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Antennas and Propagation, IEEE Transactions on

Issue 2 • Date Feb 2001

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Displaying Results 1 - 20 of 20
  • Scattering of electromagnetic waves by a metallic object partially immersed in a semi-infinite dielectric medium

    Page(s): 223 - 233
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    The concept of the generalized scattering amplitude is applied to the electromagnetic (EM) scattering of an arbitrarily shaped, perfectly conducting object partially immersed in a semi-infinite dielectric medium. The dielectric medium can have either electric loss or magnetic loss or both. In a two-dimensional (2-D) formulation, after the outgoing cylindrical wave is factored out from the scattered wave, the remaining wave envelope component in the scattered wave is defined as the generalized scattering amplitude. The transformed Helmholtz equation in terms of the generalized scattering amplitude can be solved numerically using a finite-difference method over the entire scattering domain including both the semi-infinite free-space (vacuum or air) and the semi-infinite dielectric medium. Example problems of scattering by infinitely long, perfectly conducting cylinders of circular and trapezoidal ship-shaped cross sections are solved to demonstrate the theoretical formulation and numerical method. The radial profiles of the generalized scattering amplitude and the total field over the entire scattering region are also presented, and their properties are discussed. The far-field bistatic cross section and induced current density on the obstacle's surface are also presented. These results show that the method can be used to yield complete and accurate solutions to 2-D EM scattering problems involving arbitrarily shaped metallic objects partially immersed in a penetrable semi-infinite dielectric medium. View full abstract»

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  • A cylindrical spiral patch array

    Page(s): 196 - 199
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    A simplified approach to the analysis of the patterns of a cylindrical spiral patch array is presented. In this approach, the patch element is characterized using the cavity model and the radiation patterns for each element are calculated, assuming it is mounted on a flat surface, while the curvature effects are considered in the patterns of the array. Some numerical results and experimental data are presented. The validity of the calculation is demonstrated by comparison with measured data View full abstract»

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  • Shadow boundary incremental length diffraction coefficients applied to scattering from 3-D bodies

    Page(s): 200 - 210
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (276 KB)  

    Shadow boundary incremental length diffraction coefficients (SBILDCs) are high-frequency fields designed to correct the physical optics (PO) field of a three-dimensional (3-D) perfectly electrically conducting scatterer. The SBILDCs are integrated along the shadow boundary of the 3-D object to approximate the field radiated by the nonuniform shadow boundary current (the difference between the exact and PO currents near the shadow boundary). This integral is added to the PO field to give an approximation to the exact scattered field that takes into account both PO and nonuniform shadow boundary currents on the scatterer. Like other incremental length diffraction coefficients, any SBILDC is based on the use of a 2-D canonical scatterer to locally approximate the surface of the 3-D scatterer to which it is applied. Circular cylinder SBILDCs are, to date, the only SBILDCs that have been obtained in closed form. In this paper, these closed-form expressions are validated by applying them for the first time to a 3-D scatterer with varying radius of curvature-the prolate spheroid. The results obtained clearly demonstrate that for bistatic scattering the combined PO-SBILDC approximation is considerably more accurate than the PO field approximation alone View full abstract»

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  • Effects of irregular terrain on waves-a stochastic approach

    Page(s): 250 - 259
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    This paper treats wave propagation over an irregular terrain using a statistical approach. Starting with the Helmholtz wave equation and the associated boundary conditions, the problem is transformed into a modified parabolic equation after taking two important steps: (1) the forward scatter approximation that transforms the problem into an initial value problem and (2) a coordinate transformation, under which the irregular boundary becomes a plane. In step (2), the terrain second derivative enters into the modified parabolic equation. By its location in the equation, the terrain second derivative can be viewed as playing the role of a fluctuating refractive index, which is responsible for focusing and defocusing the energies. Using the propagator approach, the solution to the modified parabolic equation is expressed as a Feynman's (1965) path integral. The analytic expressions for the first two moments of the propagator have been derived. The expected energy density from a Gaussian aperture antenna has been analytically obtained. These expressions show the interplay of three physical phenomena: scattering from the irregular terrain; Fresnel phase interference; and radiation from an aperture. In particular the obtained expected energy density expression shows contributions from three sources: self energy of the direct ray; self energy of the reflected ray; and cross energy arising from their interference. When sufficiently strong scattering from random terrain may decorrelate the reflected ray from the direct ray so that the total energy density becomes the algebraic sum of two self energies. Formulas have been obtained to show the behavior of the energy density function View full abstract»

