Showing papers by "Yahya Rahmat-Samii published in 1994"

Performance analysis of antennas for hand-held transceivers using FDTD

Abstract: The design of antennas for hand-held communications devices depends on the implementation of simulation tools that can accurately model general topologies. The paper presents the analysis of small antennas mounted on hand-held transceivers using the finite-difference time-domain (FDTD) method. The key features of the FDTD implementation are discussed, with particular emphasis placed upon modeling of the source region. The technique is used to predict the gain patterns and broadband input impedance behavior of monopole, planar inverted F, and loop antenna elements mounted on the handset. Effects of the conducting handset chassis, the plastic casing around the device, and lumped elements integrated into the antenna design are illustrated. Experimental results are provided to verify the accuracy of the computational methodology. The concept of antenna diversity is discussed, and key assumptions and expressions are provided that characterize the multipath fading fields. Several computational examples demonstrate the diversity performance of two receiving antennas on a single handset. >

Genetic algorithm optimization and its application to antenna design

Abstract: Synthesis of antenna patterns employing iterative optimization techniques has been studied by many authors. However, successful application of these approaches to pattern synthesis has usually been limited to relatively simple arrays or has required careful, intelligent selection of the optimization starting points dictated by the nature of the optimization techniques used and the functions being optimized. This is because conventional functional optimization techniques are either based on greedy, local optimization methods such as gradient methods or consist of random walk solution space searches. In either case, these conventional techniques are often poorly suited to the task of arbitrary pattern synthesis in 1D and 2D antenna arrays due to the high dimensional, multimodal functional domains involved. In addition, traditional optimization techniques usually require the object function to be, at the very least, continuous and, in many cases to be differentiable, placing severe limitations on the form and content of the object function. This paper presents a radically different and relatively new functional optimization methodology known as genetic algorithm (GA) optimization that overcomes the above-mentioned problems of the traditional techniques and discusses how GA optimization is applied to 1D and 2D antenna design. >

The bi-polar planar near-field measurement technique, part I: implementation and measurement comparisons

Abstract: A novel customized bi-polar planar near-field antenna measurement technique is presented as an alternative to the plane-rectangular and plane-polar measurement techniques. The bi-polar near-field scanner incorporates an axially rotating test antenna and a rotating probe-carrying arm to sample the near-field on a data grid consisting of concentric circles and radial arcs. This technique offers a large scan plane size with reduced "real estate" requirements and a simple mechanical implementation resulting in a highly accurate and cost-effective antenna measurement and diagnostic system. Part I of this two-part paper gives an introduction to the bi-polar near-field technique and a description of the unique hardware implementation at the University of California, Los Angeles (UCLA). Measured results are compared with those produced by a far-field range and a plane-rectangular planar near-field range to verify the implementation and validate the method. Excellent agreement was obtained for both the co-polarized and cross-polarized fields. >

The bi-polar planar near-field measurement technique, part II: near-field to far-field transformation and holographic imaging methods

Abstract: A novel customized bi-polar planar near-field measurement technique is presented in a two-part paper. This bipolar technique offers a large scan plane size with minimal "real-estate" requirements and a simple mechanical implementation, requiring only rotational motions, resulting in a highly accurate and cost-effective antenna measurement and diagnostic system. Part I of this two-part paper introduced the bi-polar planar near-field measurement concept, discussed the implementation of this technique at the University of California, Los Angeles (UCLA), and provided a comparative survey of measured results. This paper examines the data processing algorithms that have been developed and customized to exploit the unique features of the bi-polar planar near-field measurement technique. Near-field to far-field transformation algorithms investigated include both interpolatory and non-interpolatory algorithms due to the a typical arrangement of the bi-polar near-field samples. The algorithms which have been tailored for the bi-polar configuration include the optimal sampling interpolation (OSI)/fast Fourier transform (FFT), Jacobi-Bessel transform, and Fourier-Bessel transform. Additionally, holographic imaging for determination of antenna aperture fields has been incorporated to facilitate antenna diagnostics. Results for a simulated measurement of an array of infinitesimal dipoles and a measured waveguide-fed slot array antenna are included. Appropriate guidelines with respect to the advantages and disadvantages of the various processing algorithms are provided. >

