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Antennas and Propagation, IEEE Transactions on

Issue 4 • Date April 1996

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Displaying Results 1 - 22 of 22
  • Comments on "On the use of /spl rho/-algorithm in series acceleration"

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    The author draws the attention of the readers to the paper of Singh and Singh (see ibid., vol.39, p.1514-16, Oct. 1991) that an extremely simple and elegant method of handling this age-old problem may be found in Morse and Feshbach (1953). Suggested by the geometry of the original problem for the Helmholtz equation, one first determines the solution of the Poisson's equation (setting the wavenumber to zero) with appropriate boundary conditions and sources in closed form, as well as an infinite series. A useful feature of the series solution is that each term resembles a corresponding term in the original series solution of the Helmholtz equation, and for large values of the summation index, term by term, equality is approached. By adding the closed form answer, subtracting the series solution, and rearranging, one realizes fast convergence. Yet another gratifying feature is that convergence of the transformed series can be easily proven. View full abstract»

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  • A novel quasi-optical monopulse-tracking system for millimeter-wave application

    Page(s): 466 - 477
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    A novel, low-loss, polarization-independent quasi-optical monopulse comparator (QOMC) has been developed as an alternative to the commonly used waveguide comparator. It has been implemented as an experimental model with an aperture of 23×23 wavelengths for a 140-GHz monopulse tracking antenna fed by a beam waveguide. The QOMC consists of two cascaded identical stages (sections) to perform azimuth and elevation angle measurements. Each comparator stage is composed of only two plane-parallel reflectors and a low-loss quasi-optical 3-dB beam splitter symmetrically positioned in between them. Extensive model calculations of a single QOMC stage with square aperture dimensions between 10 and 25 wavelengths, and experimental investigations of the realized two-stage QOMC in a beam-waveguide field were performed. Owing to its sufficient high dynamic range at bandwidths of up to about 300 MHz, and particularly, its low absorptive loss of less than 3.5% per stage at 140 GHz, this QOMC is suited ideally for millimeter-wave applications View full abstract»

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  • A three-wave FDTD approach to surface scattering with applications to remote sensing of geophysical surfaces

    Page(s): 504 - 514
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    A major difficulty in physical interpretation of radio wave scattering from geophysical surfaces is the lack of detailed information on the signatures of geologically plausible discrete objects. Although the aggregate response will never be dominated by any single object, differences in the population of discrete objects on or near the surface (their sizes and shapes, for example) can change the character of a radio echo markedly. When the average surface is modelled as a flat, homogeneous half-space, the field that “drives” the scattering process is a composite consisting of the incident plane wave and the reflected and transmitted plane waves, all of which are known quantities; the total field can then be defined as the sum of the driving field and the scattered field. When a discrete object is near the surface, the total field can be calculated using finite-difference time-domain (FDTD) techniques, and the scattered near field can be calculated accordingly. The Green's functions for electric and magnetic currents above and below the surface, obtained by Sommerfeld theory and employed in conjunction with Huygens' principle, transform the local scattered fields to the far field. The FDTD implementation accommodates discrete lossy dielectric and magnetic scatterers in the vicinity of a dielectric surface; extension to a lossy half-space is straightforward. Two-dimensional results for scattering from perfectly conducting circular cylinders above and below a dielectric surface agree with moment method solutions within a few percent. Results for scattering from a dielectric wedge exhibit expected forward diffraction and internal reflection phenomena View full abstract»

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  • Effects of the variability of atmospheric refractivity on propagation estimates

    Page(s): 460 - 465
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    The temporal and spatial sampling requirements of atmospheric refractivity used to calculate electromagnetic (EM) propagation effects (propagation factor, equivalently path loss, etc.) is a subject of debate. Results are presented from a propagation experiment designed to address these questions. The objective in propagation estimation is assumed to be the minimization of the error of propagation estimates over the entire time interval for which individual estimates are used. It is found that the minimum error of propagation estimates over the interval of their use can be calculated from radio measurements. The error of propagation estimates calculated from horizontally homogeneous refractive structures is compared to the minimum error. It is found that for beyond-line-of-sight overwater EM propagation factor calculations at VHF and UHF in the southern California coastal region, the assumption of horizontal homogeneity leads to little more error than the described minimum error. It is also found that estimates based upon range-dependent refractive structures will provide substantially less error than estimates based upon homogeneous refractive structures only if they are sampled at intervals of two hours or less View full abstract»

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  • Minimization of the plane-wave scattering contribution of inverted-Y strut tripods to the noise temperature of reflector antennas

