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Antennas and Propagation, IEEE Transactions on

Issue 2 • Date Feb. 2000

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Displaying Results 1 - 25 of 28
  • Transformation of surface waves in homogeneous absorbing layers

    Page(s): 214 - 222
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    Surface waves in homogeneous absorbing layers are studied. The transformation of surface waves into other types of guided waves with frequency or layer's parameters variations is analyzed. It is found that in absorbing layers the standing damped surface waves do not exist and continuous transformation of surface waves into leaky waves is forbidden. Surface waves can only transform continuously into nonphysical waves with field strengths increasing exponentially in both the direction of propagation and in normal direction away from the layer into free-space. The frequency at which surface waves transform into nonphysical waves can be considered as cutoff frequency of surface waves. New and more general definitions previously proposed by the authors for the phase and energy velocities of guided waves are analyzed. It is shown that these velocities are identically equal to each other and never exceed the lightspeed in free-space. Several new physical phenomena are discovered. These are the upper frequency cutoff, its shifting to higher frequencies for materials with lower losses and the merging of high TE wave modes. View full abstract»

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  • T-matrix determination of effective permittivity for three-dimensional dense random media

    Page(s): 317 - 327
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    In this paper, we present a full wave method for determining the effective permittivity for random media in three dimensions. The type of media addressed is composed of spherical dielectric particles in a homogeneous dielectric background. The particle volume fraction ranges from 0 to 40% and dielectric contrast may be significantly different from the background medium. The method described relies on the T-matrix approach for solving Maxwell's equations using a spherical wave expansion in conjunction with a Monte-Carlo simulation for calculating the mean scattered field confined within a prescribed fictitious boundary. To find the effective permittivity, the mean scattered field is compared with that of a homogeneous scatterer whose shape is defined by the fictitious boundary and its dielectric constant is varied until the scattered fields are matched. A complete description of the T-matrix approach is given along with an explanation of why the recursive form of this technique (RATMA) cannot be used for addressing this problem. After the method development is completed, the results of our numerical technique are compared against the theoretical methods of the quasi crystalline approximation and the effective field approximation to demonstrate the region of validity of the theoretical methods. The examples contained within the paper use between 30 and 120 included spheres (with radii ranging from from ka=0.6 to 0.8) within a larger, fictitious sphere of diameter kD=8.4 View full abstract»

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  • A dual-beam micro-CPW leaky-mode antenna

    Page(s): 310 - 316
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    A quasi-planar leaky-mode antenna employing the second higher order leaky mode of even symmetry in its leaky-wave regime has been proposed for a new antenna configuration consisting of a microstrip and a coplanar waveguide (CPW) on both sides of the substrate. The new antenna, also known as the micro-CPW antenna, is directly fed by a CPW line without matching circuit. One particular K-band antenna of 36 mm long [about 2.7 free-space wavelengths (λ0) at 22.1 GHz] shows 5.3% impedance bandwidth for VSWRs less than 2.0 for frequencies between 21.927-23.110 GHz, 11.0-dB antenna gain, and 90.02% efficiency. Detailed analyses show that the dual-beam antenna is linearly polarized along two slanted lines, which lie in the longer axes of the ellipses that approximate the radiation contours. Both theoretical and experimental data for the micro-CPW antenna agree very well for the particular design. The proposed micro-CPW antenna is suitable for active integrated antenna integration at higher microwave and millimeter-wave frequencies View full abstract»

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  • Coplanar waveguide-fed circularly polarized microstrip antenna

    Page(s): 328 - 329
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    Design of a square microstrip antenna for obtaining circular polarization (CP) radiation using a coplanar waveguide (CPW) feed is presented. This CP design is achieved by insetting a slit to the boundary of the square microstrip patch, which makes possible the splitting of the dominant resonant mode into two near-degenerate orthogonal modes for CP radiation and introducing an inclined slot in the CPW feed line for coupling the electromagnetic (EM) energy of the CPW to the square patch. Good impedance matching for CP operation can be obtained by adjusting the inclined slot length and the tuning-stub length of the CPW feed line. Typical experimental results are presented and discussed View full abstract»

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  • Electromagnetic penetration into 2-D multiple slotted rectangular cavity: TM-wave

    Page(s): 331 - 333
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    Electromagnetic wave penetration into the two-dimensional (2-D) rectangular cavity with multiple slots in an infinite conducting plane with a finite thickness is investigated. The Fourier transform and the mode-matching technique are used to obtain simultaneous equations, which are solved to represent the scattered and the penetrated fields in series forms that are suitable for numerical computations View full abstract»

