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Antennas and Propagation, IEEE Transactions on

Issue 6 • Date Jun 1992

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Displaying Results 1 - 21 of 21
  • Short-pulse wave scattering from a cylindrical shell with slit-coupled coaxial interior

    Page(s): 585 - 596
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (888 KB)  

    In a previous frequency domain study by the authors (see ibid., vol.39, p.1085-97, Aug. 1991), it was shown that the hybrid ray-mode system format introduced for scattering from cylindrical targets with aperture-coupled waveguide-like interiors continues to provide a cogent observable-based parameterization of the scattering in the time domain. Utilizing rigorous as well as phenomenological constructs, causal arrivals and physical turn-on times for well convergent resonant expansions are identified for the wavefront-dominated exterior as well as for the guided-mode-dominated interior wave processes. The validity of this parameterization of the scattered field is confirmed by interpretation of reference data generated via direct numerical Fourier inversion from the frequency domain to the time domain. Although applied so far only to concentric cylindrical geometry and single mode propagation in the interior, it is anticipated that the formalism can also accommodate nonconcentric and noncircular configurations and higher mode operating regimes View full abstract»

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  • Ocean imaging with two-antenna radars

    Page(s): 597 - 605
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    Synthetic-aperture radar (SAR) measurements are affected both by the radar backscattering cross section (σ0) and by the radial velocity (Vr) of the surface. An analysis of the capabilities of two-antenna SARs using a method for forming radial-velocity images to provide a general framework for evaluating the performance of different possible measurements is presented. Several key results are derived. One can measure σ0 and V r separately, given a properly designed SAR. The error in Vr depends on the thermal noise in the receivers, the spacing of the two antennas, the coherence time of the surface (τc) and the spatial resolution of the measurement. There is an optimal separation of the antennas proportional to vτ c, where v is the speed of the aircraft. In cases where radial velocities are unimportant, two-antenna SARs can be used to image azimuth-traveling waves with wavenumbers larger than the usual azimuth cutoff. The authors show how phase errors affect the accuracy of the measurement of Vr. In this development, they also provide a simple explanation for velocity bunching, a subject that has caused much controversy View full abstract»

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  • E-plane analysis of a modified horn antenna with suppressed far-out sidelobe level

    Page(s): 620 - 627
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    It is shown that the far-out sidelobes of a horn antenna can be suppressed by curving the edges in the aperture. A three-dimensional uniform geometrical theory of diffraction (UTD) analysis of the E -plane radiation pattern of the modified horn antenna was performed and compared with straight edges. This analysis indicates that the curvature of the edges does indeed influence the sidelobes to the effect that the modified horn antenna has significantly lower far-out sidelobes. The radiation patterns for modified horns were measured and compared with the calculated patterns and were found to be in good agreement View full abstract»

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  • Improved numerical diffraction coefficients with application to frequency-selective surfaces

    Page(s): 606 - 612
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    A method for numerically determining diffraction coefficients for arbitrary scattering centers is described. In this method finite bodies possessing scattering centers of the type of interest are first analyzed via the moment method. The various contributions to the total scattered fields are then isolated by solving low-order simultaneous equations obtained by writing expressions for the fields in terms of unknown diffraction coefficients. The method yields numerical diffraction coefficients in angular sectors where previous methods fail (e.g., near grazing angles), and can be applied in the context of measured as well as simulated scattering data. Finite frequency-selective surfaces are shown to be amenable to analysis with ray-optics techniques, and several two-dimensional examples are given with comparisons to far- and near-field moment method results View full abstract»

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  • A periodic moment method solution for TM scattering from lossy dielectric bodies with application to wedge absorber

