Showing papers in "IEEE Antennas and Propagation Magazine in 2000"

Smart antennas

Abstract: Smart antennas have received increasing interest for improving the performance of wireless radio systems. These systems of antennas include a large number of techniques that attempt to enhance the received signal, suppress all interfering signals, and increase capacity, in general. The main purpose of this article is to provide an overview of the current state of research in the area of smart antennas, and to describe how they can be used in wireless systems. Thus, this article provides a basic model for determining the angle of arrival for incoming signals, the appropriate antenna beamforming, and the adaptive algorithms that are currently used for array processing. Moreover, it is shown how smart antennas, with spatial processing, can provide substantial additional improvement when used with TDMA and CDMA digital-communication systems. The material presented is tutorial in nature, leaving the details for further study from the papers appearing in the reference list.

Electromagnetic mixing formulas and applications

Abstract: he overwhelming flavor of this book is an amezing simplicity. T Avoiding . . recent rcsearch resulls on percolation as a critical phenomcnon, linear and nonlincar bianisotropic-in-biallisolropic composite niaterds, fractal aggregates, erfective wavcnumber approaches, etc., this book takes the reader on a simplc tour of the homogenization of particulate conrposile materials (PCMs) comprising (generally) linear, isotropic, diclcctric component materials. A few forays into deeper rcsults that the reader may discern in this book, however, ought to whet thcir appctitc for clcctroinagnetic research on complex media. A very large part of this book is just right for cithcr a junior-level physics course or a senior-Icvel materials science course on PCMs at US undergraduate institutions.

Synthesis of antenna arrays using genetic algorithms

Abstract: In this article, two particular methods for the synthesis of the complex radiation pattern of a linear and a planar array, based on genetic algorithms (GAs), are discussed. First, we present a combination of Schelkunoff's method and GAs for linear arrays with arbitrary radiation patterns; then, we consider the synthesis of planar arrays with rectangular cells. The results show the flexibility of GAs to solve complex problems related to antenna synthesis, subject to many restrictions which are difficult to treat by classical optimization techniques. There is good agreement between the desired and calculated radiation patterns.

Antenna arrays in mobile communications: gain, diversity, and channel capacity

Abstract: Are antenna arrays in mobile communications different from arrays in other applications? The answer is yes, sometimes, and it is the purpose of this paper to explain, in a tutorial fashion, when this is the case, and what this means for path loss in link calculations One aspect is the classical gain of an antenna, which we have to understand in a new way Another aspect is the possibility for two arrays, in a scattering environment, to create parallel channels, and thus, in effect, act as many independent antennas at the same time, carrying much more traffic over the same bandwidth

The field equivalence principle: illustration of the establishment of the non-intuitive null fields

Abstract: The field equivalence principle, one of the fundamental concepts in electromagnetics, has numerous applications. However, for a beginning student, it is not easy to understand this concept thoroughly and to appreciate it. The dilemma faced by beginning students is illustrated. We have sources in a finite Region I, and an arbitrary mathematical surface separating Regions I and II. The equivalent problems for the exterior and interior regions are specified with the use of electric and magnetic equivalent currents impressed on the boundary surface. The acceptance of the establishment by the equivalent sources of the non-intuitive null field for the exterior problem (by the equivalent sources and the original source for the interior problem) is commonly bothersome and not comfortably realized. In order to clarify this, we revisit Love's and Schelkunoff s forms of the equivalence principle. Subsequently, we discuss two simple, analytically tractable illustrative examples, consisting of plane-wave fields in two half-space regions, separated by an infinite planar surface. In particular, the emphasis is on the establishment of the non-intuitive null fields developed by these equivalent sources. Various forms of equivalence are illustrated by simple analytical field expressions.

A conformal FDTD software package modeling antennas and microstrip circuit components

Abstract: This paper describes a conformal finite-difference time-domain (CFDTD) software package, and presents its applications to RF antennas and microstrip circuit components. The program includes a Visual Basic GUI, for graphically inputting the object geometries, setting source and boundary conditions, generating a non-uniform mesh, and post-processing of the data. A robust CFDTD technique is employed to handle conductors with curved surfaces and edges. Illustrative examples that show the application of the code for modeling antennas as well as microstrip discontinuities are presented in the paper.

