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Antennas and Propagation Society International Symposium, 1990. AP-S. Merging Technologies for the 90's. Digest.

Date 7-11 May 1990

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Displaying Results 1 - 25 of 477
  • Application of absorbing boundary conditions to TLM simulations

    Page(s): 2 - 5 vol.1
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    It is demonstrated that the traditional method of match terminating the TLM (transmission line matrix) mesh does not accurately model a free space boundary. It is found that the match termination of the TLM mesh is accurate only if waves strike the boundary with normal incidence. A successful implementation of one type of FD-TD (finite-difference time-domain) absorbing boundary condition in TLM simulations is described. This boundary condition is superior to the traditional match termination of the TLM mesh.<> View full abstract»

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  • Application of time domain diakoptics to 3-D TLM method with symmetrical condensed nodes

    Page(s): 6 - 9 vol.1
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    The diakoptics (segmentation) technique is applied to TLM (transmission-line matrix) analysis of microwave circuits using Johns's (1987) 3-D symmetrical condensed node. It is shown how a microwave structure can be partitioned into substructures which are solved independently and later reassembled (all in the time domain). Diakoptics leads to considerable reduction in memory and CPU requirements for big structures since it allows numerical preprocessing of those parts of a large electromagnetic structure which remain unchanged during repeated analysis. To check the validity of the proposed approach, the algorithm was applied to compute the scattering parameters of a symmetrical capacitive iris with a gap width equal to 1.659 mm in the WR28 waveguide.<> View full abstract»

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  • On a correction to the voltage/field relationship used in the condensed node TLM method

    Page(s): 10 - 13 vol.1
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    A new relationship, based in part on Maxwell's equations, between the voltages pulses used in the condensed node TLM (transmission line matrix) method and the electromagnetic fields is given. A numerical comparison between the old relationships given by Johns (1987) is made using the impulse response of a perfect electric conductor (PEC) cavity. It is found that the formulas of Johns yield time-domain response which is highly oscillatory near the source region, whereas those given in the present work are more physically correct. In addition, the response based on the results given is found to be in good agreement with the finite-difference time-domain response.<> View full abstract»

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  • Finite-difference time-domain technique for antenna radiation

    Page(s): 14 - 17 vol.1
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    The finite-difference time-domain (FD-TD) technique is used to predict the radiation pattern of antennas mounted on ground planes, either perfectly conducting or of composite material, and other complex structures. The results obtained using the FD-TD technique are compared with results using the geometrical theory of diffraction (GTD) and measurements. The normalized power pattern of a quarter-wavelength monopole on a square perfectly conducting ground plane is displayed. In addition, results obtained using a circular perfectly conducting ground plane are exhibited. The FD-TD method predicts extremely well, as does the GTD, the ring source contributions of the circular ground plane which reflect in higher intensity lobes near the axis of the system ( theta =0 and 180 degrees ) as compared to those of the straight-edge ground plane.<> View full abstract»

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  • Time domain finite element analysis of 2-D shields

    Page(s): 18 - 21 vol.1
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    A two-dimensional time-domain application of a general finite-element mathematical package, IMSL:PDE/PROTRAN, is considered for two-dimensional shielding problems. The software runs on the IBM 3090 mainframe computer. The shielding problem is formulated via the magnetic vector potential and the Coulomb gauge. Validation of the program is performed via a simple test problem (plane wave incident on a perfect conductor). Results are presented for the case of a plane wave with double exponential time dependence incident on the shield.<> View full abstract»

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  • Time-domain scattering in 2 1/2 dimensions

    Page(s): 22 - 25 vol.1
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    A finite-difference time-domain (FDTD) method is used to solve the problem of the response of an arbitrary source, in particular, an impulsive point source, in a two-dimensional isotropic inhomogeneous medium. The field is three-dimensional, whereas the inhomogeneity is two-dimensional; hence, this is called a 2-1/2-dimensional problem. Taking advantage of the invariance of the geometry in one dimension, cosine and sine transforms are used to eliminate one of the spatial derivatives in Maxwell's equations, thereby reducing the problem to two dimensions. A rectangular staggered grid is used to discretize the equations. The complete solution is obtained by linearly superimposing several transformed field components. This provides great savings in terms of computer storage and run time over a three-dimensional FDTD method. The subsurface interface radar, in which an impulsive transmitter and an accompanying receiver are used to detect reflections from subsurface objects, is discussed as an example. Effects of increasing conductivity and depth of the buried objects on the quality of the measured signals are studied.<> View full abstract»

