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Antennas and Propagation, IEEE Transactions on

Issue 6 • Date June 2004

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Displaying Results 1 - 25 of 37
  • Table of contents

    Page(s): c1 - 1385
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  • IEEE Transactions on Antennas and Propagation publication information

    Page(s): c2
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  • Improved microwave imaging procedure for nondestructive evaluations of two-dimensional structures

    Page(s): 1386 - 1397
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    An improved microwave procedure for detecting defects in dielectric structures is proposed. The procedure is based on the integral equations of the inverse scattering problem. A hybrid genetic algorithm (GA) is applied in order to minimize the obtained nonlinear functional. Since in nondestructive evaluations the unperturbed object is completely known, it is possible off-line to numerically compute the Green's function for the configuration without defects. Consequently, a very significant computation saving is obtained, since the "chromosome" of the GA codes only the parameters describing the unknown defect. View full abstract»

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  • Study of resolution and super resolution in electromagnetic imaging for half-space problems

    Page(s): 1398 - 1411
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    It has been observed that super resolution is possible in the electromagnetic imaging. In the first part of the paper, the possible resolution of image is investigated in the inversion of far-field data using the diffraction tomographic (DT) algorithm, where two cases are considered when the object is in a homogeneous space and in an air-earth half space. The study shows that the resolution of image for inversion of far-field data has been limited theoretically to 0.3536-0.5 wavelength using the DT algorithm in homogeneous-space problems, and it is even worse in half-space problems. If the transmitters and receivers are located in the near-field regime, however, the image resolution is less than 0.25 wavelength, which is the super-resolution phenomenon. In the second part of the paper, the physical reason for the super-resolution phenomenon is investigated using different electromagnetic inverse scattering methods. The study has demonstrated that the information of evanescent waves in the measurement data and its involvement in inversion algorithms is the main reason for the super resolution. Four inversion algorithms are considered for half-space problems: the DT algorithm, the spatial-domain Born approximation (BA), the Born iterative method (BIM), and the distorted BIM (DBIM). The first two belong to linear inverse scattering, while the last two belong to nonlinear inverse scattering. Further analysis shows that DBIM provides a better super resolution than BIM, and BIM provides a better super resolution than BA. Numerical simulations validate the above conclusions. View full abstract»

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  • Meshed patch antennas

    Page(s): 1412 - 1416
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    Conventional microstrip patch antennas are printed with continuous solid copper shapes and ground planes. The general properties of meshed patches are presented in this paper where both the patch itself and the ground plane are meshed. The gain, cross-polarization, bandwidth and radiation patterns are discussed for different combinations of patch and ground plane. The radiation patterns are not significantly affected by meshing the patch alone, but meshing the ground plane increases the back radiation. The gain can suffer by up to 3 dB or more when compared to a standard patch. Cross-polarization is improved providing that the correct mesh line geometry is chosen for the excitation mode. Meshing lowered the resonant frequency in some cases by up to 30% and also improves the bandwidth by a factor of up to 2.5 in some modes. Overall, the meshed patch offers a complex tradeoff between parameters but gives opportunities for improving the bandwidth and reducing the cross polarization and the antenna dimensions at the expense of the gain. View full abstract»

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  • A spiral antenna sandwiched by dielectric layers

    Page(s): 1417 - 1423
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    An infinitesimally thin spiral antenna, sandwiched by bottom and top dielectric layers having the same relative permittivity, is analyzed under the condition that the dielectric layers are of finite extent and the antenna is backed by an infinite conducting plane. As the thickness of the top dielectric layer increases, the input impedance and axial ratio (AR) vary in an oscillatory fashion, with a period slightly larger than one-half of the guided wavelength of a wave propagating in an unbounded dielectric material. These oscillatory variations are reduced by adding a layer, called the "anti-reflection layer (ARL)," to the top dielectric layer. A representative spiral antenna with an ARL shows a frequency bandwidth of approximately 11% for a 3-dB AR criterion, having a gain of approximately 13 dBi (6 dBi higher than a printed spiral antenna without the top dielectric and ARL) and a voltage standing wave ratio of less than 1.2. View full abstract»

