Showing papers in "IEEE Antennas and Propagation Magazine in 1998"
TL;DR: Several novel elements are presented that offer significant enhancements to parameters such as impedance bandwidth, circular-polarization bandwidth, gain, or coupling to various feed structures in the dielectric-resonator antenna environment.
Abstract: This paper features some of the advances in dielectric-resonator antenna technology at the Communications Research Centre. Several novel elements are presented that offer significant enhancements to parameters such as impedance bandwidth, circular-polarization bandwidth, gain, or coupling to various feed structures. Several linear and planar arrays are also presented, to illustrate the performance of dielectric-resonator antenna elements in the array environment.
450 citations
TL;DR: In this article, an attempt is made to describe the ground-wave mechanism that is omni-present, and it is shown that the trapped surface wave can be a significant contribution to the total groundwave field, when the Earth boundary is sufficiently inductive.
Abstract: Radio-wave transmission over the surface of the Earth is a subject of enquiry going back to the beginning of the century. In this review, an attempt is made to describe the ground-wave mechanism that is omni-present. We first call attention to the early analytical contributions of Zenneck and Sommerfeld, based on a flat-Earth model. The subsequent controversies, particularly with regard to the role of the Zenneck surface wave, are outlined. Further developments by other pioneers, such as van der Pol, Fock, Bremmer, Norton, and Millington, are reviewed, and an attempt is made to put these in a modern context. We also show that the trapped surface wave can be a significant contribution to the total ground-wave field, when the Earth boundary is sufficiently inductive. Mixed-path theory and confirming model tests by Ray King are described briefly, along with calculated propagation curves for twoand three-section paths. The bibliography includes references to related topics, such as tropospheric refraction and topographic influences.
306 citations
TL;DR: After a review of the most commonly used ray-tracing techniques, a new method, called the angular z-buffer (AZB) technique, appears to be extremely efficient for GTD/UTD applications.
Abstract: The application of several ray-tracing techniques, in combination with GTD/UTD (geometrical theory of diffraction/uniform theory of diffraction), for an efficient analysis of propagation in urban scenarios is presented. The frequency of the analysis is in the UHF band, and a three-dimensional model of the geometry, using flat facets, is considered. After a review of the most commonly used ray-tracing techniques, a new method, called the angular z-buffer (AZB) technique, is presented. As is shown and validated with results, the AZB appears to be extremely efficient for GTD/UTD applications.
223 citations
TL;DR: FISC (Fast Illinois solver code), co-developed by the Center for Computational Electromagnetics, University of Illinois, and DEMACO, is designed to compute the RCS of a target described by a triangular-facet file.
Abstract: FISC (Fast Illinois solver code), co-developed by the Center for Computational Electromagnetics, University of Illinois, and DEMACO, is designed to compute the RCS of a target described by a triangular-facet file. The problem is formulated using the method of moments (MoM), where the Rao, Wilton, and Glisson (1982) basis functions are used. The resultant matrix equation is solved iteratively by the conjugate gradient (CG) method. The multilevel fast multipole algorithm (MLFMA) is used to speed up the matrix-vector multiplication in the CG method. The complexities for both the CPU time per iteration and the memory requirements are of O(Nlog N), where N is the number of unknowns. A 2.4-million unknown problem is solved in a few hours on the SGI GRAY origin 2000 at NCSA of the University of Illinois at Urbana-Champaign.
192 citations
TL;DR: In this paper, two methods of determining antenna efficiency are presented, one of which is a generalization of the "Wheeler cap method" and the other is a reflection method.
Abstract: An examination of the available literature shows that the accurate measurement of antenna efficiency is a difficult task, and that the available methods have potential for a substantial amount of error. This paper refines a method already presented, and gives detailed results with some newly defined reference antennas. Two methods of determining antenna efficiency are presented. One method may be considered to be a generalization of the "Wheeler cap method." The two methods presented show excellent agreement with each other, and show excellent agreement with the reference antennas. The reflection method appears to give the better results of the two methods, except under special circumstances.
179 citations
TL;DR: In this article, the authors provide a background and motivation for using model based parameter estimation (MBPE) in electromagnetics, focusing on the use of fitting models that are described by exponential and pole series.
Abstract: This article first provides a background and motivation for using model based parameter estimation (MBPE) in electromagnetics, focusing on the use of fitting models that are described by exponential and pole series. How data obtained from various kinds of sampling procedures can be used to quantify such models, i.e., to determine numerical values for their coefficients is also presented. The paper continues by illustrating applications of MBPE to various kinds of EM observables. It concludes by discussing how MBPE might be used to improve the efficiency of first-principles models based on frequency-domain integral equations.
