the creeping wave term should be included View full abstract»

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  • Finite element analysis of electromagnetic scattering from inhomogeneous cylinders at oblique incidence

    Publication Year: 1991 , Page(s): 645 - 650
    Cited by:  Papers (8)
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    The method of weighted residuals is used for the derivation of a convenient weak statement that is utilized for a finite-element solution of the boundary value problem for the time-harmonic electromagnetic scattering from linear, isotropic, composite cylinders at oblique incidence. The weighted-residual statement of the problem involves the z components of the electric and magnetic field and is such that the continuity of the tangential electric and magnetic fields at media interfaces is directly enforced. The finite-element solution is carried out using the bymoment method. Numerical results for echo widths and field distributions within cylinders of various shapes and compositions are presented. Comparisons of some of the results with those obtained from exact eigenfunction expansion and integral equation solutions demonstrate the validity of the method and the associated computer program View full abstract»

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  • Electromagnetic scattering from arbitrary shaped conducting bodies coated with lossy materials of arbitrary thickness

    Publication Year: 1991 , Page(s): 627 - 631
    Cited by:  Papers (35)
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    A simple and efficient numerical technique is presented to solve the electromagnetic scattering problem of coated conducting bodies of arbitrary shape. The surface equivalence principle is used to formulate the problem in terms of a set of coupled integral equations involving equivalent electric and magnetic surface currents which represent boundary fields. The conducting structures and the dielectric materials are modeled by planar triangular patches, and the method of moments is used to solve the integral equations. Numerical results for scattering cross sections are given for various structures and compared with other available data. These results are proved accurate by a number of representative examples View full abstract»

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  • Critical distance for grating lobe series

    Publication Year: 1991 , Page(s): 677 - 679
    Cited by:  Papers (6)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (236 KB)  

    Estimating a transition or critical distance above a planar periodic array of point sources radiating into an unbounded medium is considered. It is shown that as an observation point approaches a periodic planar phased array of point sources, the corresponding spectral-domain Green's function, or grating lobe series, converges much more slowly than an equivalent mixed-domain representation exhibiting Gaussian convergence. For a given argument of the Green's function, the critical transition distance above the array for which both representations take the same time to compute can be estimated numerically. Since phased array antenna structures with cavity- or waveguide-type backings often have dimensions that fall well below the critical distance, investigations of such structures would seem to benefit significantly from formulations incorporating such hybrid Green's functions View full abstract»

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  • Electromagnetic fields and the quasistationary approximation near a moving mirror

    Publication Year: 1991 , Page(s): 673 - 676
    Cited by:  Papers (1)
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    A detailed study of the electromagnetic fields generated by illuminating a plane mirror, in inertial motion, by a plane wave is described. A new and complete power series is obtained for the fields, and convergence conditions depending on the distance between the mirror and an observer are established. This series is shown to converge in bandpass systems provided the distance between the mirror and the observer is limited. The power series provides simple expressions for assessing the validity of quasi-stationary (QS) field approximations near the mirror. An example confirms that, with the exception of the immediate vicinity of wavefronts, QS approximations are valid under conditions arising in practical urban radio systems View full abstract»

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  • Scattering of electromagnetic waves by a system of two dielectric spheroids of arbitrary orientation

    Publication Year: 1991 , Page(s): 680 - 684
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (424 KB)  

    By means of modal series expansion of the incident, scattered, and transmitted electric and magnetic fields in terms of appropriate vector spheroidal eigenfunctions an exact solution is obtained to the problem of electromagnetic scattering by two dielectric spheroids of arbitrary orientation is obtained. The incident wave is considered to be a monochromatic uniform plane electromagnetic wave of arbitrary polarization and angle of incidence. To impose the boundary conditions at the surface of one spheroid, the electromagnetic field scattered by the other spheroids is expressed as an incoming field to the first one, in terms of the spheroidal coordinates attached to it, using rotational-translational addition theorems for vector spheroidal wave functions. The solution of the associated set of algebraic equations gives the unknown expansion coefficients. Numerical results are presented in the form of plots for the bistatic and backscattering cross sections of two lossless prolate spheroids having various axial ratios, center-to-center separations, and orientations View full abstract»

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  • The vector effective length of slot antennas

    Publication Year: 1991 , Page(s): 705 - 709
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (352 KB)  

    A suitable definition for the vector effective length of an arbitrary slot receiving antenna placed in a large conducting plane is presented, and a general formula is obtained for its derivation. To derive the definition and formula, the effective length of a general wire antenna is derived, and then an analogous method is applied to the slot problem. The relationship of the slot's effective length to that for a flat strip wire antenna driven by a specified current is obtained. Formulas for the lengths of some specific common slot antennas are derived from the general expression. The current sampling property of a small straight slot is discussed View full abstract»