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  • Radiation patterns of sources placed near the truncation of a semi-infinite dielectric structure: the demonstration case of a magnetic line current

    Page(s): 240 - 245
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    Numerical methods are presented to estimate the radiation pattern of a magnetic line current placed near the truncation of a dielectric structure. The problem is solved using the integral equation technique, expansion wave concept, and physical optics approximation. The comparison between different methods permits to understand better the possibilities of each of the methods View full abstract»

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  • Adaptive ISAR image construction from nonuniformly undersampled data

    Page(s): 329 - 331
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    An adaptive approach is proposed to construct ISAR images from nonuniformly undersampled data in the angular domain. The algorithm uses an adaptive scattering feature extraction engine in place of the Fourier transform in the image construction procedure. The algorithm entails searching and extracting out individual target scattering features one at a time in an iterative fashion. The interference between different target scattering features is thus avoided and a clean ISAR image without the aliasing effect can be obtained. The algorithm is verified by constructing the ISAR image from the chamber measurement data of the model VFY-218 airplane View full abstract»

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  • The coaxial beam-rotating antenna (COBRA): theory of operation and measured performance

    Page(s): 299 - 309
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    Many microwave generators, especially high-power sources, utilize an azimuthally symmetric output mode such as the TM01 circular waveguide or the coaxial TEM mode. If such a mode is projected into an antenna aperture and radiated directly, then a doughnut-shaped radiation pattern with a boresight null will result. Antenna designs to directly accommodate an azimuthally symmetric output mode and the high electric fields of high-power sources have been considered, but they tend to be low gain, do not radiate a boresight peak along the axis of the source, and the pattern peak direction changes with frequency. Mode conversion techniques to alter the aperture field distribution have also been explored, but losses and weight, size and cost additions impact negatively on total system design. This paper describes a novel antenna we call the coaxial beam-rotating antenna (COBRA) that mitigates many of the problems normally associated with the azimuthally symmetric output modes of high-power microwave sources. The COBRA accepts directly an azimuthally symmetric guided mode of a microwave source and radiates a high-gain pattern with a boresight peak. In addition, the COBRA operates with a wide bandwidth, is compatible with the intense electric fields associated with high-power microwave sources, and the geometry of the antenna can be easily configured to produce an arbitrarily (elliptically) polarized boresight field. This paper presents the fundamental theory of operation, derives pertinent design and performance equations, and gives the measured operating characteristics of a COBRA prototype View full abstract»

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  • Propagation modeling over terrain using the parabolic wave equation

    Page(s): 260 - 277
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    We address the numerical solution of the parabolic wave equation over terrain using the Fourier/split-step approach. The method, referred to as a shift map, generalizes that of Beilis and Tappert (1979) who introduced a coordinate transformation technique to flatten the boundary. This technique is extended to a wide-angle form, allowing larger propagation angles with respect to the horizon. A new impedance boundary condition is derived for electromagnetic waves incident on a finitely conducting surface that enables solution of the parabolic wave (PWE) using the previously developed mixed Fourier transform. It is also shown by example that in many cases of interest, the boundary may be approximated by discrete piecewise linear segments without affecting the field solution. A more accurate shift map solution of the PWE for a piecewise linear boundary is, therefore, developed for modeling propagation over digitally sampled terrain data. The shift-map solution is applied to various surface types, including ramps, wedges, curved obstacles, and actual terrain. Where possible, comparisons are made between the numerical solution and an exact analytical form. The examples demonstrate that the shift map performs well for surface slopes as large as 10-15° and discontinuous slope changes on the order of 15-20°. To accommodate a larger range of slopes, it is suggested that the most viable solution for general terrain modeling is a hybrid of the shift map with the well-known terrain masking (knife-edge diffraction) approximation View full abstract»

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  • Two-step inverse scattering method for one-dimensional permittivity profiles

    Page(s): 293 - 298
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    A numerical method to invert the dielectric permittivity profile from the Riccati equation using the Newton-Kantorovich iterative scheme is described. Instead of handling the equations in terms of usual geometrical depth, we determine the profile as a function of the electromagnetic path length since the convergence and the stability of the solution are found to be significantly better in this case. The initial profile used as a starting point for the inversion is obtained by another method employing successive reconstruction of dielectric interfaces and homogeneous layers in a step-like form. This method, though not always accurate, is fast and well suited for the approximate reconstruction of the profile, thus creating ideal starting conditions for the previous approach. As a result, the computation time is considerably reduced, without using any a priori information. The approach is applicable to both continuous and discontinuous profiles of high contrast and exhibits a good stability of the solution with respect to noisy input data. A lossy medium profile can also be inverted provided the overall thickness of the inhomogeneous slab and the background permittivity are known View full abstract»