Smart antennas in wireless communications: base-station diversity and handset beamforming

Abstract: We review diversity and smart antenna research applied to both base-stations and terminals. To illustrate the performance gains possible, the paper describes research being conducted by the Smart Antenna Group at Virginia Tech, in both smart base-stations and smart handheld terminals.

Propagation prediction techniques and antenna modeling (150 to 1705 kHz) for intelligent transportation systems (ITS) broadcast applications

Abstract: This paper discusses the basic aspects of radio-wave propagation and antenna modeling at 150 to 1705 kHz The paper contains descriptions of both sky-wave and ground-wave propagation-prediction models, in addition to the methodology used to analyze antennas that operate in this band A method of calculating and normalizing antenna gain for system computations is also discussed The sky-wave models described in this paper are valid from 150 to 1705 kHz The ground-wave models described in this paper are valid from 10 kHz to 30 MHz The propagation of radio waves from 150 to 1705 kHz includes both a ground wave and a sky wave, and is quite different from propagation at any other frequency The methods used for antenna modeling and analysis in this band are also quite unlike those in other bands The AM broadcast band of 535 to 1605 kHz is planned to be used in the advanced traveler information systems (ATIS) of intelligent transportation systems (ITS), to provide information such as road conditions, road hazards, weather, and incident reporting for rural travelers The band of frequencies from 285 to 325 kHz is presently being used in another application of ITS, called the Differential Global Positioning System (DGPS), which will be used for precision location of vehicles The propagation-prediction models and antenna-analysis methods described here can be used for designing systems and making performance predictions for both of these ITS applications, or for any other systems that operate in this band of frequencies from 150 to 1705 kHz Some examples of comparisons of measured and predicted data are also presented A computer program that includes all of these propagation-prediction models and antenna-modeling techniques was used

Distributed power control in CDMA cellular systems

Abstract: In wireless cellular communication, it is essential to find effective means for power control of signals received from randomly dispersed users within one cell. Effective power control will heavily impact the system capacity. Distributed power control (DPC) is a natural choice for such purposes, because, unlike centralized power control, DPC does not require extensive computational power. Distributed power control should be able to adjust the power levels of each transmitted signal using only local measurements, so that, in a reasonable time, all users will maintain the desired signal-to-interference ratio. In this paper, we review different approaches for power control, focusing on CDMA systems. We also introduce state-space methods and linear quadratic power control (LQPC) to solve the power-control problem. A simulation environment was developed to compare LQPC with earlier approaches. The results show that LQPC is more effective, and is capable of computing the desired transmission power of each mobile station in fewer iterations, as well as being able to accommodate more users in the system.

A pragmatic approach to adaptive antennas

Abstract: This paper presents a novel approach for efficient computation of adaptive weights in phased-array antennas. The fundamental philosophical differences between adaptive antennas and adaptive signal-processing methodology are also delineated. This approach, unlike the conventional statistical techniques, eliminates the requirement for an interference covariance matrix, and represents a rethinking of the entire conventional approach to adaptive processing. This approach provides greater flexibility in solving a wider class of problems, at the expense of a slightly reduced number of degrees of freedom. It is important to note that the application of a deterministic approach to address stochastic problems with an ergodic structure can be seen in the works of Wiener (1949) and Kolmogorov (1939). This paper presents examples to illustrate the effectiveness and uniqueness of this new pragmatic approach.

SuperNEC: antenna and indoor-propagation simulation program

Abstract: SuperNEC is a hybrid MoM-UTD antenna and electromagnetic simulation program, developed by Poynting Software (Pty) Ltd. The UTD primitives available in the code are dielectrically coated, multi-faceted plates and elliptical cylinders. The MoM primitives supported are wire segments. The program is capable of running in parallel on a heterogeneous network of processors. A Matlab-based, interactive graphical user interface is used to define the geometry to be simulated, as well as to view the simulation results. The program has been extensively verified using a multitude of test cases, which include comparison to published results and measurements.

