bluuebunny/crossref_metadata_2025 huggingface dataset:
Score functions for structure prediction
Authors: Goldstein Richard A.
Published: April 2005

Protein structure prediction methods often make use of a score function, which describes how compatible a given fold is for the target sequence. There are a wide variety of methods for constructing these score functions, ranging from physics‐based potentials that try to calculate the free energy, to simplified potentials, to functions drawn from machine‐learning techniques. Often, these score functions have a number of parameters whose values must be determined. For physics‐based potentials, these may be available through ab initio calculations and experimental observations of small molecules. For all of the different functional forms, it is possible to determine the values of these parameters on the basis of an analysis of available protein structures. Three different methods to do this – based on thermodynamics, statistics, and machine learning – are described.
Protein domains
Authors: Heringa Jaap
Published: April 2005

Many cellular processes involve proteins composed of multiple domains. The modular nature afforded by breaking up a protein's three‐dimensional structure into discrete domains has many evolutionary advantages. First, it allows proteins to evolve to complex cooperative functions that require elaborate, large, and stable structural scaffolds, which would be impossible to form otherwise within the structural constraints associated with the protein‐folding process. Second, it can bring active sites in proximity in a fixed stoichiometric ratio. Third, otherwise unstable intermediates can be accommodated within interdomain clefts. Fourth, the flexibility afforded by protein domain linker regions, for example, hinge regions, can greatly aid the activity of the cooperative catalytic process carried out by a multidomain enzyme. Fifth, the repeated and combinatorial use of domains in multidomain proteins has provided nature both an evolutionary economic and versatile system to maintain and enhance cellular activity.
Modeling by homology
Authors: Mizuguchi Kenji
Published: April 2005

The structures of proteins can be modeled reasonably accurately on the basis of their relationships with homologous proteins, the structures of which have been experimentally determined. Recent advances in techniques for detecting distant homologs have increased the potential of this approach considerably. Combined with the availability of increasing numbers of genome sequences and experimentally defined protein structures, new methods for modeling by homology now provide great opportunities in areas raging from research into molecular and cellular processes to structure‐based drug design.
Threading for protein‐fold recognition
Authors: Lin Kuang
Published: April 2005

Protein three‐dimensional structures, which provide vital biological information, can often be solved using X‐ray crystallography and NMR techniques (Thornton
et al.
,

; Orengo
et al.
,

). These structure‐solving techniques have been greatly improved in the last two decades. However, protein sequences from large‐scale gene‐sequencing projects are emerging at extraordinary rates. The gap between the number of known protein sequences and the known protein structures is continuously enlarging. Methods of predicting protein structure from its amino acid sequence are therefore of great importance to many biological researches.
The Gene Ontology project
Authors: Harris Midori A. , Lomax Jane , Ireland Amelia , Clark Jennifer I.
Published: April 2005

The main goal of the Gene Ontology (GO) project is to support the construction and use of structured, controlled vocabularies to address the growing need for meaningful annotation of genes and their products in different organisms. There are three key aspects of the GO project: the development of dynamic, controlled vocabularies that can be applied to all organisms even as knowledge of the roles of gene products is accumulating and changing; the application of GO terms in annotating genes or gene products; and the development and maintenance of databases and software for querying, displaying, and manipulating ontologies and associated annotation sets. The GO vocabularies are four nonoverlapping, structured networks of terms that describe key aspects of biology.Molecular functiondescribes the activities or tasks performed by individual gene products at the molecular level;biological processdescribes broad biological goals that are accomplished by ordered assemblies of molecular functions;cellular componentencompasses subcellular structures, locations, and macromolecular complexes;sequence ontologyincludes genome feature terms. The GO project's resources are available to the public athttp://www.geneontology.org.
Ontologies for natural language processing
Authors: Lussier Yves A.
Published: April 2005