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  • An enhanced bandwidth design technique for electromagnetically coupled microstrip antennas

    Page(s): 161 - 164
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    This paper describes a method of enhancing the bandwidth of two different electromagnetically coupled microstrip antennas by utilization of a tuning stub. An approximate theory and equations are developed to demonstrate the potential bandwidth improvement and required stub impedance characteristics. A novel dual-stub design is presented that achieves better characteristics than a conventional quarter wavelength open-end stub. As examples, the bandwidth (VSWR<2) of a conventional proximity-coupled microstrip antenna is increased from 4.8 to 8.4% and the bandwidth of a stacked aperture-coupled microstrip antenna is increased from 27.5 to 34.5% using this technique View full abstract»

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  • Directional macro-cell channel characterization from urban measurements

    Page(s): 137 - 146
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    We measured the angular power distribution at the mobile station in downtown Paris at 890 MHz. The transmit antenna was omnidirectional and placed high above rooftops. The receiver antenna, a 21×41 element rectangular synthetic array, was located on the roof of a van. The refined high-resolution evaluation method, particularly robust against nonstationary signal components, allows an angular resolution of better than 1° in both azimuth and elevation and a delay resolution of 33 ns. Combined angular/temporal domain measurements are crucial for the understanding of the propagation mechanisms. The evaluated sites showed strongly street-dominated propagation. We found a combined circular and rectangular distribution of scatterers around the mobile station in street-dominated environments. Propagation over the roofs was significant; typically 65% of energy was incident with elevation larger than 100. Our results corroborate the hypothesis on the importance of multiple reflections/diffractions in urban macro cells. We explain this behavior by two reasons: narrow streets favoring a canyon effect and strong scatterers without line-of-sight (LOS) to the mobile station View full abstract»

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  • A planar Van Atta array reflector with retrodirectivity in both E-plane and H-plane

    Page(s): 173 - 175
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    A planar antenna array reflector with retrodirectivity in both the E-plane and the H-plane is analyzed and demonstrated at X band. The reflector consists of six pairs of slot-coupled patch antennas arranged using the Van Atta approach. The total reflected field from the reflector is separated into three primary components; that is, the reradiation field from the patch antennas (RFPA), the scattering field from the patch antennas (SPPA), and the scattering field from the ground plane (SFGP). The first two components are calculated by using the method of moments together with a mixed potential integral equation and the last one is by the physical optics (PO) method combined with the method of equivalent currents (MEC). By tuning the microstrip-line lengths, the total reflected field contributed by the three components is designed to possess a broad-beamed pattern in both the E-plane and the H-plane. The measured patterns show good agreement with the designed ones View full abstract»

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  • Near-field signature prediction using far-field scattering centers extracted from the shooting and bouncing ray technique

    Page(s): 337 - 338
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    We present a technique to predict the near-field radar cross section (RCS) of a target using the far-field scattering centers extracted from the shooting and bouncing ray (SBR) technique. The results generated using this methodology are verified against the brute-force SBR calculations for near-field scenarios. It is demonstrated that this technique is a fairly accurate and very efficient way to generate near-field RCS data provided that the transceiver is not very close to the target View full abstract»

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  • Wide-band airborne radar operating considerations for low-altitude surveillance in the presence of specular multipath

    Page(s): 176 - 191
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    Reliable detection of low-altitude platforms while simultaneously maintaining a desired search rate can be extremely difficult due to the presence of multipath. Wide-band operation in combination with frequency diversity is a sensible approach to not only mitigate, but in some cases exploit multipath channel characteristics. While a great deal of knowledge exists for characterizing the frequency dependencies of complicated multipath channels, relatively little attention has been given to examining how this knowledge could be exploited with wide-band radar sensors. The utilization of multipath channel characteristics is considered for the scenario of an airborne wide-band radar sensor performing low-altitude surveillance in a maritime environment. A brief overview of applicable multipath phenomenology is presented leading to a description of the propagation conditions selected for the construction of a representative channel. A generalized wide-band model of the sensor engagement applicable to the resolved and unresolved domains of the interference regime is utilized in combination with the simulated channel. Wide-band short pulse and linear frequency modulation waveforms are employed to consider waveform modulation characteristics in combination with desirable sensor bandwidth and frequency diversity for nominal operation at X-band. A brief discussion on implementation possibilities is also included View full abstract»

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  • Exact surface impedance/admittance boundary conditions for complex geometries: theory and applications