Faster than Fourier: ultra-efficient time-to-frequency-domain conversions for FDTD simulations

Abstract: This tutorial compares several methods of converting from the time to the frequency domain for FDTD simulations. Applications include calculations of field or power distributions, antenna impedance, and radiation patterns. The traditional Fourier-transform methods are compared to two methods based on the solution of linear equations. This tutorial describes how to program and use these techniques, and evaluates their effectiveness for several applications, including analysis of a millimeter-resolution human model underneath a 60-Hz power line, antenna-radiation pattern and impedance calculations, the calculation of the coupling of a cellular telephone to the human head, and geophysical-prospecting simulations.

A summary of the transmitting and receiving properties of antennas

Abstract: An expression for the electric field radiated by an arbitrary current distribution is given, which leads to a straightforward characterization of an antenna by a complex vector called the equivalent length. All classical transmitting parameters-such as the radiated power density, power intensity, radiation resistance, directional gain, and gain-are simple functions of the equivalent length. The same is true for the receiving parameters: induced voltage, available power, equivalent surface, and antenna factor. Eliminating the equivalent length between the gain and an equivalent surface provides a simple, direct, and general proof of the fundamental relation for antennas.

Torque-bias profile for improved tracking of the Deep Space Network antennas

Abstract: Measurements at the drives of the NASA Deep Space Network (DSN) antennas indicated that the small gap between gear teeth was causing backlash at the gearboxes and elevation bullgear Left uncorrected, backlash will deteriorate the antenna's pointing precision At DSN, the backlash was eliminated by implementing two identical drives that impose two nonidentical torques (aka torque bias, or counter-torque) The difference between these two torques depends on the antenna load, and is shaped by the drive's electronic circuits The paper explains the shaping principles of the circuit, and shows how the circuits can be modified to improve the antenna dynamics under external disturbances

Fading distributions and co-channel interference in wireless systems

Abstract: This paper describes the physical models behind the various fading distributions applicable in wireless-communications systems, and presents the probability density functions, both for the signal envelope and for the power of the mobile radio signal. It addresses the problem of co-channel interference in terms of its mean value, for macrocellular and microcellular structures, and illustrates how the outage probability is influenced by a series of factors such as reuse pattern, modulation scheme, traffic load, and others.

On the design of a dual-band base station wire antenna

Abstract: A dual-band base station wire-antenna design is presented. The MoM and the simulated-annealing technique, for optimizing the antenna geometry, are used. The procedure starts from the design of a primary radiator, consisting of a dielectric covered driven dipole and a parasitic element. A Chebyshev antenna array is checked in both the 900 and 1800 MHz frequency bands, with and without the appropriate feed network. All the electrical characteristics, as well as the patterns of the antenna, are presented.

Potentials of ultra-short-pulse time-domain scattering measurements

Abstract: This article illustrates the potentials of ultra-short-pulse time-domain scattering measurements, and describes a facility to perform such measurements. The main advantages of measuring in the time domain are the high range resolution and the relatively simple measurement setup. A time-domain radar cross section measurement of a flat plate is performed, to illustrate the advantages of such methods over a conventional frequency-domain setup. The measurement was performed with a sampling oscilloscope, a pulse generator, and two 2-12 GHz ridged-horn antennas. Because the horns were not designed for transmitting transient signals, an additional system-response measurement, in combination with a software deconvolution algorithm, restored the impulse response of the object under test. Further processing separated the response of the object from clutter. A comparison of the time-domain data with calculated and measured frequency-domain radar cross sections shows good agreement. The high range resolution (100 ps) enabled the separation of scattering mechanisms (i.e., reflection, single and multiple diffraction). It is concluded that ultra-short-pulse time-domain measurements could be very beneficial.