177 citations
166 citations
TL;DR: In this paper, balanced antennas are viewed as two-ports, the S-parameters of which are determined by standard network-analyzer techniques, and simple formulas then lead to the differential input impedance.
Abstract: The input impedance of a balanced antenna is conventionally measured by using a balun that forces opposite currents in each part of the radiator. In this paper, balanced antennas are viewed as two-ports, the S-parameters of which are determined by standard network-analyzer techniques. Simple formulas then lead to the differential input impedance. The method is illustrated by measurements on a dipole antenna, which are compared to the results for monopoles over a ground plane.
134 citations
TL;DR: One of the goals of this tutorial is to illustrate how the wavelet decomposition is carried out, starting from the fundamentals, and how the scaling functions and wavelets are generated from the filter-theory perspective.
Abstract: The objective of this paper is to present the subject of wavelets from a filter-theory perspective, which is quite familiar to electrical engineers. Such a presentation provides both physical and mathematical insights into the problem. It is shown that taking the discrete wavelet transform of a function is equivalent to filtering it by a bank of constant-Q filters, the non-overlapping bandwidths of which differ by an octave. The discrete wavelets are presented, and a recipe is provided for generating such entities. One of the goals of this tutorial is to illustrate how the wavelet decomposition is carried out, starting from the fundamentals, and how the scaling functions and wavelets are generated from the filter-theory perspective. Examples (including image compression) are presented to illustrate the class of problems for which the discrete wavelet techniques are ideally suited. It is interesting to note that it is not necessary to generate the wavelets or the scaling functions in order to implement the discrete wavelet transform. Finally, it is shown how wavelet techniques can be used to solve operator/matrix equations. It is shown that the "orthogonal-transform property" of the discrete wavelet techniques does not hold in numerical computations.
114 citations
TL;DR: In this paper, spectral domain model based parameter estimation (MBPE), waveform-dominated MBPE, and adapting and optimizing the sampling of the generating model are discussed with reference to antenna theory including scattering.
Abstract: Discusses spectral domain model based parameter estimation (MBPE), waveform-dominated MBPE, and adapting and optimizing the sampling of the generating model. These are discussed with reference to antenna theory including scattering.
96 citations
TL;DR: A method of optimizing antenna performance is presented that exploits the natural or characteristic modes of the structure on which the antenna is mounted, and extensive use is made of the method of moments to compute these modes.
Abstract: Radio communications over relatively short distances, but beyond line-of-sight, present few problems nowadays, except in certain situations when no suitable, artificial means of relaying the signals exist. A mode of propagation, known as NVIS, or near vertical-incidence skywave, which relies only on the ionosphere, provides a solution. However, to exploit it requires careful choice of operating frequency, as well as the use of appropriate antennas. If the terminals are mobile, then the antennas, which are usually electrically small, can be the limiting factor. A method of optimizing antenna performance is presented that exploits the natural or characteristic modes of the structure on which the antenna is mounted. Extensive use is made of the method of moments, in the form of both the NEC and MININEC codes, to compute these modes, and to design and test antenna systems that exploit them. Measurements, made using an instrumentation package flown from a tethered balloon, compare very favorably with computed results.
TL;DR: In this paper, the authors describe the process of gain evaluation and give several simple gain-estimation formulas, together with recommendations for their use, and explain how to calculate the gain of an antenna.
Abstract: Gain is the most important performance parameter of an antenna. However, in many practical situations it is not possible to measure or calculate the gain of an antenna. Also, the interest in wireless applications has increased the need of system engineers to accurately estimate antenna gain. Many simple formulas are available for estimating gain. But, each formula has a range of applicability, and inappropriate use of these formulas will result in inaccurate gain values. This paper explains the process of gain evaluation, and gives several simple gain-estimation formulas, together with recommendations for their use.
TL;DR: This article presents an automated technique to extract the three-dimensional scattering-center model of a target from its geometrical CAD model, based on the shooting and bouncing ray (SBR) method.
Abstract: We present an automated technique to extract the three-dimensional scattering-center model of a target from its geometrical CAD model. The technique is based on the shooting and bouncing ray (SBR) method. In this article, we first review the basic concepts behind the three-dimensional scattering-center-extraction algorithm. Next, we present application examples of signature-data compression and radar-feature extraction, based on the scattering centers extracted from complex targets using such a methodology. We conclude by identifying some future areas of research.
TL;DR: In this article, the authors considered the rationale and illustrated the application of model-based parameter estimation (MBPE) to achieve reduced-order representations of electromagnetic observables via fitting models, the model based part of MBPE, that derive from the physics of EM fields.