Electromagnetic characteristics of superquadric wire loop antennas

Abstract: The analysis of antenna configurations in the form of a generalized superquadric loop, which includes circular, elliptical and rectangular loop geometries, is presented in this paper. Use of a Galerkin form of the moment method with piecewise sinusoidal subsectional basis and testing functions provides rapid numerical convergence and accurate representation of the antenna current. A convenient parametric representation for the superquadric curve is developed to allow a subsectional formulation using curved wire segments, rather than the commonly employed piecewise linear segments, to construct the geometry. Both magnetic frill and delta gap source models are implemented to allow a detailed study of input impedance, directivity, radiation pattern and current distribution as a function of various geometrical parameters. The results are shown to compare well with previous results for the special case of a circular loop antenna. Some useful curves are presented to aid in the design of practical superquadric loop antennas. >

The electromagnetic interaction between biological tissue and antennas on a transceiver handset

Abstract: In the design of antennas for use with hand-held transceivers, the electromagnetic interaction between the antenna and the nearby biological tissue is a key factor which must be considered when selecting the antenna configuration. The engineer must consider the effect of the body on the antenna performance as well as the rate at which energy is absorbed in the tissue. Investigation of such issues can be performed through numerical simulation provided that the antenna, handset, and adjacent tissue are represented with detailed models and that accurate solution methodologies such as the finite-difference time-domain (FDTD) approach are used. Past studies have appeared which use the FDTD method either with simple models of the head (e.g. a homogeneous sphere) or simple antenna models to predict the interaction between the antenna and the nearby tissue. The present paper combines detailed models of the head and hand with real-life models of antennas on transceiver units to predict the input impedance, radiation patterns, gain, and specific absorption rate (SAR) which result when the handset operates in the proximity of a human. Results are shown for both the monopole and planar inverted F antenna (PIFA) to illustrate the interaction for external and internal antennas respectively. >

A Low-Power Handheld Frequency-Hopped Spread Spectrum Transceiver Hardware Architecture

Abstract: The overall goal of this research project is to develop low-power personal communications transceiver hardware technologies, coupled with advanced systems techniques such as antenna diversity, channel coding, and adaptive power control, for achieving robust wireless digital data transmission over multipath fading channels. A frequency-hopped spread spectrum (FH/SS) code division multiple access (CDMA) technique was chosen over other multiple access schemes because a) it provides an inherent immunity to multipath fading; and b) the signal processing is performed at the hopping rate, which is much slower than the chip rate encountered either in a direct sequence (DS) CDMA or time division multiple access (TDMA) system, thereby potentially resulting in much lower receiver power consumption. Furthermore, frequency-shift keying (FSK) modulation with noncoherent detection in a frequency-hopped system results in a much simpler transceiver architecture as compared with coherent amplitude and phase modulation methods commonly used in DS and TDMA systems. Architectural innovations as well as advanced circuit techniques will be incorporated into the design of a portable handset to achieve minimum power and size without sacrificing robustness in performance. A 3-V CMOS implementation is being developed for the entire transceiver including the 900 MHz radio frequency (RF) front-end. Multiple miniature antennas will also be integrated into the handset to achieve the maximum diversity benefit for robust data transmission. The hardware and system technologies developed for the transceiver design will be general enough to be applicable to a wide variety of commercial and military wireless communications applications such as cellular and micro-cellular radios and telephones, wireless LANs, and wireless PBX systems. In order to validate the design techniques being proposed, a prototype all-CMOS transceiver handset with dual antenna diversity is being developed to demonstrate pedestrian-to-pedestrian communication over the 902-928 MHz band. This paper overviews the proposed FH/SS transceiver hardware architecture. The system rationale, analog and digital circuits, and antenna design techniques for the transceiver are also presented in the following sections.

Theory of physical optics hybrid method (POHM)

Abstract: Demonstrates that POHM is an approximation of an exact hybrid integral equation solution. This establishes the proper theoretical framework to address fundamental questions such as the continuity of current between the moment method and physical optics regions. In addition, it provides a basis for more refined approximations. The Fredholm series iteration is given as a simple example. The technique has been successfully applied to several radiation and scattering configurations.