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  • Electrically small unbalanced four-arm wire antenna

    Page(s): 1424 - 1428
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    An electrically small antenna element is presented. It is an unbalanced antenna, consisting of four wire arms arching over a groundplane. One of the wire ends is the feed, and the other three are connected to the groundplane, via a capacitance, short, and load, respectively. The resonance frequency is tuned by varying the capacitance, which can be set fixed or used to accomplish electronic tunability with a tuning range of several octaves. The momentary bandwidth of the antenna is approximately 3%. Applications are typically antennas for very small terminals in narrowband systems, or alternatively broadband systems where electronical tuning can be implemented in the antenna. The exact shape of the wires is not critical and can be chosen to conform to mechanical design requirements. View full abstract»

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  • Numerical and experimental analysis of leaky-wave microwave applicators

    Page(s): 1429 - 1433
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    The theory of leaky-wave antennas, suitably modified, is applied for the evaluation of the field intensity induced in a lossy medium by two applicator schemes, the grounded dielectric layer and the channel guide. The comparison of calculated and measured results confirm the effectiveness of the leaky-wave approach for the analysis of such microwave power applicators. View full abstract»

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  • A reconfigurable aperture antenna based on switched links between electrically small metallic patches

    Page(s): 1434 - 1445
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    A reconfigurable aperture (RECAP) antenna is described in which a planar array of electrically small, metallic patches are interconnected by switches. The antenna can be reconfigured to meet different performance goals by changing the switches that are open and closed. The switch configuration for a particular goal is determined using an optimizer, such as the genetic algorithm. First, the basic concept for the RECAP antenna is verified by comparing theoretical results with measurements for configurations in which the switches are simply wires connecting the patches. Next, details are given for a prototype antenna in which field-effect transistor based electronic switches are used with optical control. Theoretical results for the prototype antenna are then compared with measurements for cases in which electronic reconfiguration is used to change the bandwidth of operation or steer the pattern of the antenna. Finally, an overview of alternate switch/control strategies, some of which were tested, is given along with suggestions for improving the next generation of this antenna. View full abstract»

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  • Microstrip antenna phased array with electromagnetic bandgap substrate

    Page(s): 1446 - 1453
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    Uniplanar compact electromagnetic bandgap (UC-EBG) substrate has been proven to be an effective measure to reduce surface wave excitation in printed antenna geometries. This paper investigates the performance of a microstrip antenna phased array embedded in an UC-EBG substrate. The results show a reduction in mutual coupling between elements and provide a possible solution to the "blind spots" problem in phased array applications with printed elements. A novel and efficient UC-EBG array configuration is proposed. A probe fed patch antenna phased array of 7×5 elements on a high dielectric constant substrate was designed, built and tested. Simulation and measurement results show improvement in the active return loss and active pattern of the array center element. The tradeoffs used to obtain optimum performance are discussed. View full abstract»

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  • Relations between the reflectance and band structure of 2-D metallodielectric electromagnetic crystals

    Page(s): 1454 - 1464
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (824 KB) |  | HTML iconHTML  

    In this paper, we present two different approaches to the problem of wave propagation in two-dimensional (2-D) periodic structures; one is the search of dispersion roots of which the real and imaginary parts represent the phase and decay constants of an unbound 2-D periodic medium, respectively, and the other is the investigation of the scattering characteristics of a finite 2-D periodic structure that is treated as a stack of 1-D periodic layers. Specifically, the rigorous mode-matching method is employed for both approaches, and the class of 2-D periodic structures with metal rods of rectangular cross-section is considered explicitly for mathematical formulation and quantitative analysis of associated physical phenomena. The mutual verifications of the results by the two different approaches facilitate the understanding of band structure of the 2-D periodic medium. In addition to the stopbands that can be easily identified to be due to the individual periodicity in either x or y direction, particular attention is directed to the combined effect of both periodicities, which results in the extra stopbands that are slanted at an angle on the Brillouin diagram. This provides the physical basis for the explanation of the unusually strong reflection of an incident plane wave in certain range of frequency or incident angle. View full abstract»