    Page(s): 223 - 230
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    A methodology useful to derive exact and higher order surface impedance/admittance boundary conditions (HOI/ABC's) for complex geometries is presented. It is shown that exact surface boundary conditions are always expressed through dyadic integral operators involving the tangential magnetic and electric fields all over the surface of the body. Quasi-local surface boundary conditions that include curvature effects are shown to be computable through an asymptotic approximation of the integral operators. Finally, an example of a surface admittance boundary condition useful to analyze a structure exhibiting discontinuities along its surface boundary is presented. Practical examples to demonstrate the feasibility of the proposed methodology, as well as the accuracy of the resulting surface boundary conditions are also presented View full abstract»

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  • Two time-derivative Lorentz material (2TDLM) formulation of a Maxwellian absorbing layer matched to a lossy medium

    Page(s): 192 - 199
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    A two time-derivative Lorentz material (2TDLM) is introduced to define polarization and magnetization fields that lead to an absorbing layer that can be matched to a lossy dielectric medium. The 2TDLM is a generalization of the successful uniaxial polarization and magnetization time-derivative Lorentz material (TDLM) which has been introduced as an absorbing boundary condition for simulation regions dealing with lossless materials. Expressions are derived to describe the propagation of an arbitrary plane wave in this 2TDLM Maxwellian absorbing material. They are used to study the scattering from a semi-infinite 2TDLM half-space of an arbitrary plane wave incident upon it from a lossy isotropic dielectric medium. Matching conditions are derived which produce reflectionless transmission through such an interface for any angle of incidence and frequency. Numerical tests are given which demonstrate the effectiveness of the resulting 2TDLM absorbing layer View full abstract»

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  • A parallel finite-element tearing and interconnecting algorithm for solution of the vector wave equation with PML absorbing medium

    Page(s): 278 - 284
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    A domain decomposition method based on the finite-element tearing and interconnecting (FETI) algorithm is presented for the solution of the large sparse matrices associated with the finite-element method (FEM) solution of the vector wave equation. The FETI algorithm is based on the method of Lagrange multipliers and leads to a reduced-order system, which is solved using the biconjugate gradient method (BiCGM). It is shown that this method is highly scalable and is more efficient on parallel platforms when solving large matrices than traditional iterative methods such as a preconditioned conjugate gradient algorithm. This is especially true when a perfectly matched layer (PML) absorbing medium is used to terminate the problem domain View full abstract»

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  • Radar reflection from clouds: gigahertz backscatter cross sections and Doppler spectra

    Page(s): 254 - 259
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    This work deals with reflection of GHz radar signals from typical clouds over The Netherlands. Four principal mechanisms of reflection are identified. While the backscatter cross sections for these are mostly well known, there is a need to identify which, if any, are dominant in each frequency range. Numerical studies of superpositions of the main backscatter mechanisms are presented for a range of parameter values thought to occur commonly. These studies confirm previous results, but are generalized to incorporate gamma-function particle drop-size distributions. The results are relatively insensitive to the power of the diameter in the distribution function. The Doppler spectra of the reflected signals sometimes exhibit a bimodal form. One possible mechanism investigated here is the observation of reflections that occur simultaneously from turbulently moving globules of particles and from incoherent reflections from particles with diameter-dependent spreads in velocities View full abstract»

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  • Pulse radiation from an insulated antenna: an analog of Cherenkov radiation from a moving charged particle

    Page(s): 165 - 172
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    Cherenkov radiation arises when a charged particle moves with a constant velocity that is greater than the speed of light in the surrounding medium. This radiation has distinctive characteristics. Near the charge, the electric field is most intense along a conical surface with apex at the charge-the Mach cone. In the far field, the radiation occurs predominantly in one direction-at the Cherenkov angle. An insulated antenna consists of a metallic cylindrical conductor covered by a concentric sheath of dielectric. In use, this antenna is embedded in a medium whose permittivity is often much greater than the permittivity of the insulation. When the antenna is excited by a pulse of voltage, a pulse of charge appears to travel along its length. The apparent velocity of this charge is close to the speed of light in the insulation, which, because of the difference in the permittivities, is greater than the speed of light in the surrounding medium. Thus, the radiation from the pulse excited, insulated antenna should be analogous to Cherenkov radiation from the moving charged particle. In this paper, the pulse-excited, traveling-wave insulated linear antenna is accurately analyzed using the finite-difference time-domain (FDTD) method. Results are obtained for the charge on the conductor, the near field, and the far field. These results show the striking similarity of the radiation from this antenna to Cherenkov radiation from the moving charge View full abstract»