RCS characterization of a finite ground plane with perforated apertures: simulations and measurements

Abstract: The monostatic radar cross section of a finite-size perfectly conducting flat plate with perforated apertures is investigated by simulations and measurements. The geometry of a finite ground plane with triangular apertures resembles airplane and automobile windows. The method of moments surface patch formulation is used to compute the radar cross section of a solid plate, a plate with two widely spaced apertures, and a plate with two closely spaced apertures. The characteristics of the triangular patch mesh can impact the accuracy of the computed results with this formulation. The paper presents a methodology to achieve high quality meshes to ensure that the time and convenience gained by developing the general method of moments code is not lost in mesh construction and convergence tests. The results obtained using the method of moments are compared with results obtained by measurements and physical optics. It is shown that the method of moments simulations and measurements are in good agreement. The key features of the influence of the aperture separation on the RCS patterns are discussed. >

Characterisation of electromagnetically coupled superquadric loop antennas for mobile communications applications

Abstract: Analysis of two mutually coupled loop antennas with arbitrary relative orientation and position and possibly different geometries is presented. The antennas are represented by generalised superquadric curves, which include circular, elliptical, and rectangular loop geometries, and they may be located either in a homogeneous region or next to an infinite ground plane. A Galerkin-type moment method with piecewise sinusoidal subsectional basis and weighting functions is used. Special consideration is given to implement the solution using curved wire segments instead of the commonly employed linear segments to improve computational efficiency. This very general computational tool is used to investigate the behaviour of coupled loops in configurations suitable for personal communications applications. A discussion of the use of antenna diversity to increase the received signal-to-noise ratio for communications equipment used in a multipath fading environment is also presented. Computational examples show that antenna diversity can provide significant improvements even for closely spaced loop antennas used in mobile communications applications.

Time-parallel computational strategy for FDTD solution of Maxwell's equations

Abstract: A major emphasis within the computational electromagnetics (CEM) community concerns the solution of Maxwell's differential equations using finite-difference time-domain (FDTD) techniques. Because of the computational time and memory requirements associated with these time-stepping algorithms, their application to very large problems has been somewhat limited. To alleviate these computational obstacles, some efforts have previously been aimed at the implementation of space-parallelism-the concurrent computation of unknowns at different points in the spatial mesh using multiple processors-in the FDTD algorithms. For these schemes, however, communication and synchronization requirements have limited the amount of computational speed-up provided by the use of additional processors. This limited potential for enhanced computational efficiency implies that if full exploitation of the capabilities of emerging multiple instruction multiple data (MIMD) architectures is to be realized, approaches must be developed which represent a drastic departure from traditional FDTD techniques. The aim of the paper is to present such a computational strategy. Unlike traditional approaches which use space-parallelism this methodology exploits the decoupling mechanism of an eigenvalue-eigenvector (EE) decomposition of the FDTD matrix to allow the efficient implementation of time-parallelism-the simultaneous computation of field values at multiple time steps. The resulting algorithm is highly coarse grain and has minimum communication and synchronization requirements. >

Comparison of MOM and FDTD for radiation and scattering involving dielectric objects

Abstract: The influence of dielectric bodies on electromagnetic fields is a key consideration in many practical engineering problems. In many applications, the scattering from dielectric bodies for known incident fields is of interest. In other situations where a dielectric object interacts with a radiator, such as biological tissue near a wireless transceiver or a dielectric load on an antenna, the resulting input impedance, radiation patterns and power absorption characteristics are important quantities to be determined. Simulation of such configurations often requires the use of computational tools such as the method of moments (MOM) or finite difference time domain (FDTD) approach. Because these techniques are based on different solution methodologies, they provide an independent check of computed results. This paper provides a comparative study of these two approaches. Results from various scattering and radiation examples are presented, and the advantages of each technique are discussed.