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  • Time-domain physical-optics analysis of large reflector antennas

    Page(s): 26 - 29 vol.1
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    The time-domain analysis of large reflector antennas is presented. Physical-optics time-domain analysis is used to determine the antenna characteristics under pulse-modulated RF excitation. The resulting far fields for a front-fed paraboloid are shown. The waveforms of the incident wave and scattered field are plotted. From these patterns one can determine buildup and decay transient time as a function of reflector diameter or observer look angle. Using the peak of the steady-state field for different look angles, one can determine the gain pattern of the reflector, which agrees exactly with the gain pattern obtained directly from frequency-domain analysis.<> View full abstract»

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  • Time domain scattering from curved surfaces

    Page(s): 30 - 33 vol.1
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    The time-dependent form of the electric field integral equation (EFIE) is used to solve transient scattering problems for perfectly conducting surfaces. The basis functions of Rao et al. (1982) and a time marching method-of-moments technique are used to solve the time-dependent EFIE. A special testing procedure and an averaging technique are used to overcome solution stability problems at late times. Numerical results obtained using the proposed algorithm are presented.<> View full abstract»

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  • Transient electromagnetic wave scattering by a dissipative dielectric sphere

    Page(s): 34 - 37 vol.1
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    The scattered field from a step-excited dielectric sphere is evaluated by the numerical Bromwich integration and a recently developed asymptotically compensated singularity expansion method. Both lossless and dissipative cases are considered. Excellent agreement is observed between the numerical results obtained using the two techniques. An interpretation of the various features in the return signal in terms of their physical origin is presented. It is concluded that spectral summation methods provide an effective means for the treatment of dissipation and dispersion phenomena which are encountered in realistic scatterers and wave supporting media. The use of proper asymptotics or its compensation allows not only a considerable reduction in the computation time required to achieve a given accuracy but also the determination of unknown entire functions, making the recovery of the early-time response possible.<> View full abstract»

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  • Application of the finite-difference time-domain method to electromagnetic scattering from 3-D chiral objects

    Page(s): 38 - 41 vol.1
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    It is shown how the finite-difference time-domain (FDTD) method has been adapted to handle electromagnetic wave propagation through chiral media. Two solutions to this problem are presented, enabling the FDTD method to handle propagation through chiral, as well as achiral, media. One solution is exact, involving the recursive (but still parallel) solution of the field update equations. This method produces results comparable to those obtained by theoretical means, as well as by other computational methods (the method of moments). The second solution has been implemented via a scattered-field formulation and has been successfully applied to problems of scattering from three-dimensional chiral objects.<> View full abstract»

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  • Solving the Helmholtz equation using multiply-propagated waves

    Page(s): 44 - 47 vol.1
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    The Helmholtz equation multiple propagator (HEMP) for solving system matrices from partial differential equation (PDE) models is described. It will be of order N/sup 1/2/ and N/sup 2/3/ faster than banded techniques commonly used for two-dimensional and three-dimensional problems having N unknowns. It is pointed out that HEMP could improve the efficiency of the only modeling technique suitable for problems involving penetrable, inhomogeneous objects. The two-point boundary-value problem analogy for HEMP is considered, and a feasibility test for HEMP is discussed.<> View full abstract»

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  • A recursive algorithm for reducing algorithmic complexity of scattering problems

    Page(s): 52 - 55 vol.1
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    A recursive algorithm for calculating the exact solution of field scattering from a dielectric object is proposed. As in the method of moments, the object is first divided into N subobjects. Then, every subject is treated as a single scatterer in an N-scatterer problem. The recursive algorithm is then employed to calculate the (n+1)-scatterer tensor-T matrix from the n-scatterer tensor-T matrix. With this recursive algorithm, the N-scatterer tensor-T matrix can be derived. From this N-scatterer tensor T-matrix, the scattered field from the object can be obtained. This results in an N/sup 2/ and a linear in N algorithm in the long wavelength limit rather than the N/sup 3/ algorithm as in the method of moments.<> View full abstract»

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  • On the scattering matrix formalism for electromagnetic modelling

    Page(s): 56 - 59 vol.1
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    The use of a collection of scattering matrices coupled with addition theorems as a framework for general electromagnetic problems is discussed. The author presents the basic machinery, illustrates some assumptions necessary to make this approach viable, and presents some directions for further research. This work began as an investigation of the effect of mutual coupling and was motivated by the desire to have an analytic theory that was nearly as accurate as the moment methods. Scalar modes and a scattering matrix were used instead of the traditional vector modes. The vector problem was solved inside a closed surface (such as a sphere) and the scattering matrix acted as an accountant of the relative strengths of the modes. In addition, the method is very closely related to the generalized multipole techniques.<> View full abstract»