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  • Numerical simulation of scattering from rough surfaces: a wavelet-based approach

    Page(s): 246 - 253
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    In this paper, a preliminary study is carried out to demonstrate the application of wavelets for improving the computation time and reducing computational memory required for evaluating the statistics of the scattered field from rough surfaces using the method of moments (MoM) in conjunction with a Monte Carlo simulation. In specific, Haar and the first order B-spline wavelet basis functions are applied to the MoM formulation of one-dimensional rough surfaces in order to compare the computation time and sparsity for wavelets in the same family but of higher order. Since the scattering coefficient (the second moment of the backscatter field per unit area) is a gentle function of the surface parameters and the radar attributes, it is demonstrated that a relatively high thresholding level can be applied to the impedance matrix, which leads to a sparser impedance matrix and faster computation time. It is also shown that applying a high threshold level the coefficients of the high-order wavelets would increase out of proportion, however, the effect of these current components averages out when computing the scattering coefficients. The resulting sparse impedance matrices are solved efficiently using fast search routines such as the conjugate gradient method. A systematic study is carried out to investigate the effect of different threshold levels on the accuracy versus computing speed criterion. The computed scattering coefficients are compared to previous results computed using a conventional pulse basis function as well as the existing theoretical solutions for rough surfaces. It is shown that wavelet basis functions provide substantial reductions in both memory requirements and computation time View full abstract»

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  • Matrix-exponent formulations for wave radiation and propagation in anisotropic stratified ionosphere

    Page(s): 339 - 341
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    A matrix-exponent method is developed to treat wave radiation and propagation in anisotropic stratified ionosphere. By using matrix-exponent formulations, the present approach is relatively compact for expressions and reliable for computations. As an example, the method is used to calculate the ELF excitation of the Earth-ionosphere waveguide by an equivalent dipole resulted from heating the lower ionosphere with modulated HF waves. Simulation results are presented and compared for different latitudes View full abstract»

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  • Exact geometrical optics solution for an isorefractive wedge structure

    Page(s): 335 - 336
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    A structure consisting of four right-angle wedges with a common edge, made of four different materials isorefractive to one another is considered. If a certain relation is satisfied among the four intrinsic impedances of the materials, then an incident plane wave generates reflected and transmitted plane waves that constitute the exact solution to the boundary-value problem. This geometrical-optics exact solution is valid for arbitrary polarization and arbitrary direction of incidence of the primary wave View full abstract»

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  • High-gain low-sidelobe double-vee dipoles

    Page(s): 333 - 335
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    This paper introduces a novel variation of the vee dipole antenna consisting of two coplanar vee dipoles with a common feed point. It is referred to as a double-vee dipole. Radiation characteristics of the double-vee dipole antenna are investigated numerically and experimentally. It is shown that the double-vee dipole can provide significantly higher directivity and lower sidelobes and back radiation than the conventional vee dipole. With increasing arm length, the directivity of the double-vee dipole exhibits a series of local maxima. Measured and predicted radiation patterns for a fabricated double-vee dipole are presented and shown to be in good agreement View full abstract»

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  • Some convergence considerations in space-domain moment-method analysis of a class of wide-band microstrip antennas

    Page(s): 147 - 160
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    The method of moments (MoM) analysis of probe-fed rectangular microstrip patches requires the inclusion of a probe-to-patch attachment mode-expansion function when the substrate thickness d⩾0.02λ, where λ is the free-space wavelength. The results for the input impedance showed increased divergence with measurements when the attachment mode was omitted from the full-wave analysis. The attachment mode can be expressed as an infinite eigenfunction series that increases the fill time of the impedance matrix in an MoM analysis. In an earlier investigation, the infinite eigenfunction series was reduced to a residue series that required one or two terms compared to about 55 terms for the eigenfunction series. In this paper, the convergence properties of the eigenfunction and residue series are investigated in view of rigorous MoM analysis. The relative errors resulting from replacing the eigenfunction by the residue series for the attachment mode, are compared by numerically evaluating a class of two-dimensional (2-D) spatial integrals shown to be closely related to the elements of an MoM impedance matrix. Additionally, the computation times for the evaluation of these integrals for the two forms of the attachment mode-expansion function are also included. Based on the superior convergence properties of the residue series for the attachment mode-expansion function, it is mathematically justified that this form can readily be used for analytic reduction of the spatial, reaction integrals from four to 2-D forms. This feature allows further reduction of the fill time of the MoM impedance matrix, suggesting the possibility of developing an efficient space-domain MoM technique for modeling of wide-band microstrip antennas 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