Rapid technological improvements of biomedical computational semantics and natural language processing (NLP) are leading to a profound transformation in the reuse of knowledge found in scientific journal articles or semistructured biomedical databases. Indeed, both contain textual entries, which can be transformed and annotated with formal language representation, description logic and ontologies (such as Gene Ontology), and can be stored in ontology‐anchored databases. Recent advances in the standardization of electronic document interchange formats (e.g., XML, RDF, OWL) also contribute to the reuse of information and knowledge. While scientific biomedical literature remains the pinnacle of knowledge in terms of breadth and depth, as compared to derivative databases (e.g., Pubmed), its role is challenged by alternate receptacles of original knowledge, such as biomedical databases containing semistructured textual entries and highly computable data (e.g., Genbank). As a result of the development of digital libraries and semistructured textual databases, automated tools are increasingly researched to create data structures indeclarative knowledge, a highly computable data structure using logic. Though current generation ofliterary knowledgetrumps declarative knowledge in quantity, its noticeable value is compromised at aretrieval cost, owing to literary knowledge being buried in an overwhelming growth of scientific articles. Similarly, data entry of biomedical databases derived from scientific journals, generally accomplished via manual annotation, is a rate‐limiting and costly process. In contrast, automated data structures derived from NLP portend instantaneous and comprehensivelinguistic knowledgeacross boundless scientific articles and research communities. Increasing efforts have been invested to translate linguistic data structures generated by NLP into ontology‐anchored declarative data sets to obtain otherwise unattainable large‐scale or cross‐disciplinary inferences. Additionally, NLP based on Harris' Sublanguage Theory challenges the very nature by which we conceive and maintain biomedical ontologies. This article focuses on the challenges brought on by the convergence of biomedical ontologies, originally developed to structure databases, and linguistic data structures produced by NLP operating on unstructured or semistructured corpora. Theories upon which the convergence of NLP, biomedical informatics, and ontologies are being conducted will first be addressed, followed by a description of the properties of ontologies that make them suitable for NLP, and a succinct analysis of their readiness for use by NLP systems.
Bioinformatics pathway representations, databases, and algorithms
Authors: Karp Peter D.
Published: April 2005

Management of pathway information is of great importance in bioinformatics because pathways place gene products in a causal, mechanistic context showing how those gene products contribute to a larger biological process, and allow us to reason about perturbations to a living system. In the postgenomic era, pathways are also an important organizational and information‐reduction device that decreases the cognitive complexity of a biological system by allowing us to reason about a system at the level of pathways instead of at the level of proteins.

Management of pathway data is a challenging area of bioinformatics because it involves a host of representational and algorithmic problems. This article explores pathway representations and reviews existing pathway databases. It also presents a number of algorithms for computing with pathways, including algorithms for pathway prediction, algorithms for predicting which genes code for missing enzymes in metabolic pathways, algorithms for flux‐balance analysis, and algorithms for qualitatively propagating metabolites through a metabolic network.
Frame‐based systems: Protégé
Authors: Yeh Iwei
Published: April 2005

An ontology is the formal specification of concepts and their attributes in a particular domain of interest. Frames provide an intuitive representation structure for ontologies and knowledge bases (KB) (ontologies with specific instances of concepts and attributes). Protégé is a frame‐based Knowledge Base Management System (KBMS) that provides users with tools for knowledge modeling, knowledge acquisition, consistency checking, and concurrency control. In addition, Protégé provides tools for analysis of knowledge bases and a Java API that allows users to develop and share their own tools.
Bioconductor: software and development strategies for statistical genomics
Authors: Gentleman Robert , Carey Vincent
Published: April 2005

Bioconductor is an open source initiative for the creation and dissemination of methods in statistical genomics and computational biology based on R. This article describes the requirements, language features, and methodology of design and development guiding the evolution of this project. Commitments to software interoperability, computable task‐oriented documentation, and full transparency of algorithm development and use are found to be valuable in reducing barriers to access faced by statistical, computational, or biological researchers attempting interdisciplinary work. These commitments are expected to foster the propagation of standards of transparency and explicit reproducibility from wet‐lab science, where they are well accepted, to in silico biology, where explicit reproduction of important published results is often very difficult.
E
nsembl and
U
ni
P
rot (
S
wiss‐
P
rot)
Authors: Jackson David , Schneider Reinhard
Published: April 2005

Two of the premier resources for genomic and proteomic information are the Ensembl and UniProt (Swiss‐Prot) database projects. In this chapter, we discuss their principle features and structures and show how they can help biologists separate the data sizzle from the true steaks of knowledge and information.
Acedb genome database
Authors: Durbin Richard , Griffiths Edward
Published: April 2005

