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Antennas and Propagation, IEEE Transactions on

Issue 1 • Date Jan 1990

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Displaying Results 1 - 21 of 21
  • Case study of intense scintillation events on the OTS path

    Page(s): 107 - 113
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    Selected events of enhanced field amplitude fluctuations recorded on the Orbital Test Satellite (OTS) path have been analyzed. The statistical properties of the beacon signals together with their correlation with the concurrent radiometric signal suggest that both scintillation and a variable attenuation mechanism act to produce the field fluctuations. Analysis of the coherence between the beacon and the radiometric signals gives an indication of the fluctuation frequency range over which each effect prevails. The comparison between scintillation data collected by a large (17 m) and a small (3 m) antenna indicates that the turbulence of the refractive index was particularly high during the course of the considered events View full abstract»

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  • Effect of error correlations and systematic errors on average array sidelobes

    Page(s): 124 - 129
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    The effect of error correlations on the average error-induced sidelobe level of a planar array is studied. It is found that correlations among the random errors rapidly decrease the average error-sidelobe level (relative to the level when all errors are uncorrelated) as the correlation radius is increased. However, even though the average error-sidelobe level decreases, there are certain types of correlation that produce large local sidelobe increases. In particular, it is found that a one-dimensional correlation produces a large ridge of sidelobes in the plane orthogonal to the direction of the correlation. Simple results are derived from both the amplitude and the angular width of this ridge View full abstract»

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  • On the analysis of frequency-selective surfaces using subdomain basis functions

    Page(s): 40 - 50
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    The problem of scattering from frequency-selective surfaces (FSSs) has been investigated by expanding the unknown current distribution with three different sets of basis functions, namely the roof top, surface patch, and triangular patch. The boundary condition on the total electric field on the FSS due to this current distribution is tested either by a line integral or by the Galerkin procedure. This results in an operator equation that can be solved either by a direct matrix inversion method or by an iterative procedure, namely the conjugate gradient method (CGM). The performance of each of these basis and testing functions is evaluated. It is found that the roof-top and the surface-patch basis functions in conjunction with the Galerkin testing are superior in computational efficiency to other combinations of basis and testing functions that have been studied. Comparison of the CPU times on a Cray X-MP/48 supercomputer in solving the operator equation by the direct matrix inversion method and the CGM is provided. Frequency responses of free-standing, periodic arrays of conducting and resistive plates are also presented View full abstract»

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  • Modeling folded dipoles and feedlines for radiation and scattering

    Page(s): 30 - 39
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    Dipole-like antennas, suited for use as radiating elements in large phased arrays, are modeled for efficient analysis by decomposing the antenna into a radiating mode part and a transmission line mode part. All the essential features of folded dipoles, integrated balun feedlines, and feedline scattering are contained in the models. Feedline scattering, in particular, is an important consideration in analyzing phased arrays. The models are suited for moment-method analysis of arrays of such antennas, because the models entail simplified radiation mode parts. Extensive results of exercising the models were obtained for assessing model accuracy. Agreement between full-moment-method analyses and combined-mode analyses was reasonably good View full abstract»

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  • The theory of chirowaveguides

    Page(s): 90 - 98
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    The theory of chirowaveguides is discussed, and their salient features are analyzed. It is shown that the Helmholtz equations for the longitudinal components of electric and magnetic fields in chirowaveguides are always coupled and that, consequently, in these waveguides individual transverse electric (TE), transverse magnetic (TM), or transverse electromagnetic (TEM) modes cannot be supported. As an illustrative example, the parallel-plate chirowaveguide is analyzed in detail and the corresponding dispersion relations, cutoff frequencies, and propagating and evanescent modes are obtained. In the dispersion (Brillouin) diagram for a chirowaveguide, three regions are identified: the fast-fast-wave region, the fast-slow-wave region and the slow-slow-wave region. For each of these regions, the electromagnetic field components in a parallel-plate chirowaveguide are analyzed and the electric field components are plotted. Potential applications of chirowaveguides in integrated optical devices, communications systems, and printed circuit antennas are mentioned View full abstract»