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  • Input impedance of the slot-fed dielectric resonator antenna with/without a backing cavity

    Page(s): 307 - 309
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    Slot-fed dielectric resonator antennas with or without a backing cavity are studied theoretically and experimentally. A coaxial cable is used to excite the slot. The input impedances and return losses of the antennas are found and discussed View full abstract»

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  • Adaptive multiresolution antenna modeling using hierarchical mixed-order tangential vector finite elements

    Page(s): 211 - 222
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    Hierarchical mixed-order tangential vector finite elements (TVFEs) for tetrahedra are attractive for accurate and efficient finite element or hybrid finite element/boundary integral (FE/BI) simulation of complicated electromagnetic problems. They provide versatility in the geometrical modeling of physical structures, guarantee solutions free of spurious modes, and allow a local increase of resolution by combination of mixed-order TVFEs of different orders within a computational domain. Regions with higher order mixed-order TVFEs can be selected a priori or be found adaptively via methods for a posteriori error estimation or indication. This paper demonstrates the merits of various explicit residual methods for a posteriori error indication via adaptive refinement of FE/BI solutions of electromagnetic radiation problems using hierarchical mixed-order TVFEs for tetrahedra View full abstract»

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  • Efficient mode-matching analysis of discontinuities in finite planar substrates using perfectly matched layers

    Page(s): 185 - 195
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    A new method to determine the reflection of substrate modes in finite substrate planar circuits is proposed. The perfectly matched layer (PML) concept is used to transform the open problem into a closed one. The discrete set of substrate, evanescent, and Berenger modes of the resulting anisotropic waveguides are then used in a mode-matching scheme to deduce the scattering coefficients of the substrate modes for oblique incidence on the edge of the substrate. We show results for single- and double-layered substrates and compare with finite-difference time-domain (FDTD) results. The combined perfectly matched layer (PML) mode-matching technique turns out to be very efficient View full abstract»

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  • Closed-form expressions for the numerical dispersion and reflection in FEM simulations involving biaxial materials

    Page(s): 158 - 164
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    Closed-form expressions for the numerical errors caused by finite-element discretization of problems involving materials of biaxial permittivity and permeability tensors are developed. In particular, we derive expressions for the numerical dispersion and reflection in both first-order node and edge basis function finite-element formulations in an equilateral triangular mesh. Results using these closed-form expressions are compared to practical numerical simulations. The application of these expressions to the analysis of the performance of the perfectly matched layer boundary is suggested View full abstract»

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  • A complete physics-based channel parameter simulation for wave propagation in a forest environment

    Page(s): 260 - 271
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (332 KB)  

    At HF through UHF frequencies, wave propagation in a forest environment is mainly attributed to a lateral wave which propagates at the canopy-air interface. Due to the existence of tree trunks, significant multiple scattering also occurs which is the dominant source of field fluctuations. Basically, the current induced in the tree trunks by the source and the lateral wave reradiate and generate higher order lateral waves and direct scattered waves. Using a full-wave analysis based on the method of moments in conjunction with Monte Carlo simulations, the effect of multiple scattering among a very large number of tree trunks is studied. It is shown that only scatterers near the source and the observation points contribute to the field fluctuations significantly. This result drastically simplifies the numerical complexity of the problem. Keeping about 200 tree trunks in the vicinity of the transmitter dipole and the receiver point, a Monte Carlo simulation is used to evaluate the statistics of the spatial and spectral behavior of the field at the receiver. Using a wide-band simulation, the temporal behavior (impulse response) is also studied as is performance of antenna arrays and the effects of different spatial diversity combining schemes in such a multipath environment View full abstract»

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  • A hybrid BEM/WTM approach for analysis of the EM scattering from large open-ended cavities

    Page(s): 165 - 173
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (192 KB)  