Acedb is an integrative database system that has been widely used for management of genome oriented biological data. Since 1990, it has been used to manage and display the results of many of the large genome projects, starting with that for the nematode

C. elegans

from which it derives its name: A C Elegans DataBase. Acedb has many features to manipulate genomic sequence and other high genetic data, but also a flexible design that accommodates novel data types and attributes, and has been used for many other types of information, including those unrelated to biology. It provides an integrated research tool that requires no programming by the user to produce sophisticated representations and displays of genomic data.
Cluster architecture
Authors: Dagdigian Chris
Published: April 2005

Life science informatics is one of the many fields in which researchers faced with vast computational and data analysis tasks have embraced the use of cluster‐computing techniques to solve large problems.
A brief Perl tutorial for bioinformatics
Authors: Moorhouse Michael J.
Published: April 2005

The use of the Perl programming language is widespread in modern Bioscience and Bioinformatics. This tutorial uses a series of examples to highlight the strengths and weaknesses of the core language. Additional functionality using precoded Bioperl “modules” as developed for the Ensembl genome browser is described and demonstrated.
Acoustic Microwave Devices
Authors: Smith Peter M.
Published: April 2005

Acoustic wave devices are often used in radiofrequency and microwave circuits because of their very small size and the performance that they offer. In this article, we review the basic theory of acoustic wave propagation in crystals. We describe some of the implementations of acoustic wave devices and show how they can be employed in microwave systems. We then present some of the models that are commonly used in the design of acoustic wave devices.
Active Antennas
Authors: Mickelson Alan
Published: April 2005

An antenna is a structure that converts electromagnetic energy propagating in free space into voltage and current in an electrical circuit and vice versa. In a transceiver system, the antenna is used to both receive and transmit free‐space waves. At minimum, a transceiver then must consist of a signal source that serves to drive the antenna as well as a receiver circuit that reads out the signal from the antenna. Until relatively recently, practically all antenna systems operating in the microwave frequency regime (operation frequencies greater than 1 billion cycles per second, or 1 GHz) went to great lengths to isolate the antenna from the circuitsthat is, to find ways to make system operation independent of the antennas electrical characteristics. In contradistinction, an active antenna is one in which the antenna actually serves as a circuit element of either the driver or readout circuit.
Active Filters: Overview of Active Filter Structures
Authors: Billonnet Laurent , Jarry Bernard
Published: April 2005

This article is dedicated to an overview of existing active‐filter structures. We first talk about the different families of filters. We evaluate different ways of classifying microwave active filters (MAFs) and show that two main categories can be finally considered. For the first category, we describe, from a general point of view, various compensation techniques and detail more specifically filters based on the use of gyrators and negative‐impedance converters (NICs). In this step, we emphasize different problems that must be faced by RF and microwave designers in comparison to low‐frequency classical designs. The second category is based on filters resulting from the application, in the microwave domain, of methods and concepts used primarily at low frequency. As an example, this section sums up the different ways of considering and implementing recursive and transversal (R&T) filters for the microwave purpose.
Direction of Arrival Estimation and Adaptive Processing Using a Conformal Phased Array
Authors: Sarkar Tapan K. , Salazar-Palma Magdalena
Published: April 2005

The objective of this article is to present a general methodology for carrying out direction of arrival (DoA) estimation and adaptive processing using conformal arrays. The antenna elements in a conformal array can be unequally spaced, and they need not be situated over a planar surface. In addition, the antenna elements can have a directive gain. The goal is to present a signal processing methodology coupled with electromagnetic physics that can deal with nonuniformly spaced directive antenna elements over a conformal surface.
Anisotropy and Bianisotropy
Authors: Mackay Tom G. , Lakhtakia Akhlesh
Published: April 2005