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  • Impedance matrix localization produces a sparse moment method matrix

    Page(s): 60 - 63 vol.1
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    Basis and testing functions for moment-method calculations that make the resulting matrix sparse are discussed. Recent numerical results have verified theoretical predictions that a moment-method matrix with N/sup 2/ elements can be replaced by one with 100N nonzero elements. This technique is unlike some other techniques (e.g. k-space, differential equation, etc.) in that this is done without increasing N from its usual moment-method value. This technique also differs from many of these techniques (e.g. k-space. Rokhlin's factorization, etc.) because the resulting matrix is truly sparse, permitting solutions using efficient preconditioners for iterative techniques and possibly sparse direct techniques. For large moment-method problems (N>10000), the resulting savings in memory and execution time are both greater than a factor of 100. This impedance matrix localization technique applies to any moment-method formulation.<> View full abstract»

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  • Convergence acceleration techniques in the iterative solution of integral equation problems

    Page(s): 64 - 67 vol.1
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    The authors present some techniques that, when applied judiciously, can speed up convergence rates considerably or make convergence possible in otherwise impossible cases in connection with the iterative solution of integral equation problems. Particular attention is given to automatic renumbering (matrix, method), extrapolation schemes, and a multilevel approach. It is noted that the combined effect of these techniques would speed up the iterative process further.<> View full abstract»

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  • Iteration, multiple scattering and the sphere of influence technique

    Page(s): 68 - 71 vol.1
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    An iterative scheme, the sphere of influence (SOI) technique, specifically designed around the electromagnetic (EM) scattering problem is described. This method presents an alternative to other classical iterative methods as well as to newer schemes e.g., small domains coupled by Schelkunoff-type sources. The SOI technique is based on an iterative interpretation of multiple scattering first described by Tai (1970). As a simple example, SOI was used to compute backscatter from a small spherical cloud (random array) of 100 randomly oriented resonant wires.<> View full abstract»

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  • Diakoptic theory and the moment method

    Page(s): 72 - 75 vol.1
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    The modified diakoptic theory is described, and its relationship to the method of moments is explained. Utilization of the theory is illustrated by an example application of the simple curved-wire antenna. A numerically efficient iteration method for computing open-port currents on a structure is outlined, as is the use of these currents to obtain open-port impedance elements characteristic of the diakopted structure. It is pointed out that the rank of the diakoptic matrix is far smaller than that of the moment-method matrix. For those antennas and circuits to which the modified diakoptic theory is applicable, the lower rank of the matrix admits greater numerical efficiency in complex and/or large structures.<> View full abstract»

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  • Application of integral equation and method of moments for electrically very large scatterers using spatial decomposition technique

    Page(s): 76 - 79 vol.1
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    The spatial decomposition technique is successfully implemented to analyze electromagnetic scattering by both two-dimensional and three-dimensional perfectly conducting and dielectric scatterers. Two canonical scattering cases are reported. Electromagnetic scattering by electrically large two-dimensional perfectly conducting thin strip scatterers is illustrated. The distribution of the bistatic radar cross section for a 10 lambda strip scatterer with transverse magnetic excitation is also given. The strip scatterer is divided into 10 subzones for implementation of the spatial decomposition technique. The spatial decomposition data for the first sweep are compared. Results of the monostatic radar cross section are given for the same scatterer, but with transverse electric excitation. Good agreement with the direct method-of-moments solution is also shown.<> View full abstract»

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  • Rapid solution of integral equations fast numerical algorithms

    Page(s): 80 - 81 vol.1
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    Summary form only given. The author discusses an algorithm for the rapid solution of boundary value problems for the Helmholtz equation in two dimensions based on iteratively solving integral equations of scattering theory. The algorithm requires an amount of work proportional to n/sup 4/3/, where n is the number of nodes in the discretization of the boundary of the scatterer, and, when it is combined with a generalized conjugate residual type algorithm, the resulting process takes very few iterations to converge, leading to an order n/sup 4/3/ algorithm for the solution of the original scattering problem. A fairly straightforward refinement of the scheme, reducing its CPU time requirements from O(n/sup 4/3/), to O(n log (n)), has also been considered.<> View full abstract»

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  • Frequency selective scattering properties of dielectric gratings