Specific absorption rates (SARs) induced in head tissues by microwave radiation from cell phones

Abstract: The cellular/mobile phone industry has been reluctant to disclose SAR values to the public in the past. The decision by the Cellular Telecommunication Industry Association (CTIA) to require manufacturers to disclose SARs emerged after it became clear that SAR information was going to appear on the FCC Web site in a user-friendly form. It was also prompted by a British government panel's call, on May 11, 2000, for the SAR of each mobile phone to be printed on the box in which the phone is sold. The wireless telephone industry is concerned about how the public may use the SARs, because, according to one industry source, "SARs by themselves may be misleading and variations in SARs do not represent a variation in safety." Indeed, SAR values can vary as a function of the carrier frequency-for example, 850 or 1900 MHz, used to transmit the telephone message-or how the handset is held. The article presents some insights into SAR, what it is, how is it determined, and what it means.

Evaluation of directivity for planar antenna arrays

Abstract: In this paper, the directivity, including phase shift factors, for several types of uniformly excited planar arrays is obtained. Four types of dipole arrays are considered: arrays of collinear short dipoles, and of parallel short dipoles; and broadside and end-fire arrays of crosses of short dipoles. Curves of directivity versus inter-element spacing and scan angle for planar arrays with these element power patterns are presented.

Pencil-beam pattern synthesis with a uniformly excited multi-ring planar antenna

Abstract: In this paper, we describe the synthesis of pencil-beam power patterns for antennas composed of concentric rings of uniformly excited circular patches. The synthesis is performed by simulated annealing, varying the radii of the patch rings so as to optimize performance parameters. Examples illustrate the potential of the method.

Azimuth-track level compensation to reduce blind-pointing errors of the Deep Space Network antennas

Abstract: The 34-meter antennas of the NASA Deep Space Network are "wheel and track" antennas. The latter term refers to a set of wheels at the base of the structure, which roll on a circular steel track supported by a concrete foundation ring. The track is assumed flat; however, its level varies due to manufacturing imperfections, structural loads, non-uniformity of the soil, and temperature variations. It is specified that the deviations of the azimuth-track level shall not exceed /spl plusmn/0.5 mm. During tracking, this amplitude of deviations causes deformations of the antenna structure, resulting in pointing errors of /spl plusmn/2 mdeg, which exceed the required accuracy for 32-GHz (Ka-band) tracking. However, structural deformations caused by the azimuth track unevenness are repeatable; therefore, a look-up table can be created to improve the blind-pointing accuracy. This paper presents the process for creation of the look-up table, describes the instrumentation necessary for determining the pointing errors, and describes the processing of inclinometer data. It derives algorithms for the pointing-error estimation, and for the azimuth-axis tilt using the inclinometer data. It compares the error corrections based on the created look-up table and actual measurements of pointing errors using the conical scanning (conscan) technique. This comparison shows a satisfactory convergence that justifies the implementation of the approach in forthcoming NASA missions.

Genetic design of linear antenna arrays

Abstract: Discusses a way to optimize both the topology and the numerical parameters of an antenna design. The approach relies on an "antenna language" to define how antennas are constructed, and a genetic algorithm to create new designs using this language. The grammatical rules of a language can be very vague or very specific, depending on the purpose of the designer. With a vague grammar, genetic algorithms search a very large design space, and can occasionally find unexpected solutions to a design problem. Other times, they completely fail to find a reasonable solution because of the vastness of the search space. In this case, including knowledge about the problem into the grammar narrows the search to a region expected to yield good results. This yields more conventional design solutions that usually perform reasonably well. In an example, two languages were used to design a linear antenna array. The general language allowed a wide variety of designs, while the Yagi-log language confined the search to topologies known to perform well. The performance of the antennas produced by both languages was superior to that of a conventional log-periodic design. Further, the Yagi-log design was more fit than the unconventional design from the general language, illustrating the benefits of including knowledge in the grammar.