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  • Adaptive beam-space nulling of multipath signals

    Page(s): 129 - 134
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    A beam-space adaptive nulling technique that has potential for applications in communication circuits degraded by multipath signals is described. The technique is based on a well-known vector relationship in beam space. A linear transformation is used to prevent the degradation of the desired signal reception. Results are presented from tests of the technique that were conducted using real multipath data. The data used in these tests were recorded using a 32-element sampled aperture antenna (SAMPAR). These measurements were carried out on an over-water path. The direct signal, i.e. the signal that propagates via the shortest path from the transmitter to the receiver antenna, is selected as the desired signal; its reception is enhanced by suppressing the unwanted signals, i.e. the multipath signals that propagate via a reflection from the surface of the water. Examples are given in which the wanted and unwanted signals are separated by less than a beamwidth View full abstract»

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  • Studies on wide inclined slots in the narrow wall of rectangular waveguide

    Page(s): 24 - 29
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    Investigations of the resonant length and admittance of inclined slots in the narrow wall of a rectangular waveguide that are sufficiently wide for applications involving high power handling capacity are presented. For high accuracy, the integrals appearing in the expression for the self-reaction are evaluated numerically instead of attempting to derive closed-form expressions, which are applicable only to thin slots. Numerical data on the variation of resonant length with slot width and also on the variation of real and imaginary parts of the admittance with 2L/λ as well as frequency for wide slots are presented. Comparison between theory and experimental results is presented for a slot with a width of 1.6 mm and a depth of cut in both the broad walls of 2.23 mm View full abstract»

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  • Scattering from irregular-edged planar and curved surfaces

    Page(s): 137 - 141
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    An electric field integral equation (EFIE) formulation is used to describe the electromagnetic scattering from finite planar and curved perfect electrical conducting surfaces truncated by an irregular edge. The edge can have an arbitrary form if it satisfies certain differentiability requirements. Similarly, the generating curve describing the surface can be convex, concave, or a combination of both. An edge-dependent entire domain Galerkin expansion is used for the current variation along the surface in the direction of translation. A subdomain expansion is used along the orthogonal direction. The backscatter cross sections obtained from the method of moments are compared with experimental data View full abstract»

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  • A unified approach to the design of robust narrow-band antenna array processors

    Page(s): 17 - 23
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    A unified approach to the design of robust narrowband antenna array processors is presented. The approach is based on the idea of minimizing the weighted mean-square-deviation between the desired response and the response of the processor over variations in parameters. Three specific examples of robust design are considered: robustness against directional mismatch, robustness against array geometry errors, and robustness against channel phase errors. Initially, a general quadratic constraint on the weights is developed. However, it is then shown that the quadratic constraint can be replaced by linear constraints or at most linear constraints plus norm constraint. These latter constraints are no more complex than those required for designs which do not incorporate robustness features explicitly. Numerical results show that the proposed approach appears to offer a unified treatment for directly designing narrowband processors which are robust against various types of errors and mismatches between signal model and actual scenario View full abstract»

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  • The RCS of a rectangular microstrip patch in a substrate-superstrate geometry

    Page(s): 2 - 8
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    A general scattering formula is derived for an arbitrary resonant conductive body within a layered medium, which shows that the body radar cross section (RCS) is directly related to the radiation efficiency of the body. The radar cross section of a rectangular microstrip patch antenna in a lossy substrate-superstrate configuration is then investigated as a specific case. Results are presented to show the effects of loss in the substrate, a lossless superstrate, and a lossy superstrate View full abstract»

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  • Note on interpolational basis functions in the method of moments [EM scattering]

    Page(s): 134 - 137
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    Efficient basis sets for the method of moments may be obtained using quasi-localized bandlimited interpolational functions that, broadly speaking, are defined relative to a mean sampling rate that is adjusted to curvature and proximity to edges, thus reflecting the local spatial-frequency bandwidth. Computed scattering data in a number of structures, including perfectly conducting circular and elliptical two-dimensional cylinders as well as a flat infinite strip, indicate that reasonable accuracy can be obtained with an average rate of between 2.5 and 3 basis functions per wavelength. Average sampling rates need not correspond strictly to the bandwidth of the basis functions, and there is considerable latitude with respect to random variation of sampling intervals. Although each basis function typically extends over several sample points, required integrals can be obtained speedily by the use of standard sampling-theoretical methods View full abstract»