    Page(s): 492 - 499
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    The article is concerned with determining the metallic strut cross section of inverted-Y tripods used to minimize the plane-wave scattering contribution to the noise temperature of ground-station reflector antennas and radio telescopes. This is accomplished by numerically optimizing the cross section for minimum ground-noise pickup over the antenna elevation-angle operation range. Advantage is taken of the fact that although the struts' cross-section perimeter can be comparable to the operation wavelength, their electrical lengths are usually long. This allows the struts' scattering characteristics to be determined by solving standard two-dimensional field integral equations. The article concentrates on the top strut of an inverted-Y configuration since it is responsible for the dominant ground noise pickup associated with the plane-wave scattering. Numerical simulations are presented to substantiate this fact and a simple and convenient broadband closed-form representation for the minimum-noise top-strut cross section is derived and tested View full abstract»

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  • Matching score properties between range profiles of high-resolution radar targets

    Page(s): 444 - 452
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    In this paper, how the statistical properties of the matching scores are affected by the carrier frequency, the aspect variation, the range resolution, and the target complexity are studied. The statistical parameters (mean and standard deviation) of the matching scores among range profiles of independent objects are derived, and their values are used as references to determine the threshold values for target identification. It is found that the range profile obtained at a certain carrier frequency can also be used as the feature vector for radars operated at shifted frequencies if the range resolution is fine enough or the target is simple. It is also found that a radar with higher range resolution can tolerate more aspect variation, yielding a significant advantage in saving memory space for establishing the data base. The results obtained can have several applications such as target identification, data association in multiple target tracking, and target direction determination when widely-spaced high-resolution radars are employed View full abstract»

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  • The theory of polarization diversity systems: the partially polarized case

    Page(s): 425 - 433
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    A detailed consideration is presented of copolarized and cross-polarized radar response for partially polarized scattering. A technique using an adjustable polarization basis is used where appropriate. Conditions for maximum and minimum copolarized and cross-polarized power return are obtained. Necessary and sufficient conditions for the realization of the polarization fork are presented View full abstract»

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  • Combining an extrapolation technique with the method of moments for solving large scattering problems involving bodies of revolution

    Page(s): 548 - 553
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    The purpose of this work is to combine an extrapolation technique with the method of moments (MoM) to solve scattering problems involving large bodies. It has been shown in a previous work that the current induced on the smooth parts of large scatterers may be represented as a series of complex exponential functions with a few terms. Based on this concept, a hybrid set of basis functions is constructed using entire domain functions of complex exponential type on the smooth portion of the scatterer, complemented by subdomain basis functions near edges and discontinuities. An extrapolation procedure is developed in which the scattering problem is first solved for a portion of the scatterer using the conventional MoM. Next, a set of entire-domain basis functions, whose behaviour could be extrapolated with an increase in the size of the scatterer, is extracted from this original solution. The procedure outlined has the very desirable feature that the total number of basis functions remains unchanged even as the scatterer size is increased, allowing for large scatterers to be handled with a relatively small number of unknowns. The extrapolation technique is applied to scattering problems from bodies of revolution (BORs), and numerical results for an open cylinder and a barrel-shaped BOR are presented View full abstract»

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  • A multiresolution study of two-dimensional scattering by metallic cylinders

    Page(s): 572 - 579
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    The application of wavelet transforms in method of moments (MoM) solutions for scattering problems is extended to cases involving metallic cylinders whose periphery contains a variety of length-scale features ranging from smoothly varying large-scale features (characterized by a radius of curvature of several wavelengths) to rapidly varying small scale ones (characterized by a radius of curvature that is small compared with the wavelength). The basic idea is to first consider a periodic extension of the equivalent current in the arc-length variable with a period identical to the scatterer circumference, and then to expand this representation using a set of periodic wavelets derived from a conventional basis of wavelets by a periodic extension. Using a Galerkin method and subsequently applying a threshold operation, a substantial reduction in the number of elements of the moment-method matrix is attained without virtually affecting the solution accuracy. The proposed extension is illustrated by a numerical analysis of TM (transverse magnetic) and TE (transverse electric) scattering from a cylinder of elliptic cross section. A thorough study is carried out showing how the solution accuracy improves with increasing resolution level, and how this accuracy is affected by a thresholding process which renders the moment matrix sparsely populated View full abstract»

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  • A 12 GHz-band planar waveguide array antenna for compact range application-a preliminary study

    Page(s): 588 - 589
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    For the purpose of generating a quasi-plane wave and realizing small-antenna pattern measurements in the 12 GHz band, a planar-waveguide array antenna consisting of 45 WRJ-120 rectangular waveguides with 57 longitudinal slots on the broad wall of each waveguide is fabricated and tested. The dimensions of the antenna are about 1100 mm×1300 mm×850 mm, including the waveguide-supporting frame, the feed network, and the antenna-supporting pedestal. As an example, the H-plane pattern measurement of a small pyramidal horn antenna is conducted in a small chamber at 12 GHz. The pattern obtained in the compact range is compared with the corresponding data obtained outdoors View full abstract»