Abstract: For pt.II see ibid., vol.40, no.2, p.51-66 (1998). This article has considered the rationale and illustrated the application of model-based parameter estimation (MBPE) to achieve reduced-order representations of electromagnetic observables via fitting models, the model-based part of MBPE, that derive from the physics of EM fields. The parameter-estimation part of MBPE is the process of obtaining numerical values for the coefficients of the fitting model by matching or fitting it to sampled values of the EM observable of interest. Although a wider range of fitting models are feasible, attention here is focused on what are termed waveform-domain models, comprised of exponential series, and spectral-domain models, comprised of pole series. These kinds of fitting models are shown to provide natural basis functions for many kinds of EM observables, whether these observables are based on experimental measurement or numerical computation.
TL;DR: In this article, a procedure to design axially symmetric Cassegrain or Gregorian dual-reflector antennas from various combinations of prescribed geometric parameters is presented, and the overall geometry of the antenna is derived in closed form.
Abstract: A procedure to design axially symmetric Cassegrain or Gregorian dual-reflector antennas from various combinations of prescribed geometric parameters is presented. From these input parameters, the overall geometry of the antenna is derived in closed form. This procedure can be used as the starting point of a synthesis procedure, where both main reflector and subreflector are shaped to create the desired aperture field distribution.
TL;DR: A survey of the methods of constructing phased-array antennas with optimum radiating structures that provide shaping of the flat-topped array-element patterns is given in this article, which corresponds to maximization of the array gain in a given limited scanning sector, using a corresponding minimum number of controlled elements.
Abstract: A survey is given of the methods of constructing phased-array antennas with optimum radiating structures that provide shaping of the flat-topped array-element patterns. This corresponds to maximization of the array gain in a given limited scanning sector, using a corresponding minimum number of controlled elements. The methods considered include the use of passive multiport networks, coupled dual-mode waveguides, protruding dielectric elements, corrugated structures, and quasi-optical networks that provide excitation of the necessary over-lapped subarrays.
TL;DR: In this article, the authors give a tutorial on the electromagnetic analysis and design of three key components of an MRI system, namely, the magnet, the gradient coil, and the radiofrequency (RF) coil.
Abstract: Magnetic resonance imaging (MRI) is a powerful new imaging method, which produces cross-sectional tomographic and three-dimensional images similar to those of x-ray computed tomography (CT). However, rather than relying on harmful ionizing radiation, MRT is based on the interaction between RF fields and certain atomic nuclei in the body, when they are in the presence of a strong magnetic field. An MRI system is one of the few complete systems in which the design relies heavily upon a knowledge of electromagnetics. We give a tutorial on the electromagnetic analysis and design of three key components of an MRI system, namely, the magnet, the gradient coil, and the radiofrequency (RF) coil. We also discuss the analysis and characterization of the interactions of RF electromagnetic fields with biological subjects.
TL;DR: A significant speed improvement is realized over that of the standard LU factorization of this matrix, and the method presented is referred to as the LU sparse integral factored representation (LUSIFER).
Abstract: All of the matrices which arise in the method-of-moments solution of scattering and antenna problems have a hidden structure. This structure is due to the physics of electromagnetic interactions. Matrix-algebra routines are used to uncover this structure in moment-method matrices, after they have been calculated. This structure is used to create a sparse representation of the matrix. Although this step involves an approximation, the error involved can be nearly as small as the precision of the calculation. Then, without further approximation, a sparse representation of the LU factorization of this matrix is computed. A significant speed improvement is realized over that of the standard LU factorization of this matrix. The resulting method can be added to any of a variety of moment-method programs to solve the matrix problem more quickly, and with less computer memory. For large problems this is the time-critical operation, so this allows larger problems to be solved. The computer program we have written can be used immediately with most moment-method programs, since it amounts to simply a better matrix-inversion package. The method presented is referred to as the LU sparse integral factored representation (LUSIFER).
TL;DR: Validation results show that the facility is capable of characterizing the radar cross section of a point target to within /spl plusmn/1 dB in magnitude and /splplusmn/5/spl deg/ in polarization phase difference over a wide range of bistatic angles.
Abstract: A fully polarimetric bistatic-radar facility has been constructed at the University of Michigan, to serve as a research tool for improved understanding of the nature of bistatic scattering for point and distributed targets. The facility is capable of operation at 10, 35, and 94 GHz, but only the 10-GHz system is described in the presentation. To meet both the size and design constraints both a horn antenna operating in the far-field, and a parabolic-dish antenna operating in a near-field focused mode, are utilized. A newly developed bistatic-calibration technique, using a flat metal plate, is used to calibrate the facility. Validation results, using a hemisphere over a conducting metal plate, show that the facility is capable of characterizing the radar cross section of a point target to within /spl plusmn/1 dB in magnitude and /spl plusmn/5/spl deg/ in polarization phase difference over a wide range of bistatic angles. Sample data for a point target and a distributed target are presented.