    Page(s): 652 - 660
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    The periodic moment method (PMM) solution for the scattering from two-dimensional lossy dielectric bodies is developed. The purpose is to design a microwave wedge absorber for low reflectivity so that one can improve the performance of anechoic chamber measurements. With PMM, the reflection and transmission coefficients of periodically distributed bodies illuminated by a plane wave have been accurately calculated using a Cray Y-MP supercomputer. Through these studies, some wedge absorber configurations have been designed, fabricated, and then tested in the OSU/ESL compact range measurement facility. Two 8-in commercial wedges, a curved wedge, and a four-layer wedge, were studied. In all cases, good agreement between calculations and measurements was obtained View full abstract»

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  • Green's functions analysis of planar circuits in a two-layer grounded medium

    Page(s): 690 - 696
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    A moment method analysis of planar circuits in a layered medium is developed. The Green's functions of a two-layer grounded medium are used in order to take into account the effect of the surface wave, coupling, and radiation. Interpolation techniques are used to increase computational efficiency. The embedded conductors are modeled with triangular patches. Results for several configurations, including direct and proximity coupled radiators, are in good agreement with measurements and other calculations View full abstract»

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  • Modeling of resistive sheets in finite element solutions [EM scattering]

    Page(s): 727 - 731
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    A variational formulation is presented for modeling a resistive card in the context of the finite-element method (FEM). To validate this formulation, results based on a physical modeling of the resistive sheet are also presented. In this case, the resistive sheet is equivalently replaced by a thin dielectric layer. The modeling of such a layer in the usual manner leads to larger and consequently inefficient linear systems, which is the primary reason for resorting to a mathematical modeling of the resistive sheet. Results based on the mathematical and physical modeling are presented in connection with the scattering by a metal-backed cavity in a ground plane. These are used to validate the proposed mathematical model View full abstract»

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  • Near-field analysis of a thick lens and horn combination: theory and measurements

    Page(s): 613 - 619
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    At millimeter-waves quasi-optical systems are commonly designed based on the Gaussian beam and thin lens approximation. The accuracy of the Gaussian beam and thin lens approximation was studied in the case of a corrugated horn and a thick Teflon lens (focal length to lens diameter ratio f/D=1.2) combination at 87 GHz. A special near-field measurement system was constructed. A large disagreement between the measured and theoretical values with the approximative method was obtained. The measurements showed that the thin lens approximation breaks down when the distance from the input beam waist to the lens is less than 1.5 D. Theoretical values obtained with a thick lens model based on ray tracing and use of exact Huygens' aperture integration principle agree well with the measurements View full abstract»

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  • Inward-looking and outward-looking formulations for scattering from penetrable objects

    Page(s): 714 - 720
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    A number of approaches for solving the problem of scattering from two-dimensional penetrable objects using the finite-element method or other differential-equation-based solutions are reviewed. Different ways of joining the numerical solution on a finite domain to a field representation that satisfies the Sommerfield radiation condition in the infinite exterior domain are also reviewed within the paradigm of the equivalence principle of electromagnetics. Depending on the region where the solution is deduced first in an inherently sequential process, these approaches can often be classified as either inward-looking or outward-looking. Although inward-looking formulations may exhibit computational advantages, outward-looking formulations appear preferable for electrically large domains because of internal resonance difficulties that may arise when solving the isolated interior problem first in an inward-looking formulation View full abstract»

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  • Complementary reciprocity theorems in electromagnetic theory

    Page(s): 675 - 681
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    Two complementary reciprocity theorems are derived that are distinct from the reciprocity theorems of Rayleigh-Carson and Lorentz. An application of one of the theorems to a radiation problem is given. A one-dimensional version of the theorems is introduced first by using transmission lines as the models to illustrate some of the key concepts in the theory View full abstract»

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  • The use of curved segments for numerically modeling thin wire antennas and scatterers

    Page(s): 682 - 689
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (556 KB)  