    An effective hybrid boundary-element method (BEM) and wavelet-transform method (WTM) is proposed to analyze electromagnetic scattering from three-dimensional (3-D) open-ended cavities with arbitrary shapes. This hybrid technique formulates the original cavity problems by a magnetic field integral equation. The BEM is employed to establish the mapping between the original complex integral surface and the unit square. The WTM is used to reduce the density of the moment matrix. Since a surface integral equation has to be solved, the WTM requires a two-dimensional (2-D) wavelet basis to implement the numerical computation. The previous fast iterative algorithm for 2-D wavelets has been extended for efficiently constructing various 2-D wavelet basis functions by a tensorial product from two one-dimensional (1-D) regular multiresolution analyses. Unlike the conventional method of moments, the proposed hybrid technique can always obtain sparse moment matrix equations, which can be efficiently solved by sparse solvers. As the level scales for numerical discretization of cavities increase, larger compression rates can be obtained, which makes it possible for the hybrid BEM/WTM technique to efficiently solve scattering from large open-ended cavities with complex terminations. Numerical results are presented to demonstrate the merits of the proposed method View full abstract»

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  • A low-sidelobe partially overlapped constrained feed network for time-delayed subarrays

    Page(s): 280 - 291
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (248 KB)  

    Completely overlapped space-fed subarrays have been shown to provide sufficient pattern control to enable (modestly) wide-band arrays using time delays at the input to each subarray and phase shifters at the array face. These configurations are bulky, but have been proposed for space-based use as well as for certain ground-based applications that do not have severe volume constraints. Other applications for space-based and airborne radar require much more compact, constrained array feed networks, but until now there have been few appropriate constrained networks for inserting time delay at the subarray ports without causing high sidelobes. This paper describes one such network that, at the outset, provided far lower sidelobes than the usual contiguous subarrays, but retained closely spaced high lobes near the main beam. This paper presents a synthesis procedure that alters the subarray patterns and reduces nearly all array sidelobes to levels determined by tolerance errors. Several examples are presented that synthesize sidelobes at -40 dB. The resulting network operates over 70% to 80% of the maximum theoretical bandwidth View full abstract»

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  • Two-dimensional array beam scanning via externally and mutually injection-locked coupled oscillators

    Page(s): 243 - 249
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (192 KB)  

    The use of arrays of injection-locked voltage-controlled oscillators coupled to nearest neighbors has been proposed as a means of controlling the aperture phase of one and two-dimensional (2-D) phased-array antennas. It has been demonstrated both theoretically and experimentally that one may achieve linear distributions of phase across a linear array aperture by injection locking to an external oscillator the end oscillators of an array of a mutually injection-locked oscillators. These linear distributions cause steering of the radiated beam. It is demonstrated theoretically here that one may achieve beamsteering in a similar manner in two dimensions by injecting appropriately phased signals into the perimeter oscillators of a 2-D array. The analysis is based on a continuum representation of the phase previously developed in the context of beamsteering via tuning of the perimeter oscillators View full abstract»

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  • Design and performance of large phased arrays of aperture stacked patches

    Page(s): 292 - 297
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    We present a technique to achieve large-bandwidth and low-profile printed phased arrays that can give good sky coverage; that is, a reasonable scan range and, therefore, may be suited to applications such as radio astronomy. The microstrip arrays consist of aperture stacked patches and can exhibit VSWR <2:1 bandwidths in excess of an octave over a scanning range of ±45° in the principal planes. A thorough investigation of these microstrip phased arrays is given using an infinite array analysis and a design strategy to achieve broad-band characteristics is proposed. It is shown that the element spacing and the dielectric material thicknesses are critical in optimizing the scan/bandwidth performance View full abstract»

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  • Analysis and design of L-probe proximity fed-patch antennas

    Page(s): 145 - 149
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    In this paper, the finite-difference time-domain (FDTD) method is employed to analyze L-probe proximity-fed rectangular patch antennas. Numerical results for the input impedance, co- and cross-polarization radiation patterns are presented and compared with the measurements. Good agreement between the computed and measured results is obtained. The effects of geometric parameters on the characteristics of the L-probe patch antenna are extensively studied. For design purposes, the variation of input impedance at resonance with different geometric parameters is plotted on a Smith chart. Mutual coupling between two L-probe patch antennas is also investigated View full abstract»

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  • On the dynamics of two-dimensional array beam scanning via perimeter detuning of coupled oscillator arrays

    Page(s): 234 - 242
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (256 KB)  