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  • An asymptotic solution of EM backscattering from a conducting sphere coated with a composite material

    Page(s): 1465 - 1472
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    An asymptotic high frequency solution for the electromagnetic (EM) backscattered field produced by a plane wave incident on a perfectly conducting sphere coated with a thin composite material is derived in this paper. For the formulation of the incident and the reflected field the characteristics of the wave transformation and the line integral via the stationary phase method were applied, respectively, and the obtained results are cast in the ordinary ray formats of a geometrical optics field. Based on the Watson transform technique, the diffracted field is also formulated from the residue series solution of the problem and presented in a form suitable for the numerical calculation. The numerical results obtained from the derived asymptotic solution show excellent agreement with those from the rigorous eigenfunction solution. View full abstract»

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  • Exact penetration, radiation, and scattering for a slotted semielliptical channel filled with isorefractive material

    Page(s): 1473 - 1480
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    A semielliptical channel flush-mounted under a metal plane and slotted along the interfocal distance of its cross-section is considered. The channel is filled with a material isorefractive to the medium that occupies the half-space above the metal plane. The boundary-value problem is solved exactly in terms of Mathieu functions, when the primary source is a plane wave with arbitrary direction of incidence and polarization, or an electric or magnetic line source parallel to the channel and located either above the channel or inside it. View full abstract»

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  • Ku-band long distance site-diversity (SD) Characteristics using new measuring system

    Page(s): 1481 - 1491
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    This paper deals with the short (10 km) and long distance (300-1400 km) site-diversity (SD) characteristics by using a newly developed measuring system. In the proposed measuring system, six earth stations transmit 14-GHz band QPSK signals, and one measuring earth station receives 12-GHz band signals and processes them to determine SD characteristics. As a result, easy operation and maintenance, low-cost measuring system construction and highly accurate data have been obtained. By comparing those measured results with the SD joint probability approximation equation in ITU-R Rec.P.618-7, a good agreement can be obtained. Furthermore, the effect of typhoons on SD characteristics were measured. View full abstract»

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  • Observed effects of cloud and wind on the intensity and spectrum of scintillation

    Page(s): 1492 - 1498
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    This paper examines the observed effects of wind and cloud presence on the intensity and power spectrum of tropospheric scintillation, using propagation data recorded during a three-month period (May-July 1997) at Sparsholt from the ITALSAT satellite beacons at 39.59 and 49.49 GHz. The results show strong correlation between corner frequency and transverse wind speed, a weaker correlation with overall wind speed and a negligible correlation between scintillation intensity and transverse wind speed. Furthermore, it was found that the presence of cumulus clouds led on average to increased scintillation levels and a reduced correlation of the corner frequency with wind speed. View full abstract»

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  • Comparison of measurements with prediction methods for propagation by diffraction at 88-108 MHz

    Page(s): 1499 - 1504
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    Three methods for predicting attenuation due to diffraction are tested against a large database of 115 614 measurement points, representing 115 614 different path profiles of 100-m horizontal resolution, and vertical root mean square error of about 6 m. The signal level at each geographic measurement point is calculated as the median of about 40 basic measurements, and the measurement database is thus reduced from about four million basic measurements. The mobile measurements are taken from main FM (88-108 MHz) broadcasting emitters located in southern Norway, and the corresponding broadcasting antenna diagrams have been measured by helicopter. Path profiles are categorized by number of terrain obstructions between emitter and receiver, in order to study their effect on each propagation loss method. The current ITU method and Picquenard's construction, with a variable number of included terrain obstructions, are compared with the measurements, and difference statistics are calculated. A particular version of Picquenard's construction is shown to be better than the current ITU method in terrain of Norwegian type. This new method is, in contrast to the ITU method, within the estimated expected errors resulting from using Norwegian digital terrain elevation data. View full abstract»