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  • Time-domain analytic and numeric analysis of vertical dipoles in front of a dielectric planar interface

    Publication Year: 1991 , Page(s): 663 - 667
    Cited by:  Papers (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (388 KB)  

    Potentials excited by impulsive vertical electric and magnetic dipoles in a geometry of two dielectric media with a planar interface are studied both analytically and numerically. Closed-form expressions for the Hertzian potential are derived in cases when the observation point is on the axis of the dipole or when both the dipole and the observation point are located at the interface. Numerical results are given for more general observation points View full abstract»

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  • On internal higher order mode coupling in slot arrays

    Publication Year: 1991 , Page(s): 694 - 698
    Cited by:  Papers (11)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (432 KB)  

    Internal higher-order mode coupling between adjacent longitudinal radiating slots cut in a broad wall of a rectangular waveguide is examined rigorously. Method-of-moments solutions to pertinent coupled integral equations are investigated. Higher-order mode coupling effects on the amplitude and phase of the aperture electric field are studied as a function of waveguide and slot parameters. It is shown that higher-order mode coupling effects are accounted for, to a substantial extent in standard-height waveguides and almost completely in reduced-height waveguides, by including the TE20 mode coupling in the analysis View full abstract»

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  • Plane-wave reflection properties of two artificially hard surfaces

    Publication Year: 1991 , Page(s): 651 - 656
    Cited by:  Papers (23)  |  Patents (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (384 KB)  

    The concepts of artificially hard and soft surfaces are interpreted in terms of the plane-wave reflection properties of the surfaces, in particular the reflection phase angles for two orthogonal polarizations. Numerical results are presented for two types of nearly hard surfaces. The first is a surface with longitudinally oriented and dielectrically filled corrugations. The second is a surface with longitudinally oriented strips on a grounded dielectric substrate. Neither of the two realizations is ideally hard in the strictest sense of the concept, but a good approximation can be achieved over a limited bandwidth. The numerical results demonstrate how various geometrical parameters affect the behavior of the surfaces. One important result is that the strip-loaded surface is the most favorable with respect to bandwidth. Another is that the strip-loaded surface can be made arbitrarily thin, though at the expense of bandwidth View full abstract»

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  • Numerical analysis of finite frequency selective surfaces with rectangular patches of various aspect ratios

    Publication Year: 1991 , Page(s): 569 - 575
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (524 KB)  

    The finite frequency selective surface studied consists of a two-dimensional array, finite in one dimension and infinite in the other, of perfectly electrically conducting plates in free space. The analysis uses a Green's function formulation in the spectral domain, and the resulting equation is solved using the method of moments. Techniques are demonstrated for increasing the speed of the numerical computation so that the problem can be solved on a VAX 11-780 in a reasonable amount of time. Two types of basis functions, one subsectional and the other complete domain, are used to represent the currents on the patches. Both yield similar results and both are suitable for use in this problem. Techniques are demonstrated for approximating the tails of the infinite integrations which allow a more accurate solution than truncation. Results for scattered power density and patch currents illustrate the edge effect and incident angle dependencies. Results showing 3 dB bandwidth, resonance peak versus frequency, and scattered power versus frequency are plotted for patches with aspect ratios varying from 0.1 to 1 View full abstract»

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  • Analysis of mutual coupling between an infinite phased array of horizontal dipoles and vertical wires

    Publication Year: 1991 , Page(s): 591 - 599
    Cited by:  Papers (1)
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    A theoretical method is presented to analyze the radiation characteristics of an infinite phased array of horizontal dipoles in the presence of adjacent vertical wires protruding out of the ground plane. Green's functions for vertical and horizontal δ-sources in a unit cell are derived in terms of Floquet modes. By superposition of the Green's functions, the E- and H-fields in a unit cell are obtained. The current distributions and the input radiation impedances of the dipole array are obtained by making use of the method of moments. Numerical examples are given to show the coupling effects. Measurements of the radiation impedances in a waveguide simulator with and without the coupling effects due to the vertical wires have been conducted to verify the theoretical model. The general trend of measured data has been predicted quite well by the theoretical computation View full abstract»

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  • On the theory of the synthesis of offset dual-shaped reflectors-case examples

    Publication Year: 1991 , Page(s): 620 - 626
    Cited by:  Papers (8)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (536 KB)  