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  • Variational nature of Galerkin and non-Galerkin moment method solutions

    Page(s): 500 - 503
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    There is renewed interest in the use of variational methods in conjunction with numerical solutions of electromagnetic radiation and scattering problems. The variational aspects of secondary calculations based on a method of moments (MoM) solution are investigated. These calculations exhibit a type of second-order accuracy, regardless of whether or not the operator being discretized is self-adjoint, and regardless of whether or not testing functions are identical to the basis functions (Galerkin's method). Numerical results support these conclusions and suggest that the advantage of Galerkin's method in actual calculations is grossly overstated View full abstract»

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  • Main-beam polarization properties of four-element sequential-rotation arrays with arbitrary radiators

    Page(s): 515 - 522
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    A description for the on-axis polarization of four-element sequential-rotation arrays with arbitrary radiators is presented, characterizing the contributions from the feed network and the antenna elements separately. This separation provides criteria for an optimization of the individual radiators including mutual coupling effects without an impact of the applied feed network on the optimization process. Moreover, severe specifications for the polarization purity of a four-element sequential-rotation array can be subdivided into separate, less severe requirements for the elements and the excitation-structure, using the description presented. Measurement methods for the determination of the identified characteristic parameters are discussed along with possible limiting factors of the theory. Experimental results are presented for a four-element array of helix antennas which shows the impact of mutual coupling on the element performance and illustrates the excellent polarization properties of the discussed array type for the entire mainbeam. The predicted figures for the cross-polarization of the array and the measurement results are in good agreement View full abstract»

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  • Modeling three-dimensional scatterers using a coupled finite element-integral equation formulation

    Page(s): 453 - 459
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    Finite-element modeling has proven useful for accurately simulating scattered or radiated fields from complex three-dimensional objects whose geometry varies on the scale of a fraction of a wavelength. To practically compute a solution to exterior problems, the domain must be truncated at some finite surface where the Sommerfeld radiation condition is enforced, either approximately or exactly. This paper outlines a method that couples three-dimensional finite-element solutions interior to a bounding surface with an efficient integral equation solution that exactly enforces the Sommerfeld radiation condition. The general formulation and the main features of the discretized problem are first briefly outlined. Results for far and near fields are presented for geometries where an analytic solution exists and compared with exact solutions to establish the accuracy of the model. Results are also presented for objects that do not allow an analytic solution, and are compared with other calculations and/or measurements View full abstract»

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  • Maximum likelihood estimation and Cramer-Rao bounds for direction of arrival parameters of a large sensor array

    Page(s): 478 - 491
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    A maximum likelihood (ML) method is developed for estimation of direction of arrival (DOA) and associated parameters of narrowband signals based on the Taylor's series expansion of the inverse of the data covariance matrix R for large M, M specifying number of sensors in the array. The stochastic ML criterion function can thus be simplified resulting in a computationally efficient algorithm for DOA estimation. The more important result is the derivation of asymptotic (large M) expressions for the Cramer-Rao lower bound (CRB) on the covariance matrix of all unknown DOA angles for the general D source case. The derived bound is expressed explicitly as a function of snapshots, signal-to-noise ratio (SNR), sensors, separation, and correlation between signal sources. Using the condition of positive definiteness of the Fisher information matrix a resolution criterion is proposed which gives a tight lower limit on the minimum resolvable angle View full abstract»

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  • A comparison of acceleration procedures for the two-dimensional periodic Green's function

    Page(s): 567 - 571
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    Alternate acceleration procedures are evaluated for the two-dimensional free-space periodic Green's function. Two of these techniques yield exponential convergence rates and are preferable if high accuracies are required. Numerical aspects of these procedures are described View full abstract»

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  • Reflection and transmission characteristics of lossy periodic composite structures

    Page(s): 580 - 587
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    A periodic surface integral formulation is proposed to analyze the reflection and transmission properties of a lossy periodic composite structure which has circular conducting fibers embedded in a dielectric matrix. This formulation is based on the equivalence principle which represents the unknown electric and magnetic currents over the material discontinuity interfaces, and uses the structure periodicity and Poisson summation formula to reduce the problem to a periodic cell. These surface integral equations are then solved numerically, using the method of moments with pulse bases and point matching. Only the transverse magnetic (TM) case is analyzed and the numerical results such as reflected, transmitted, and dissipated powers for a single-layer fiber-reinforced composite structure are presented, in detail, to discuss the effects of frequency, incident angle, fiber radius, fiber conductivity, embedding dielectric, etc. A convergence study and a comparison with the previous published results are also included to confirm the accuracy of the new formulation View full abstract»

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  • On fractional calculus and fractional multipoles in electromagnetism