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  • Rain attenuation statistics over terrestrial microwave link at 19.4 GHz at Amritsar

    Page(s): 1505 - 1508
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    Rain induced attenuation at 19.4 GHz over a terrestrial path link of 2.29 km was measured for the period of one year (i.e. from January to December 2001) at Amritsar (31°36' N 74° 52' E). An empirical model for predicting rain-induced attenuation on terrestrial line of sight microwave link has been proposed in this paper. Experimentally measured results have been compared with those obtained using international radio communication union (ITU-R). It has been observed that our results differ from those predicted using ITU-R model. View full abstract»

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  • An accurate UTD model for the analysis of complex indoor radio environments in microwave WLAN systems

    Page(s): 1509 - 1520
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    An accurate uniform theory of diffraction (UTD) model for the analysis of complex indoor radio environments, in which microwave WLAN systems operate, is presented. The model employs a heuristic UTD coefficient suitable to take into account the effects of building floors, walls, windows, and the presence of metallic and penetrable furniture. A numerical tool based on an enhanced tridimensional beam-tracing algorithm, which includes diffraction phenomena, has been developed to compute the field distribution with a high degree of accuracy. After the validation of the model, obtained by means of some comparisons with measurements available in literature, an accurate electromagnetic characterization of typical indoor environments has been performed. The numerical results show that the electromagnetic field distribution and the channel performances are significantly influenced by the diffraction processes arising from the presence of furniture. View full abstract»

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  • Neural networks-method of moments (NN-MoM) for the efficient filling of the coupling matrix

    Page(s): 1521 - 1529
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    In this paper, novel radial basis function-neural network (RBF-NN) models are presented for the efficient filling of the coupling matrix of the method of moments (MoM). Two RBF-NNs are trained to calculate the majority of elements in the coupling matrix. The rest of elements are calculated using the conventional MoM, hence the technique is referred to as neural network-method of moments (NN-MoM). The proposed NN-MoM is applied to the analysis of a number of microstrip patch antenna arrays. The results show that NN-MoM is both accurate and fast. The proposed technique is general and it is convenient to integrate with MoM planar solvers. View full abstract»

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  • Method of lines numerical analysis of conformal antennas

    Page(s): 1530 - 1540
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    This paper presents a generalization of the method of lines (MoL) numerical algorithm for the analysis of integrated structures with a finite local curvature. Starting from the derivation of the generalized transmission line equations in a generic orthogonal reference system and in a bianisotropic material, it is shown the possibility of solving the electromagnetic problem associated to an integrated structure with arbitrary curvature and/or loaded by complex dielectric materials via a MoL approach. The introduction of new algorithms that extend and complete the original method is required in the most general case. A detailed description of this new extended version of the MoL is presented through the paper and its full coincidence with the original algorithm in the case of structures with simple loading materials is also shown. Finally, some numerical results obtained analyzing microwave conformal antennas and resonators are presented and compared with analyzes performed employing other numerical methods and available in the literature. View full abstract»

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  • Accuracy analysis and optimization of the method of auxiliary sources (MAS) for scattering by a circular cylinder

    Page(s): 1541 - 1547
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    This paper presents a rigorous accuracy analysis of the method of auxiliary sources (MAS), when applied to scattering problems. A benchmark, canonical geometry, consisting of a perfectly conducting, infinite, circular cylinder, is chosen for clarity and simplicity. For this particular structure it is shown that the MAS square matrix can be inverted analytically, yielding exact mathematical expressions for the discretization error and the condition number of the pertinent linear system. It is also demonstrated that the error increases very abruptly for source locations associated with the characteristic eigenvalues of the scattering geometry, precisely as predicted in theory. Various plots depict comparisons between analytical and computational data for the boundary condition error, and all occurring discrepancies are fully explained. Among several important results of the analysis, the fundamental MAS question concerning the optimal location of the auxiliary sources is thoroughly investigated and resolved on the grounds of error minimization. View full abstract»

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  • A new representation for the Green's function of multilayer media based on plane wave expansion