    To reduce the number of segments required to model curved wire structures, a method of moments formulation using curved wire segments is presented. Piecewise quadratic wire segments are used to discretize the wire geometry. This algorithm is used to analyze a small wire loop antenna and multiple-arm Archimedian spiral antenna. For each example, a comparison is made between results obtained using this quadratic segment algorithm and a linear segment algorithm. These examples demonstrate that the memory required is significantly reduced and a reduction in the solution time is achieved when quadratic segments are used View full abstract»

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  • On the radial component of the electric field for a monopole phased array antenna focused in the near zone

    Page(s): 723 - 727
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    The near-zone radial and principal spherical components of the electric field for a linear phased array antenna that is focused at one to two aperture diameters in the near zone are investigated. The phased array antenna consists of thin monopole elements, which are analyzed by using the method of moments. A theoretical formulation is described and computer simulation results are presented. The results show that the radial component is negligible in the focused near-field region, which allows the principal component to be accurately computed from knowledge of only the tangential electric field on the focal plane View full abstract»

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  • Scanning characteristics of infinite arrays of printed antenna subarrays

    Page(s): 666 - 674
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    The analysis and scanning characteristics of several different types of infinite arrays composed of subarrayed printed dipole and microstrip patch elements are presented. The analysis is based on full-wave moment method theory, and includes mutual coupling between elements in the subarray as well as between subarrays. The effect of subarraying on scan blindness is demonstrated for arrays using two-element subarrays of printed dipoles and microstrip patches. Results are also given for the amount of power radiated in grating lobes. The effect of a subarray composed of one driven element and one parasitic element, and the use of a four-element synchronous subarray of microstrip patches to generate circular polarization are also considered. Data are given for impedance mismatch, power radiated into grating lobes, and the axial ratio; both square and rectangular patches area considered. Results are also shown for an infinite array of seven-element hexagonal subarrays of printed dipoles, and it is found that the large spacing between subarrays leads to a limited scan range View full abstract»

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  • The fast multipole method (FMM) for electromagnetic scattering problems

    Page(s): 634 - 641
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    The fast multipole method (FMM) developed by V. Rokhlin (1990) to efficiently solve acoustic scattering problems is modified and adapted to the second-kind-integral-equation formulation of electromagnetic scattering problems in two dimensions. The present implementation treats the exterior Dirichlet problem for two-dimensional, closed, conducting objects of arbitrary geometry. The FMM reduces the operation count for solving the second-kind integral equation from O(n3) for Gaussian elimination to O(n4/3) per conjugate-gradient iteration, where n is the number of sample points on the boundary of the scatterer. A sample technique for accelerating convergence of the iterative method, termed complexifying k, the wavenumber, is also presented. This has the effect of bounding the condition number of the discrete system; consequently, the operation count of the entire FMM (all iterations) becomes O(n4/3). Computational results for moderate values of ka, where a is the characteristic size of the scatterer, are given View full abstract»

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  • A stable procedure to calculate the transient scattering by conducting surfaces of arbitrary shape

    Page(s): 661 - 665
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    A solution procedure to obtain the transient scattering by arbitrarily shaped conducting objects directly in the time-domain using the marching-on-in-time method is presented. The late-time oscillations are eliminated by a simple stabilization procedure which involves a negligible amount of extra computation. Numerical results for surface current density and far-scattered fields are given for various structures and compared with other methods View full abstract»

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  • A generalized three-parameter (3-P) aperture distribution for antenna applications

    Page(s): 697 - 713
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    A versatile three-parameter (3-P) elliptical aperture distribution, which encompasses many of the existing circular aperture distributions, is presented. Characteristics of the 3-P distributions such as the edge taper, the taper efficiency, the aperture power, the closed-form far fields, the sidelobe levels, the beamwidth and beam efficiency are analyzed. Also derived is an asymptotic form of the far-field, based on which the far-angle features of the 3-P distributions such as the far-field envelope, the decay rate, and the positions of nulls are determined. Examples are presented to illustrate the improvements resulting from the enlarged parameter space for aperture models obtainable from the 3-P distributions. Issues concerning the total power associated with an aperture distribution are studied and the inadequacy of some commonly used approximation is demonstrated View full abstract»