The electromagnetic properties of an isotropic medium are the same in all directions. Accordingly, isotropic media are characterized simply by scalar constitutive parameters that relate the induction field phasorsandto the primitive field phasorsand. In contrast, anisotropic media exhibit directionally dependent electromagnetic properties, such thatis not aligned withand/oris not aligned with. Furthermore, recent advances have focused attention on bianisotropic media wherein bothandare anisotropically coupled to bothand. Therefore, dyadics (i.e., second‐rank Cartesian tensors) are needed to relate the primitive and the induction field phasors in both anisotropic and bianisotropic media. Correspondingly, these media exhibit a vastly more diverse range of phenomena than do isotropic media. A consequence of this wealth of interesting properties is that electromagnetic analyses are considerably more complicated for anisotropic and bianisotropic media than for isotropic media. In this article, anisotropic and bianisotropic media are discussed in terms of their constitutive dyadics and spacetime symmetries. Planewave propagation, dyadic Green functions, and the conceptualization of anisotropic and bianisotropic media via homogenization are described. A brief account of anisotropic and bianisotropic behavior in nonlinear media concludes this article.
Antenna Anechoic Chambers
Authors: Chung Boon-Kuan
Published: April 2005

The geometric shape of the anechoic chamber can be optimized to achieve better performance. Microwave absorbers to be installed over the entire inner surfaces of the anechoic chamber must be properly selected. The selection criteria may include the reflectivity levels for normal and oblique incident angles, operating frequency, bandwidth, type of bulk materials, size, shape, weight, mechanical stability, mounting method, fire retardant, and lifespan. The anechoic chamber must be sufficiently large to provide the required dimensions of the quiet zone. Shielding enclosure is provided to prevent extraneous energy from masking measurements of the intended signals. Ventilation, access doors, lighting, cabling, and HVAC (heating–ventilation–air conditioning) shall be incorporated in the design according to the operational purpose of the facility. Different construction methods will give rise to varying degrees of imperfection that determine the shielding effectiveness. The performance of the anechoic environment may be evaluated in terms of amplitude ripple and phase change across the quiet zone. There is much interest in trying to reduce the required investment in constructing an anechoic chamber while still maintaining adequate performance. With a suitable computer simulation tool, together with a reasonable absorber model, design of anechoic chamber can be made more economical.
Antenna Arrays for Mobile Communications
Authors: Paulraj Arogyaswami , Gesbert David , Papadias Constantinos
Published: April 2005

Smart antennas constitute a promising but still emerging technology. Spacetime processing algorithms provide powerful tools to enhance the overall performance of wireless cellular networks. Improvements, typically by a factor of 2 in cell coverage or capacity, are shown to be possible according to results from field deployments using simple beamforming. Greater improvements can be obtained from some of the more advanced spacetime processing solutions described in this article.
Antennas for High‐Frequency Broadcasting
Authors: Wilensky Ronald , Sinclair Gordon G. , Greene Richard R.
Published: April 2005

This article describes the antennas, transmission lines, switching systems and baluns used in modern high‐frequency (HF) transmitting stations. Longrange broadcasting uses 10 bands from 5.9 to 26.1 MHz that are defined by internationally established‐ or radio regulations. Short‐range coverage normally uses four international bands from 2.3 to 4.9 MHz. Power levels are typically 50–500 kW. Propagation is via sky waves which are refracted in the ionosphere and return to Earth at a distance depending on the angle of incidence and the height of the reflecting layer. Under ideal conditions HF broadcasts can be received several thousand kilometers from the transmitting station.

The principal antenna types used for HF broadcasting are horizontally polarized log‐periodics and dipole arrays. Log‐periodic antennas have a greater frequency range, while dipole arrays are capable of handling more transmitter power. HF broadcast antennas are normally supported by large towers or guyed masts on a fixed bearing; dipole arrays are capable of being steered electrically up to 30° in the azimuthal plane. HF broadcast stations typically have a number of transmitters and antennas and may require complex feeder and swtiching systems. Modern transmitters typically have coaxial outputs, while the antennas have balanced inputs. Standard practice employs coaxial feeder to connect to a matrix of coaxial switches, followed by baluns and balanced feeder to reach the antennas.
Aperture Antennas
Authors: Kozakoff Dennis J.
Published: April 2005

Aperture antennas include those having large planar areas such as parabolic reflectors. The classic technique used in the calculation of aperture antenna radiation patterns is applying the equivalence principle followed by physical optics approximations. The equivalence principle is based on replacing the physical antenna aperture with a virtual antenna aperture consisting of an ensemble of Huygens sources, each of which is a source of spherical wavelets. The total pattern is taken as a construction of these Huygens secondary waves. It can be further shown that a simple Fourier transform relation exists between the amplitude distribution of these sources and the radiation pattern in angle space. For many aperture antenna problems, the distribution can be assumed from theoretical considerations or determined empirically from near‐field probes of the aperture fields. But there are many other antennas such as horn antennas, where a virtual aperture can also be constructed in front of the physical aperture in order to model and analyze the antenna. Thus, an aperture antenna has more to do with the method used to model the antenna than its actual form.