    Page(s): 554 - 566
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    Using the concept and tools of fractional calculus, we introduce a definition for “fractional-order” multipoles of electric-charge densities, and we show that as far as their scalar potential distributions are concerned, such fractional-order multipoles effectively behave as “intermediate” sources bridging the gap between the cases of integer-order point multipoles such as point monopoles, point dipoles, point quadrupoles, etc. This technique, which involves fractional differentiation or integration of the Dirac delta function, provides a tool for formulating an electric source distribution whose potential functions can be obtained by using fractional differentiation or integration of potentials of integer-order point-multipoles of lower or higher orders. As illustrative examples, the cases of three-dimensional (point source) and two-dimensional (line source) problems in electrostatics are treated in detail, and an extension to the time-harmonic case is also addressed. In the three-dimensional electrostatic example, we suggest an electric-charge distribution which can be regarded as an “intermediate” case between cases of the electric-point monopole (point charge) and the electric-point dipole (point dipole), and we present its electrostatic potential which behaves as r-(1+α)Pα(-cosθ) where 0<α<1 and Pα(·) is the Legendre function of noninteger degree α, thus denoting this charge distribution as a fractional 2α-pole. At the two limiting cases of α=0 and α=1, this fractional 2α -pole becomes the standard point monopole and point dipole, respectively. A corresponding intermediate fractional-order multipole is also given for the two-dimensional electrostatic case. Potential applications of this treatment to the image method in electrostatic problems are briefly mentioned. Physical insights and interpretation for such fractional-order 2α-poles are also given View full abstract»

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  • Efficient direction and polarization estimation with a COLD array

    Page(s): 539 - 547
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    This paper considers angle and polarization estimation by means of a cocentered orthogonal loop and dipole (COLD) array. We show that by using the COLD array, the performance of both angle and polarization estimation can be greatly improved, as compared to using a crossed dipole array. We present an asymptotically statistically efficient method of direction estimation (MODE) algorithm that can be used with the COLD array for both angle and polarization estimation of correlated (including coherent) or uncorrelated incident signals. Numerical examples are given to show the better estimation performance of the MODE algorithm than that of the multiple signal classification (MUSIC) and the noise subspace-fitting (NSF) algorithms View full abstract»

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  • Scatter measured from impedance discontinuities

    Page(s): 532 - 538
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    The vertically-polarized (electric vector in the plane of incidence) radar cross sections of four-square doped silicon samples that were inserted in a diamond-shaped aluminum fixture have been measured at elevation angles of both 5.5° and 18° (near grazing incidence) at an azimuth angle normal to the edge of the samples. The measured scatter matches predictions for samples inserted in an infinite conductive plane that were made using two different two-dimensional models at 18°, but was 2-6 dB below predictions at 5.5° in all cases. A comparison between predictions made with and without a Leontovich impedance-boundary-condition (IBC) approximation demonstrates that the discrepancy between theory and measurement at 5.5° is not caused by the use of this approximation in the predictions. The most likely source of the discrepancy is the finite extent of the diamond-shaped fixture used in the measurements View full abstract»

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  • Scattering from coated targets using a frequency-dependent, surface impedance boundary condition in FDTD

    Page(s): 434 - 443
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    One method for reducing the radar cross section of objects such as aircraft and missiles is the application of a lossy coating. Computing scattering from targets coated with dielectric/magnetic materials is challenging due to the reduced wavelengths of an incident field inside the coating. These smaller wavelengths require finer sampling of the fields. A technique for implementing this calculation without greatly increased memory requirements or computation times has previously been developed using a finite-difference time-domain (FDTD) code which has been tested in one, two, and three dimensions. The method requires knowledge of the frequency behaviour of the complex permittivity and permeability, and the thickness of the dielectric coating and is applicable to thin coatings when one or more reflections from the conducting surface are significant. The impedance at the surface of the coating is computed based on the given information and then approximated using a summation of causal functions. The approximated impedance is Z-transformed and added to the FDTD code in special update equations for the fields at the surface of the coating. No computations are required inside the coatings so the FDTD grid can be sized based on the free-space wavelength. The result obtained is valid over the entire frequency range of interest, assuming that the approximated surface impedance is a good match over the entire range. Comparisons with measurements of a scale model coated missile show good agreement and almost no increase in resource requirements over a standard FDTD calculation for an uncoated metal target View full abstract»

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  • Accurate radiation and impedance characteristics of horn antennas-a moment-method model

    Page(s): 523 - 531
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    A moment-method-model for horn analysis is presented. The generalized reflection matrix of the aperture is determined using the spectral Galerkin method. The reflection matrix is then combined with the generalized scattering matrix of the horn to yield the input return loss and the aperture fields of the horn. It is shown that the model can predict the radiation characteristics more accurately than the models commonly used in practice 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