TL;DR: In this paper, the authors describe modifications to the integral-equation time-domain approach that offer the prospect of a reduction in cost scaling, to possibly the third power of frequency, and an associated large reduction in costs.
Abstract: Computation of scattering from multi-wavelength bodies is expensive, and costs scale with up to the sixth power of the incident frequency. Conventional integral-equation time-domain methods have costs scaling with the fifth power. Here are described modifications to the IETD approach that offer the prospect of a reduction in cost scaling, to possibly the third power of frequency, and an associated large reduction in cost. The approach exploits the pulsed nature of the illumination, which results in surface fields that are small most of the time over most of the body, on bodies that are electrically large. Neglect of these produces some modest increase in error, but allows large reductions in cost and storage requirements. In the examples shown, cost reductions by amounts approaching two orders of magnitude are obtained, with the factor by which costs are reduced itself increasing with roughly the square of the body size; storage requirements are rendered essentially negligible.
TL;DR: In this article, the wavelet transform is described from the perspective of a Fourier transform and an adaptive window is presented that may be optimally tailored to suit one's needs and hence, possibly, the scaling functions and the wavelets.
Abstract: The wavelet transform is described from the perspective of a Fourier transform. The relationships among the Fourier transform, the Gabor (1946) transform (windowed Fourier transform), and the wavelet transform are described. The differences are also outlined, to bring out the characteristics of the wavelet transform. The limitations of the wavelets in localizing responses in various domains are also delineated. Finally, an adaptive window is presented that may be optimally tailored to suit one's needs, and hence, possibly, the scaling functions and the wavelets.
IBM1
TL;DR: This paper compares the use of three programming languages, FORTRAN, C/C++, and Java, in a simple numerical computation of the product of two matrices, and examines the causes of these differences.
Abstract: This paper compares the use of three programming languages, FORTRAN, C/C++, and Java, in a simple numerical computation of the product of two matrices. The goal is to show that, although similar in expressiveness, the performance of the three languages can be vastly different with current implementations. We analyze the causes of these differences, and we discuss how state-of-the-art compilation can be used to deliver excellent performance for all three languages.
TL;DR: In this paper, a stacked microstrip antenna designed at the University of Utah is compared with a typical mobile phone equipped with the conventional monopole antenna, and the main thrust is to highlight the problems associated with typical mobile-telephone antennas and to show how microstrip-antenna technology could solve most of these problems.
Abstract: The increasing use of cellular telephones has spawned considerable research effort on the design of compact, high-efficiency antennas for mobile telephones, exhibiting low EM coupling to the human head. Microstrip antennas seem now to render possible the achievement of all these characteristics. We present some results on a stacked microstrip antenna designed at the University of Utah, comparing its performance with that of a typical handset equipped with the conventional monopole antenna. The main thrust is to highlight the problems associated with typical mobile-telephone antennas, and to show how microstrip-antenna technology could solve most of these problems.
TL;DR: In this article, a large number of VLF scattering events associated with optical sprites were analyzed, and it was shown that the average scattering distribution has a strong frontal lobe and also has strong large-angle scattering.
Abstract: Analysis of a large number of VLF scattering events associated with sprites results in an average scattering distribution having a strong frontal lobe, and also having strong large-angle scattering. This is consistent with the scattering pattern produced by a theoretical model of the sprite as an array of interacting vertical plasma columns, indicating that some part of the sprite is "hard" and contains complex structure (most likely the "stalactites" seen in optical sprites). Attempting to determine the scattering distribution due to sprites using a high threshold for event detection could result in observation of only the frontal lobe and the minima on either side of it, and not the scattering at larger angles. This problem would be worsened by the use of only amplitude, and not phase, monitoring to detect perturbations, and could lead experimenters to the conclusion that sprites are narrow-angle scatterers, with a smoothly varying diffuse structure.
TL;DR: In this paper, the radiation from two simple filamentary current distributions, traveling-wave and uniform, was considered, and the total energy radiated by the distributions, U/sub rad/, was shown to behave as ln(/spl tau/sub a///spltau/) for the traveling wave distribution, and as Ln tau//sub a/=h/c for the uniform distribution, where h/c is the time for light to travel the length of the filament.