Higher order impedance boundary conditions applied to scattering by coated bodies of revolution

Abstract: In this paper higher order impedance boundary conditions will be employed in the solution of scattering by coated conducting bodies of revolution. The higher order impedance solution reduces the total number of unknowns relative to the exact solution, and produces a system matrix which is less dense than that of the exact solution. The construction of the solution involves two distinct steps. In the first step the body of revolution is replaced by an equivalent set of electric and magnetic currents on its exterior surface which generate the true fields outside the body. An integral equation relating these currents through the free space Green's function is derived. Step two employs the higher order impedance boundary condition to relate the electric and magnetic currents on the surface of the body. This replaces the rigorous solution of the interior problem. The higher order impedance boundary conditions are derived by obtaining an exact impedance boundary condition in the spectral domain for the coated ground plane, approximating the impedances as ratios of polynomials in the transform variables, and employing the Fourier transform. The resulting spatial domain differential equations are solved in conjunction with the integral equation using the method of moments. Several examples of bistatic and monostatic radar cross section for coated bodies of revolution are used to illustrate the accuracy of the higher order impedance boundary condition solution relative to the standard impedance boundary condition solution and the exact solution. The effects of coating thickness, loss, and curvature on the accuracy of the solution are discussed. >

Characterization of antennas for personal wireless communications applications

Abstract: The advancement of antenna technology in personal wireless communication systems has been encouraged by the increasingly stringent demands placed upon these systems to provide low-power and highly reliable information transfer. The antenna designer must not only consider the cost, manufacturability, compactness, and system integrability of the radiator but also generate a product which satisfies rigid specifications concerning return loss, bandwidth, and gain while operating in a complex radiating environment. Successful, cost-effective approaches to the design of antennas for communication devices rely upon the implementation of sophisticated analysis tools, such as the finite-difference time-domain (FDTD) method, capable of predicting the electromagnetic behavior of complicated topologies. In this work, the behavior of planar inverted F, monopole, and loop antennas is investigated using tools based upon the FDTD approach. Such factors as the effects of the conducting chassis, plastic casing, and biological tissue on the antenna performance are investigated. Experimental measurements are used to validate the results obtained from computations and to provide further insight into the behavior of the different geometries. The use of antenna diversity to reduce the effects of multipath fading is discussed, and several examples of antenna diversity configurations are provided.

Diffraction shaping of reflector antennas with elliptical apertures and circular feed

Abstract: Reflector antennas with elliptical apertures are often seen to be advantageous in satellite communications and radar applications. Utilization of an elliptical aperture is typically motivated by the required radiation patterns, and the reduced weight and cost. The central issue in the design of such elliptical reflector antennas is re-distributing or re-directing the feed field to the reflectors in the manner that the desired beam characteristics are obtained. It becomes a challenging task when a more complex radiation pattern such as a contoured-beam is to be generated from an elliptical reflector antenna that uses one circularly symmetric feed. In this situation, the field-redirection can only be achieved through reflector shaping. In the paper, the diffraction shaping technique based on an orthogonal global surface expansion and optimization techniques is employed to shape an elliptical dual-reflector antenna, and the result compared with that using a circular reflector antenna. >

Characterisation of the effects of thin radomes on antennas for space applications

Abstract: For aperture type antennas in space applications, a thin radome in the mouth of the antenna is often employed. This radome is typically constructed of a layer of dielectric with a coating of paint on the exterior surface. Usually, the dielectric and paint can be made to be very thin as compared to the free space wavelength, and thus one anticipates that (for reasonably low loss material) the effect of the radome on the far-field is negligibly small. One must consider the antenna and radome interactions to properly account for the effects of the radome since the radome is typically in the near-field of the antenna. When the full interactions are taken into consideration, the loss can be considerably larger than the loss predicted by a simple plane wave transmission loss. For applications like scatterometer systems where the gain level in extended angular range is critical, or in systems for which the G/T level must be predicted accurately, the net radome loss can be significant. This radome loss is studied for a specific antenna geometry. It is shown that the loss in peak gain due to a radome is noticeably larger than that predicted by a plane wave analysis. >

Fresnel-field range design: array element pattern considerations

Abstract: Real-time antenna pattern measurements are usually performed using either a far-field range or compact range. Alternative measurement techniques have been proposed in order to reduce the measurement distance without using an optical device (i.e., reflector or lens) required for compact range implementation. Generally, this is accomplished by using an array to synthesize a uniform plane wave at near-field distances. Recently, an alternative plane wave synthesis method which employs a ring array geometry similar to that of Hald has been proposed. In this technique (McKay and Rahmat-Samii, 1993), a uniformly excited ring array is employed to compensate for the phase taper associated with a concentric radiator. Compensation is achieved using Fresnel zone approximations in conjunction with a graphical design approach carried out in the complex plane. The method results in simple design formulas and provides insight into fundamental limitations on the extent of the plane wave which can be generated. Several range design issues related to array element pattern effects are examined in this paper. >