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  • Empirical-computational methods in the study of creeping waves

    Page(s): 51 - 59
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    A method is described for the direct extraction of parameters characterizing the fundamental creeping wave modes by the application of elementary signal processing techniques to the total scattered radiation from arbitrary layered cylinders. Filtering is implemented by multiplying the field by suitable window functions designed to separate the creeping wave from the remaining radiation, and can be applied in the Fourier domain or directly in configuration space. Examples are presented for perfect electric conductors, impedance surfaces, and dielectrically coated cylinders. The method is easy, fast, and well suited to the direct observation of qualitative behavior patterns such as those associated with resonances View full abstract»

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  • Analysis of microstrip antenna structures using the `add-on' technique

    Page(s): 114 - 117
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    Microstrip antennas and antenna arrays are analyzed using the add-on technique. The goal of this work is to provide a way of handling more than 1000 unknowns while maintaining agreement with the moment method. The problem of a 1008-unknown finite array, with 11 elements and connecting and feeding lines, is an example of the capability of the method. In a CPU time of 1.5 h for each frequency point, accurate impedance results are obtained from the resultant standing-wave pattern along the feeding microstripline. The accuracy of the method is established by comparison with published results when possible, and by the computation of the root mean square (RMS) error in the boundary condition on the conductor View full abstract»

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  • Scattering from partial bodies of revolution

    Page(s): 69 - 75
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    The electromagnetic scattering from classes of partial bodies of revolution formed by the presence of perfectly electrically or magnetically conducting (PEC or PMC) planes is investigated. It is found that when the Galerkin technique is used with a harmonic circumferential expansion, there is a modal decoupling of the integral operators. The choice of these expansions is determined by the angle subtended by the intersecting planes and by whether these planes are PEC, PMC, or a combination of the two. In this analysis, the partial bodies can be either PEC or penetrable. Examples illustrating this formulation are given for conducting and dielectric hemispherical geometries and for a modified corner reflector. Measured and calculated data are compared for the case of a half-cylinder on a finite ground plane View full abstract»

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  • Identification of the natural resonance frequencies of a conducting sphere from a measured transient response

    Page(s): 141 - 143
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    It is shown that the imaginary parts of the first six natural resonant frequencies of a spherical scatterer can be determined quite accurately from measurements of both its surface charge and scattered field responses. This is an important step in verifying the practicality of characterizing low-Q targets by means of their resonance frequencies. Results for the real parts of the natural frequencies are not highly accurate. However, these values do not need to be known with great precision, since radar target discrimination schemes can be geared to use information primarily from the imaginary parts. It is important to note that target identification does not depend on the extraction of resonant frequencies in real time under adverse noise conditions. Rather, the resonance frequencies of the targets are assumed to be a priori knowledge, determined in a controlled, low-noise laboratory environment View full abstract»

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  • Procedures for noninvasive electromagnetic property and dosimetry measurements

    Page(s): 99 - 106
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    Five different formulations are presented that can be used to determine local values of the complex permittivity, electric field, polarization current density, and rate of energy deposition, within an inhomogeneous dielectric object of arbitrary shape. These formulations were obtained from the matrix equations for moment-method solutions of the electric field integral equation. Implementation of these formulations requires knowledge of the shape of the object and the incident electric field, and measurements of the scattered electric field at a number of points external to the object. The use of a new type of model, with antenna arrays for measurements, has caused a substantial improvement in matrix conditioning. Results of numerical simulations using 21-cell models with high dielectric contrast (bone/muscle) suggest that all five formulations have sufficient accuracy to warrant experimental testing using cylindrical scatterers with transverse magnetic polarization View full abstract»