Abstract: We have considered the radiation from two simple filamentary current distributions: traveling-wave and uniform. For an excitation that is a Gaussian pulse of characteristic time /spl tau/, the total energy radiated by the distributions, U/sub rad/, was shown to behave as ln(/spl tau//sub a///spl tau/) for the traveling-wave distribution, and as /spl tau//sub a///spl tau/ for the uniform distribution, where /spl tau//sub a/=h/c is the time for light to travel the length of the filament. An examination of numerical results shows that two physical interpretations can be used for the radiation. Radiation can be considered to arise at the two ends of the filament in the form of spherical wavefronts centered at the ends. The overlap of these wavefronts, which changes with the ratio /spl tau//sub a///spl tau/, is the cause of the observed dependencies for U/sub rad/. An alternative explanation is that radiation occurs along the entire length of the filament. Destructive interference of the radiation from different points on the filament then causes the radiation to be insignificant except at the times corresponding to radiation from the ends of the filament.
TL;DR: In this paper, a Ku-band linearly polarized 16/spl times/16 microstrip-patch array is presented, which is fed by a corporate network, using dog-bone shaped coupling apertures.
Abstract: The design and development of a Ku-band linearly polarized 16/spl times/16 microstrip-patch array is presented. The array is fed by a corporate network, using dog-bone shaped coupling apertures. Thin substrates and low-loss foam are used for antenna-bandwidth enhancement. The design of the radiating patches and feed network was aided by the commercial software package, Ensemble(R). Experimental results for the array, in terms of its return loss, radiation pattern, and gain, are presented.
TL;DR: The conditions derived in Granet are extended to take into account the phase-center position of the feed, to achieve the minimum-blockage condition.
Abstract: For pt.1 see ibid., vol.40, no.2, p.76-82 (1998). An easy procedure to design classical Cassegrain or Gregorian dual-reflector antennas has been presented in Granet (1998). This procedure allows the antenna designer to fully define the antenna geometry with different sets of input parameters, depending on the requirements of the antenna size and its performance. In this paper the conditions derived in Granet are extended to take into account the phase-center position of the feed, to achieve the minimum-blockage condition. This procedure can be used as the starting point of a synthesis procedure, where both main reflector and subreflector are shaped to create the desired aperture-field distribution.
TL;DR: Ka-band propagation measurements with the Advanced Communications Technology Satellite (ACTS) are entering their fifth year of data collection, with unusually strong antenna-wetting effects observed with the APTs, plus analyses related to impairment dynamics, rain-rate modeling, and clear-air effects.
Abstract: Ka-band propagation measurements with the Advanced Communications Technology Satellite (ACTS) are entering their fifth year of data collection. Beacon-signal levels and radiometric sky noise are being measured at 20.2 and 27.5 GHz, with supporting meteorological information, at seven experiment sites in the US and Canada, using ACTS propagation terminals (PSTs) supplied by NASA. Uniform system-calibration, data-collection and data-processing procedures and software, prescribed and developed by the NASA Propagation Experimenters (NAPEX) Group, promote commonality of the data sets and permit comparison among results from the various sites. Goals of the program are to develop basic Ka-band propagation knowledge, and to supply information needed to improve propagation-prediction models and to develop impairment-mitigation techniques. The APT sites were selected to sample a variety of North American climate regions and their associated propagation features. Statistical results from the propagation measurements are presented and compared to the predictions of new and existing prediction models. Other aspects of the measurements, including unusually strong antenna-wetting effects observed with the APTs, plus analyses related to impairment dynamics, rain-rate modeling, and clear-air effects, are discussed.
TL;DR: The history of diffracted rays is usually considered to be less than a century long as mentioned in this paper, however, a careful study of experiments and observations made by Newton and described in his Optiks shows that diffracted ray had been observed nearly 300 years ago.
Abstract: The history of diffracted rays is usually considered to be less than a century long. However, a careful study of experiments and observations made by Newton and described in his Optiks shows that diffracted rays had been observed nearly 300 years ago. To show this is the goal of the present paper. Optiks Book III, part I, also comprehends diffraction. Of the various observations by Newton in that book, two have been chosen in the present paper to demonstrate, by applying modern theories, that Newton was actually observing diffracted rays.
TL;DR: In this paper, the authors provide a review of existing solutions for this important topic in diffraction theory, in particular numerical and analytical techniques, suitable to properly account for the scattering properties of the anisotropic impedance faces, are considered.
Abstract: Electromagnetic scattering from the edge of an anisotropic impedance wedge, illuminated at oblique incidence, is addressed. In particular, the paper provides a review of existing solutions for this important topic in diffraction theory. Both numerical and analytical techniques, suitable to properly account for the scattering properties of the wedge's anisotropic impedance faces, are considered.