Axial field of a symmetric paraboloidal antenna: a PO/PTD solution

Abstract: Proper characterization of the radiated field along the axial direction of a symmetric paraboloidal reflector antenna is important for pencil beam applications such as ultra wideband radar systems. The challenging issues in this task is the evaluation of the edge diffracted fields from the rim of the reflector and those from other scattering structures. The goal of this study is to determine the axial-field of a symmetric paraboloidal reflector antenna using the diffraction techniques of physical optics (PO) and physical theory of diffraction (PTD). As a result, with a general spherical incident field representation, closed-form formulas are obtained for various axial field components. These formulas provide useful information on the diffraction effects of the reflector edge, and facilitate the characterization of the time-domain response of these antenna systems. >

On Crabtree's BQSP technique for significant reduction of integration points in analyzing reflector antennas

Abstract: Physical optics (PO) is commonly used for reflector antenna analysis. This is primarily due to the inherent simplicity in the PO formulation and the accuracy of PO, especially with reference to large reflectors. However, PO is computationally demanding since it involves, in general, a two dimensional numerical integration with a highly oscillatory integrand. This radiation integral can be evaluated using various numerical integration techniques which are based on either brute force approaches (Simpson's rule, Gaussian quadrature, etc.) or modified approaches (Ludwig's linear surface patch and Crabtree's (1991) biquadratic surface patch (BQSP)). In the modified approach, the number of integration points are drastically reduced compared to brute force integration leading to greater efficiency in terms of computer storage and time. The application of optical path length correction, which is vital for obtaining the full computational advantage of the BQSP technique, is discussed. >

Scattering by dielectric filled grooves using a combination of higher order impedance boundary conditions and edge conditions

Abstract: In this paper higher order impedance boundary conditions, (HOIBC), are used in combination with edge conditions to solve the problem of scattering by a two dimensional dielectric filled groove in an infinite ground plane. This object can be used to model a perturbation in an otherwise homogeneous large conducting body such as hatches, ports, and seams between panels. The conducting boundary of the groove is of arbitrary shape, and the dielectric filling the groove is assumed to be comprised of discrete layers oriented parallel to the ground plane. Higher order impedance boundary conditions which are determined using a spectral domain method, are applied to the problem of scattering by multilayer grooves with arbitrary profiles, and the effects of discontinuities on the boundary conditions, i.e. sharp corners, are addressed by incorporating the proper edge conditions. Monostatic and bistatic RCS results will be compared to those from an exact solution which is based upon a mode matching/scattering matrix approach for the interior region.

A Study of the Electromagnetic Coupling Between Handset Mounted Antennas and a Human Operator

Abstract: In the design of antennas for use with hand-held transceiver devices, the electromagnetic interaction between the antenna and the nearby biological tissue is a key factor which must be considered before a design is finalized. This paper presents an investigation of this antenna-tissue interaction using the finite-difference time-domain (FDTD) electromagnetic simulation approach. Accurate models of antennas on a handset are coupled with detailed models of the human head and hand to provide input impedance, radiation pattern, gain, and specific absorption rate (SAR) data for different antenna configurations. Antenna structures such as the monopole, side-mounted planar inverted F antenna (PIFA), and back-mounted PIFA are selected as representative examples of external and internal type radiators. Experimental results are provided which support the theoretically obtained conclusions.

Surface currents on conducting plates with multiple apertures

Abstract: The surface current distribution on a conductive scatterer gives an insight into the dominant scattering mechanisms by defining regions that contribute the most to the backscattered radar cross section. A map of the surface current density identifies the "hot spots" of current on the scatterer. It could help one establish where to place antennas to achieve minimal interference from the scatterer to the antenna. The paper discusses the characteristics of the surface currents induced by a plane wave incident on a plate with multiple apertures. The moment method surface patch formulation is used to compute the surface current density for a solid plate, plates with widely-spaced apertures, and plates with closely-spaced apertures. The effects of the aperture separation and incident wave polarization on the currents are discussed. Surface current maps determined by the method of moments (MoM) are compared with surface currents computed by the physical optics (PO) approximation. These comparisons are used to explain the difference between the MoM and PO predicted RCS results. >