Simple formulas for the calculation of the average physical optics RCS of a cylinder and a flat plate over a symmetric window around broadside

Abstract: Closed-form formulas for the arithmetic-average radar cross section (RCS) of a perfectly conducting cylinder and of a perfectly conducting rectangular plate are derived The derivations proceed under the assumption that the conditions governing the use of the physical optics approximation are satisfied Examination of the behavior of these formulas reveals simple approximations of very reasonable accuracy The approximations allow quick and easy evaluation of dynamically collected RCS data in which the scattering phenomenology can be modeled by either a cylinder or a rectangular plate

Numerical modeling of an enhanced very early time electromagnetic (VETEM) prototype system

Abstract: In this paper, two numerical models are presented to simulate an enhanced very early time electromagnetic (VETEM) prototype system, which is used for buried-object detection and environmental problems. Usually, the VETEM system contains a transmitting loop antenna and a receiving loop antenna, which run on lossy ground to detect buried objects. In the first numerical model, the loop antennas are accurately analyzed using the method of moments (MoM) for wire antennas above or buried in lossy ground. Then, the conjugate gradient (CG) methods, with the use of the fast Fourier transform (FFT) or MoM, are applied to investigate the scattering from buried objects. Reflected and scattered magnetic fields are evaluated at the receiving loop to calculate the output electric current. However, the working frequency for the VETEM system is usually low and, hence, two magnetic dipoles are used to replace the transmitter and receiver in the second numerical model. Comparing these two models, the second one is simple, but only valid for low frequency or small loops, while the first modeling is more general. In this paper, all computations are performed in the frequency domain, and the FFT is used to obtain the time-domain responses. Numerical examples show that simulation results from these two models fit very well when the frequency ranges from 10 kHz to 10 MHz, and both results are close to the measured data.

Implementation issues for three-dimensional vector FEM programs

Abstract: Theoretical and practical issues that impact on the development of (especially) three-dimensional vector finite-element (FEM) programs are discussed. The theory of vector elements is briefly reviewed, and some troublesome aspects are highlighted: in particular, the interpretation of the degrees of freedom. The choice of element shape is briefly addressed, as is the evaluation of the elemental matrices. Several useful references are provided in this context. Data structures appropriate for a three-dimensional code are outlined, as is the issue of establishing mesh interconnectivity. The topics of mesh generation and linear algebra are very briefly considered. The paper concludes with some comments on post-processing, in particular, the computation of fields within the FE mesh.

RCS measurements and results of an engine-inlet system design optimization

Abstract: A number of papers have been published on the prediction of backscatter from engine-inlet-type structures. Measured data used to verify these predictions mostly consists of backscattered data from canonical structures, e.g., cylindrical waveguide cavities. In this paper, measured backscattered data for an unmanned aerial vehicle-type engine-inlet system is presented. The effects of the application of radar-absorbing materials to the engine-inlet system on the contribution to the RCS is discussed, and data measured in a compact-range facility are presented.

Design of a wideband miniature dielectric-filled waveguide antenna for collision-avoidance radar

Abstract: This article presents a theoretical and experimental study of the design of a miniaturized wideband dielectric-filled waveguide (DFW) antenna. The operational frequency is X band, i.e., 9.0/spl les/f/spl les/10.5 GHz. The desired bandwidth is 1.5 GHz. The antenna uses an air-gap matching network to reduce its high aperture reflection. In order to ease the integration with antenna circuits and to increase the bandwidth, two E-plane steps are used. The antenna is designed to have -3 dB beamwidths of 60/spl deg/ and 100/spl deg/ for the E- and H-plane patterns, respectively. An input reflection of less than -10 dB for the desired bandwidth is observed. The gain of the antenna is 7 dBi. A cross-polar level of less than -25 dB is achieved. This article discusses the mathematical model for input reflection, the design scenarios, and the experimental results.

An implementation of the UTD on facetized CAD platform models

Abstract: The Aircraft inter-Antenna Propagation with Graphics (AAPG) 2000 computer code relies on realistic computer-aided design (CAD) platform geometrical modeling to support uniform geometrical theory of diffraction (UTD) predictions of antenna-to-antenna coupling for aircraft-mounted antennas. The code employs novel ray-tracing techniques that permit the computation of UTD propagation paths over facetized objects of general shape. Moreover, geometrical data, required by UTD formulations for wedge and smooth-surface diffraction and reflection, are computed entirely from path and facetized-object geometries. Since the entire object representation in AAPG 2000 is in terms of facets, division of path-object interactions into wedge and smooth-surface cases must be performed heuristically.