    Page(s): 84 - 87 vol.1
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    The extension of the optoacoustic scattering phenomenon to microwave and millimeter-wave applications has been investigated by many researchers using various approaches, including the coupled-wave, leaky-wave theories among the analytic methods and the unimoment, finite-difference formulations among the numerical techniques. The present study indicates that most of these methods are very time-consuming in terms of obtaining relevant data such as reflection and transmission coefficients at a single frequency. Therefore, a computationally efficient method of solution based on fundamentally sound principles needs to be chosen for the purpose of simulating the scattering process from a dielectric grating over a wide band of frequencies. Only Kong's (1977) second-order coupled-wave approach has been found to suit this purpose well. As an example, the reflection and transmission coefficients of a dielectric grating with a thickness to modulation periodicity ratio of 1.713 were determined.<> View full abstract»

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  • Design of a cascaded frequency selective surface as a dichroic subreflector

    Page(s): 88 - 91 vol.1
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    The frequency selective surface (FSS) is often used as a filter for electromagnetic radiation to separate the S- and K-band feed in a communication satellite. The problem of synthesizing such a multifrequency-band FSS as a subreflector in a communication satellite antenna system that covers the frequency range of 2-30 GHz is considered. The reflection and transmission properties of FSS screens are calculated using the spectral-Galerkin method in conjunction with the cascading technique. The frequency responses of one-layer, two-layer, and three-layer screens are shown. The frequency response of a four-legged FSS was also computed. It is shown that the square-aperture-type FSS performs better than the four-legged configuration.<> View full abstract»

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  • A generalized scattering matrix analysis for cascading FSS of different periodicities

    Page(s): 92 - 95 vol.1
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    A generalized scattering matrix for cascading frequency-selective surfaces (FSSs) of different periodicities is developed by studying the interaction of the harmonics when scattered between two screens of different periods. It is noted that the new system, formed by stacking two (or more) screens of different periods, will have a scattered spectrum which is, in general, more densely packed than the spectra of any of the individual component screens. Using this approach, one can apply the old cascading equations to generate composite systems matrices (for FSSs radomes). The procedure was carried out for a system of two stacked strip gratings with different periodicities.<> View full abstract»

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  • A frequency selective surface using aperture coupled microstrip patches

    Page(s): 96 - 99 vol.1
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    A new type of frequency-selective surface (FSS) that uses two back-to-back arrays of aperture coupled microstrip antennas is described. The resulting bandpass response is relatively narrowband, which may be an advantage in applications where frequency-selective surfaces are used as radomes, as dichroic surfaces, or for antenna radar cross section (RCS) reduction. The frequency response can be controlled, to some extent, by adjusting the size of the coupling apertures. Also, by proper arrangement of the coupling apertures, the FSS can be made to pass linear or dual-polarized waves. A full-wave infinite array analysis of this FSS has been completed, and calculated results showing the effect of aperture size on the transmission coefficient are presented, along with data on the variation of the transmission coefficient with frequency and incidence angle. Measurements from waveguide simulators were used to confirm the theory.<> View full abstract»

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  • A simple circular polarization selective surface (CPSS)

    Page(s): 100 - 103 vol.1
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    A new circular polarization selective surface (CPSS) (a surface that reflects one sense of circular polarization but transmits the other) is described. It is easy to fabricate and has excellent electrical characteristics. This CPSS element is made of a single piece of wire bent in three sections. The two end sections are on the faces of a dielectric slab and the middle section goes through it. This resonating element does not use transmission lines, and printed circuit technology can be used for fabrication above a few gigahertz. Results of a simulation of a 25-element CPSS are presented. Preliminary experimental results confirmed the theoretical findings.<> View full abstract»

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  • Frequency selective surfaces with time-varying nonlinear loads

    Page(s): 104 - 107 vol.1
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    An approach to a periodic surface loaded with nonlinear devices is presented which uses an equivalent circuit representation at the load location. This method is applicable to all angles of incidence, and it takes into account the effect of dielectric substrate and superstrate layers. It is also applicable to periodic surfaces with arbitrarily placed loading and arbitrarily shaped array elements. Specifically, a unique development combining the frequency-domain data of a frequency selective surface (FSS) with time-varying loads is demonstrated. It is based on the moment method solution of the spectral domain formulation for variable surface impedance and lumped elements. Nonlinearly loaded surfaces can disperse energy into many scattered frequencies. Applications would include frequency multipliers, frequency modulators, and beam steering.<> View full abstract»

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