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  • Focusing properties of a microwave radiator utilizing a slotted rectangular waveguide

    Page(s): 121 - 124
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    A microwave radiator, utilizing a slotted rectangular waveguide that is bent into an equiangular spiral, for focusing leaky microwaves radiated from a continuous longitudinal slot is presented. The radiation fields near the focus are derived by setting up a magnetic current on the slot. Some of the fundamental focusing properties of this spiral slotted rectangular waveguide (SSRW) radiator, such as the radiated power distribution and the phase behavior near the focus, are also discussed numerically. The power distribution near the focus in the form of intensity contours is compared with preliminary measurement of the prototype operating at 10 GHz View full abstract»

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  • Radiation from a dipole near a general anisotropic layer

    Page(s): 9 - 16
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    The radiation properties of a dipole source located near a gyrotropic layer are investigated analytically. Both electric and magnetic anisotropy of the most general form are assumed. Fourier-transform domain field representations in conjunction with matrix analysis techniques are used to facilitate the analysis. Transmission phenomena through the general anisotropic layer are investigated by examining the radiation patterns at the far-field region. The analysis is also used to derive the response of the anisotropic layer to an incident plane wave. In this case, the transmission and reflection coefficient matrices are obtained View full abstract»

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  • Limitations on reflector antenna gain by random surface errors, pointing errors, and the angle-of-arrival jitter

    Page(s): 117 - 121
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    The effects of random surface, pointing, and angle-of-arrival errors on the antenna gain are discussed. It is shown that for manufacturing tolerance ratios greater than 105, the random surface errors decrease the antenna gain only when D/λ>3000. The pointing/angle-of-arrival error causes nonnegligible loss in gain when it exceeds approximately a few tenths of the antenna beamwidth. The gain (and the size) of very large reflector antennas is generally limited by the combined effects of the above errors View full abstract»

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  • FDTD algorithm in curvilinear coordinates [EM scattering]

    Page(s): 76 - 89
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    The finite-difference-time-domain (FDTD) algorithm for the solution of electromagnetic scattering problems is formulated in generalized coordinates in two dimensions and implemented in a code with the lowest-order Bayliss-Turkel radiation boundary condition expressed in cylindrical coordinates. It is shown that, for a perfect conductor, such a formulation leads to a stable, well-posed algorithm and that, in regions where the curvature of the coordinate lines is not great, the dispersion and anisotropy effects are negligible. Such effects become more pronounced in regions of high curvature, leading to unphysical phase shifts. The magnitude of such shifts and the amount of wavefront distortion is studied via numerical experiments using a cylindrical mesh. Near-field results are given for two canonical shapes for each polarization: the circular cylinder and cylinders of square and rectangular cross sections. These results are compared with those obtained by exact eigenfunction expansion techniques, with method-of-moments (MM) solutions, and with solutions obtained from an alternate FDTD approach. In each case, agreement is excellent. The propagation of a plane wave through a polar space in the absence of a scatterer is also examined, and it is shown that the FDTD algorithm is capable of tracking the incident wave closely View full abstract»

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  • A hybrid equation approach for the solution of electromagnetic scattering problems involving two-dimensional inhomogeneous dielectric cylinders

    Page(s): 60 - 68
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    A numerical procedure for the solution of electromagnetic scattering problems involving inhomogeneous dielectric cylinders of arbitrary cross section is discussed. The cases of illumination by both transverse magnetic (TM) and transverse electric (TE) plane waves are considered. The scattering problems are modeled via a hybrid integral-equation/partial-differential-equation approach. The method of moments is applied to obtain a system of simultaneous equations that can be solved for the unknown surface current densities and the interior electric field. The interior region partial differential equation and the exterior region surface integral equation are coupled in such a manner that many existing surface integral equation computer codes for treating problems involving scattering by homogeneous dielectric cylinders can be modified easily to generate the block of the matrix corresponding to the surface current interactions. The overall system matrix obtained using the method of moments is largely sparse. Numerical results are presented and compared with exact solutions for homogeneous and inhomogeneous circular cylinders 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