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  • Implementation and application of resistive sheet boundary condition in the finite-difference time-domain method [EM scattering]

    Page(s): 628 - 633
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (432 KB)  

    Use of a resistive sheet boundary condition in the finite-difference-time-domain (FDTD) analysis of scattering problems involving a resistively coated dielectric object is described. An algorithm is introduced through an analysis of E-polarized scattering from a thin resistive strip. For a given resistance, numerical experiments indicate that algorithm stability is ensured for time sampling intervals chosen according to a specific criterion. Validity of the resultant FDTD method is verified in a comparison of computed E-polarized scattering data for several resistive strips with existing data. Results on the E-polarized scattering behaviour of a resistively coated dielectric strip as a function of surface resistances and angle of incidence are also presented. Extension to the H-polarized case and application of the present method to pulsed problems are briefly discussed View full abstract»

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  • E-polarized scattering from a conducting rectangular cylinder with an infinite axial slot filled by a resistively coated dielectric strip

    Page(s): 731 - 733
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    The potential use of resistive films for damping the resonance spikes observed in the radar cross section (RCS) spectrum of a partially open rectangular cavity is investigated using a recently developed finite-difference-time-domain (FDTD) method that utilizes the resistive-sheet boundary condition for the modeling of resistive films. Backscattering data obtained in the first resonant region for an E -polarized plane wave normally incident into the slotted side of the cavity are presented. It is shown that resonance behaviors can be eliminated completely with a low-resistance film that attenuates significantly the impinging wave. Poorer resonance damping performance is observed as the film resistance increases because more of the field is allowed to penetrate into the cavity. For the latter case, the presence of the resistive film lowers the Q-factor of the slotted cavity such that the resultant resonance spectrum is lower in strength and broader in bandwidth View full abstract»

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  • Efficient and accurate evaluation of external mutual coupling between compound broad wall slots

    Page(s): 733 - 737
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    The efficient and accurate evaluation of external mutual coupling between compound broad-wall slots cut in rectangular waveguides is addressed. The special case of longitudinal slots is also considered. Errors introduced by approximations used in some mutual coupling expressions are investigated and numerical results are presented for a range of waveguide and slot parameters. It is shown that for most applications, mutual coupling expressions can be determined in terms of single numerical integrals View full abstract»

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  • Nodal-based finite-element modeling of Maxwell's equations

    Page(s): 642 - 651
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    Weak forms are derived for Maxwell's equations which are suitable for implementation on conventional C0 elements with scalar bases. The governing equations are expressed in terms of general vector and scalar potentials for the electric field intensity vector. Gauge theory is invoked to close the system and dictates the continuity requirements for the potentials at material interfaces as well as the blend of boundary conditions at exterior boundaries. Two specific gauges are presented, both of which lead to Helmholtz weak forms which are parasite-free and enjoy simple, physically meaningful boundary conditions. A general and numerically efficient procedure for enforcing the jump discontinuities on the normal components of vector fields at dielectric interfaces and boundary conditions on curved surfaces is also given View full abstract»

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  • A note on spatially weighted subarray covariance averaging schemes

    Page(s): 720 - 723
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    In spatial smoothing, the effective correlation among the impinging sources goes to zero only asymptotically with the number of subarrays. K. Takao and N. Kimura (ibid., vol.AP-35, p.1389-96, Dec. 1987) and A. Paulraj et al. (IEEE J. Ocean Eng., vol.OE-12, p.163-72, Jan. 1987) proposed Toeplitzization and adaptively weighted subarray covariance averaging methods, respectively, wherein the impinging sources are perfectly decorrelated whenever the number of subarrays exceeds a minimum value. While both methods achieve the same result, the formulations of the two methods are different. The present work first suggests an alternate formulation of the same problem and then compares the three methods showing that the solutions obtained by them are equivalent 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