    Page(s): 1548 - 1557
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    In this paper, a new representation for the space domain Green's function of general multilayer media is presented. This approach is based on an efficient plane wave expansion of the source incident field. Using the transmission line model for the multilayer medium, the effect of the layered medium on the incident plane waves is determined by a transmission coefficient, and the amplitude and phase of each plane wave at the field point are obtained. The total field is evaluated by summing the resultant plane waves at the field point. The main advantage of the proposed method is that the spatial domain Green's function of multilayer media can be obtained easily as a summation of simple exponential functions, without the need for Sommerfeld integration or complex image approximation. The plane wave approximation is independent of the parameters of the medium and is valid over a wide frequency range. Furthermore, the exponential form of the plane wave solution makes it possible to compute the method of moments matrix elements analytically for most important types of basis functions in multilayer problems. View full abstract»

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  • Magneto-dielectrics in electromagnetics: concept and applications

    Page(s): 1558 - 1567
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    In this paper, the unique features of periodic magneto-dielectric meta-materials in electromagnetics are addressed. These materials, which are arranged in periodic configurations, are applied for the design of novel EM structures with applications in the VHF-UHF bands. The utility of these materials is demonstrated by considering two challenging problems, namely, design of miniaturized electromagnetic band-gap (EBG) structures and antennas in the VHF-UHF bands. A woodpile EBG made up of magneto-dielectric material is proposed. It is shown that the magneto-dielectric woodpile not only exhibits band-gap rejection values much higher than the ordinary dielectric woodpile, but also for the same physical dimensions it shows a rejection band at a much lower frequency. The higher rejection is a result of higher effective impedance contrasts between consecutive layers of the magneto-dielectric woodpile structure. Composite magneto-dielectrics are also shown to provide certain advantages when used as substrates for planar antennas. These substrates are used to miniaturize antennas while maintaining a relatively high bandwidth and efficiency. An artificial anisotropic meta-substrate having μrr, made up of layered magneto-dielectric and dielectric materials is designed to maximize the bandwidth of a miniaturized patch antenna. Analytical and numerical approaches, based on the anisotropic effective medium theory (AEMT) and the finite-difference time-domain (FDTD) technique, are applied to carry out the analyzes and fully characterize the performance of finite and infinite periodic magneto-dielectric meta-materials integrated into the EBG and antenna designs. View full abstract»

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  • A direct derivation of a single-antenna reciprocity relation for the time domain

    Page(s): 1568 - 1577
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    In this paper, a single-antenna reciprocity relation is derived for the time domain. First, the antenna is considered on transmission; next, the same antenna is considered when it is receiving an incident plane wave. The two states, transmission and reception, are related by the application of a modified form of the reciprocity theorem for electromagnetic fields with general time dependence due to Cheo. The derivation of the reciprocity relation for the antenna makes use of simple geometric arguments to evaluate the spatial/temporal integrals that occur in the theorem. A few extensions of the reciprocity relation are also described. View full abstract»

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  • Time-domain Green's functions for microstrip structures using Cagniard-de hoop method

    Page(s): 1578 - 1585
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    Time-domain Green's functions are required for transient analyzes of many structures using the time-domain integral equation method. In this paper, we express the generalized reflection coefficient of the microstrip structure in terms of a geometric optics series so that by applying the Cagniard-de Hoop method to each term of the series, we can derive the time-domain Green's function. It is demonstrated that this series converges rapidly and there are two contributing waves from each source image if the observation point is beyond the total internal refraction location. The two waves are the direct wave from the image and the head wave from the image to the critical point, and then laterally along the surface to the observer. Each contribution is a definite integral that is evaluated for each point in space and time. Therefore, the derived Green's function is efficient for time-domain simulations compared with conventional approach, in which for each point in space and frequency a Sommerfeld type integral is involved and then the frequency-domain data is converted into time-domain by discrete Fourier transform. This rigorous Green's function can also be used to check the accuracy of other approximate methods such as those using the discrete complex image theory. 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