    In an earlier paper by V. Galindo-Israel et al. (see ibid., vol.AP-35, p.887-96, August 1987) the geometrical optics (GO) principles, constraints, and requirements of the dual- and single-offset-shaped reflector synthesis problem were collected and developed into a set of nonlinear first-order partial differential equations (PDEs). Methods of solving these PDEs numerically were illustrated, as were certain problems that may arise. An extension of the methods by which solutions to the PDEs can be obtained is presented, together with several case examples. These examples are independently analyzed by GO and physical optics diffraction methods. The starting point for the integration over each reflector can be taken on the outer rim, at the center, or at an intermediate point-the intermediate starting point being the more general case. The utility of the speed of this synthesis method is demonstrated. This makes practical the incorporation of the synthesis into a search algorithm that can optimize one or more parameters of the reflector system. As an example, the optimization of the mapping equations for low cross polarization is discussed View full abstract»

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  • Image theory for chiral bodies

    Publication Year: 1991 , Page(s): 676 - 677
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    Image theory for a chiral body over a perfect electric or magnetic conducting ground plane is developed using the chiral volume equivalence theorem and conventional image theory for electric and magnetic currents. It is shown that the image of the chiral body has the same material parameters as the original body, except that the chirality admittance of the image is the negative of the original. In essence the mirror image of a chiral object is that object, such as a helix, with the opposite handedness. Alternatively, the same results can be obtained using tensor analysis View full abstract»

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  • Experimental verification of the 2-D rooftop approach for modeling microstrip patch antennas

    Publication Year: 1991 , Page(s): 690 - 694
    Cited by:  Papers (19)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (500 KB)  

    Radiation pattern and input impedance measurements for a rectangular microstrip patch are used to establish the accuracy of a general moment-method approach. The approach uses 2-D rooftop basis functions to model arbitrarily shaped 3-D composite, conductor-dielectric structures. The numerical results show agreement with the measurements. However, both the experimental and theoretical results show considerable differences when compared with previously published analyses using pulse-basis functions. These differences and others cast doubts on the validity of using pulse basis functions to model scattering or radiation from composite structures View full abstract»

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  • Application of spherical wave expansions to reflector antennas using truncated field data

    Publication Year: 1991 , Page(s): 639 - 644
    Cited by:  Patents (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (536 KB)  

    The electromagnetic field radiated by paraboloidal and hyperboloidal reflector antennas excited by an azimuthally independent, linearly polarized source is considered. The electromagnetic field is sampled on a spherical surface in order to numerically generate a set of spherical wave mode coefficients which, in turn, are used to compute antenna patterns. Patterns are produced in the near- and far-field regions based on both full and partial, or truncated, near- and far-field data as a function of the antenna parameters and the truncation angle location. The similarity of the patterns obtained from truncated field data to the untruncated patterns is found to depend on the pattern level at the truncation point, the location of the truncation point, the antenna parameters, and the radial distance between the spherical surfaces on which the electromagnetic field is sampled and produced View full abstract»

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  • Exact mutual impedance between sinusoidal electric and magnetic dipoles

    Publication Year: 1991 , Page(s): 684 - 686
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (220 KB)  

    Analytic expressions are presented for the exact mutual impedance of sinusoidal electric and magnetic monopoles in a homogeneous medium. Numerical computations are carried out and presented. It is shown that computed values of mutual impedances for both linear monopoles and surface-patch monopoles in the near-overlapping case exhibit an excellent convergence to the theoretical values when monopoles are physically overlapped. This demonstrates the robust accuracy of the presented expressions in the near-singular integration kernel for the mutual impedance computation. The expressions are also compatible with existing techniques. Minor modifications are needed to enable existing programs to compute such mutual impedances View full abstract»

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  • Impact of linear array geometry on direction-of-arrival estimation for a single source

    Publication Year: 1991 , Page(s): 576 - 584
    Cited by:  Papers (7)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (708 KB)  

    The analysis is conducted using the Cramer-Rao lower bound simulations and performance modeling of maximum-likelihood estimation (MLE) while assuming a single source of illumination and additive white Gaussian noise. Particular attention is paid to the implementation of the MLE, the threshold signal-to-noise ratio (SNR), probability of outlier, and high SNR mean-squared-error (MSE) performance, which are evaluated and compared for uniform and nonuniform arrays. The conditions under which tradeoffs exist in choosing a particular geometry and their significance are determined View full abstract»

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  • Amplitude and phase-shaping effects in beamwaveguides

    Publication Year: 1991 , Page(s): 687 - 690
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (332 KB)  

    Results are presented of an investigation on improvements in a geometrical optics design of a beam-waveguide antenna for operation at multiple frequency bands. Improvements might be possible by changing the design of the lower-frequency input pattern to the beam waveguide. The effects of amplitude and phase shaping the input pattern have been studied with an aperture diffraction model. Accurate vector near-field computations were made rapidly with a spherical wave expansion of the input and scattered fields. Numerical results indicate that for aperture sizes of less than 30 wavelengths, there is insufficient control on defocusing due to amplitude and phase shaping. Design tradeoffs on spillover loss and defocusing are possible by changing the amplitude and phase distribution of the input wavefront for larger size apertures 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