    Arrays of voltage-controlled oscillators coupled to nearest neighbors have been proposed as a means of controlling the aperture phase of one-dimensional (1-D) and two-dimensional (2-D) array antennas. It has been demonstrated, both theoretically and experimentally, that one may achieve linear distributions of phase across a linear array aperture by tuning the end oscillators of the array away from the ensemble frequency of a mutually injection-locked array of oscillators. These linear distributions result in steering of the radiated beam. It is demonstrated theoretically that one may achieve similar beamsteering in two dimensions by appropriately tuning the perimeter oscillators of a 2-D array. The analysis is based on a continuum representation of the phase in which a continuous function satisfying a partial differential equation of diffusion type passes through the phase of each oscillator as its independent variables pass through integer values indexing the oscillators. Solutions of the partial differential equation for the phase function exhibit the dynamic behavior of the array during the beamsteering transient View full abstract»

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  • On the quantized excitation and the geometry synthesis of a linear array by the orthogonal method

    Page(s): 298 - 303
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    A method for the geometry synthesis of a linear array is presented. We start from an initial array with quantized amplitudes. After this, we perturb the element positions by combining an iterative technique with the orthogonal method. The final position of the elements is found from the last iteration where the desired approximation of the pattern is obtained. Arrays with more constraints on the pattern need more quantized amplitudes. Several examples for different cases show the applicability of our method View full abstract»

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  • Reconfigurable contour beam-reflector antenna synthesis using a mechanical finite-element description of the adjustable surface

    Page(s): 272 - 279
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (372 KB)  

    A synthesis technique based on a mechanical finite-element (FEM) surface description of dual-offset reflector (DOSR) surfaces is described. This technique is used to implement a reconfigurable contour beam for a geostationary satellite application. The mechanical finite-element description of the surface takes into account the mechanical properties of the reflector surfaces and the effect of a finite number of forces applied at control points on the surface. Mechanical adjustment is achieved using high-deflection stacked piezoelectric actuators attached to the rear of a lightweight flexible subreflector surface. Both the main and subreflector surfaces are shaped for an initial required geographical coverage zone and then the main reflector surface is assumed to be fixed in shape. The variable shape subreflector is then synthesized to achieve the desired illumination for different geographical regions and different geostationary positions. Based on a study of the effect of the mechanical properties of the subreflector surface and actuator number and placement, a practical design has been completed and the ability to reconfigure the beam has been verified View full abstract»

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  • Surface impedance determination of a planar boundary by the use of scattering data

    Page(s): 304 - 307
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    A method to determine the surface impedance of a planar boundary through the remote field measurements is addressed. The surface impedance is recovered from the boundary condition itself, which requires knowledge of the field distribution and its derivative on the surface. The measured data is analytically continued to the boundary to obtain the required field expressions View full abstract»

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  • A system for unobtrusive measurement of surface currents

    Page(s): 174 - 184
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    A backpropagation technique is investigated that uses the plane wave spectrum to reconstruct the subwavelength features of the current distribution on flat surfaces without damaging the surface under measurement. The technique employs spectral and spatial filtering of the measured signal to capture the propagating spectrum as well as a portion of the evanescent spectrum. Special consideration is given to avoiding exponential amplification of measurement noise during backpropagation of the evanescent spectrum. This approach uses equipment commonly available in antenna measurement laboratories, for example, a network analyzer, a horn antenna, a magnetic field probe, and an automated positioning apparatus for the probe. The approach is first studied using computer simulations. The results from the simulations are then used as a guide to develop an actual measurement system. The measurement system is constructed and tested by measuring the surface current on a slotted plate. The measured current distributions are compared with results from finite-difference time-domain (FDTD) computations to demonstrate the feasibility of the technique View full abstract»

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  • A shared-aperture dual-band dual-polarized microstrip array

    Page(s): 150 - 157
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (348 KB)  

    This paper describes the design and testing of a prototype dual-band dual-polarized planar array operating at L- and X-bands. The primary objectives were to develop new antenna technology with dual-band and dual-polarization capability in a shared aperture, featuring low mass, high efficiency, and limited beam scanning. The design of a prototype planar microstrip array of 2×2 L-band elements interleaved with an array of 12×16 X-band elements that meets these requirements is discussed in detail and measured results are presented. The array is modular in form and can easily be scaled to larger aperture sizes 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