For most aperture antenna problems, these classical techniques are adequate and give reasonably accurate results. However, more modern analysis techniques such as the method of moments (MoM), the finite‐element method (FEM), and the finite‐difference time‐domain (FDTD) method are also discussed. These methods are more robust and accurate, but the complexity and large amount of computer resources required must be traded off with the accuracy desired. Generally, they are infeasible for high‐gain antennas.
Attenuation Measurement
Authors: Hilty Kurt
Published: April 2005

When a signal is sent along any transmission path, many different mechanisms degrade it. Because of finite conductivity, every cable shows a resistive loss. Scattering loss is dominated by Rayleigh scattering, and below 1600nm absorption losses are caused mainly by OH absorption. Wireless transmission paths, such as microwave links, satellite links, and broadcast or mobile communication, are affected by scattering caused by rain, clouds, and multiple reflections. Many different methods and a variety of systems for measuring attenuation have been developed.
Attenuators
Authors: Sundararajn Raji , Peterson Edward , Nowlin Robert
Published: April 2005

Attenuators are linear, passive, or active networks or devices that attenuate electrical or microwave signals, such as voltages or currents, in a system by a predetermined ratio. They may be in the form of transmission‐line, stripline, or wave‐guide components. The most commonly used method in attenuators is to place resistors at the center of an electric field. Due to the electric field, there is current induced, resulting in ohmic loss.
Bandpass Filters
Authors: Ramirez-Angulo Jaime
Published: April 2005

Bandpass filters are common building blocks of many electronic systems. They find applications in diverse fields such as communications, audio, instrumentation, and biomedicine. Bandpass filters are used on the receiver side in order to select or “filter out” only one of the signals transmitted in the channel. A process called demodulation is used to shift the spectrum of the selected signal back to baseband (to a zero center frequency).
Bandstop Filters
Authors: Choi Chiu H.
Published: April 2005

An approach to designing a circuit (a bandstop filter)with a frequency response that satisfies the bandstop requirements is described below. It consists of two steps: the approximation of the requirements by a transfer function and the synthesis of the transfer function. A bandstop filter (also known as
band-reject
,
band-elimination
, or
notch filter
) suppresses a band of frequencies of a signal, leaving intact the low‐ and high‐frequency bands.
BiCMOS Devices and RF Integrated Circuits
Authors: Chen Yupeng , Yang Xiaomin , Thomas X. Wu
Published: April 2005

In this article, BiCMOS technology development focusing on the most recent achievements of SiGe BiCMOS technology is reviewed first. Several important figures of merit of SiGe device for RFIC designers such asfT,fmax, BVCEO(collector–emitter breakdown voltage with an open base), and NFminare studied in detail. We begin with the stringent system requirements of typical modern wireless communication standards. Commercial available technology options for RFIC are compared according to performance, power consumption, level of integration, and cost with special emphasis on GaAs, RF CMOS, and SiGe BiCMOS. Passive devices are also explored with respect to their impacts on RFIC design. The design of key wireless system building blocks is investigated. Recently reported results of SiGe BiCMOS low‐noise amplifiers (LNAs), power amplifiers (PAs) and integrated voltage‐controlled oscillators (VCOs) are reviewed. Finally, future trends of RF integrated system adopting SiGe BiCMOS technology are discussed.
Biological Effects of Radiofrequency Energy as Related to Health and Safety
Authors: Foster Kenneth R.
Published: April 2005

This article reviews biological effects of radiofrequency energy with emphasis on effects with clear implications for to human health and safety. The rationale for two major exposure limits is described. Major areas of controversy about health effects of low‐level radiofrequency energy are described, both as related to uncertainities in the scientific data and in large variations in exposure limits around the world.
Medical Imaging with Microwave: Thermoacoustic Tomography
Authors: Xu Yuan , Wang Lihong V.
Published: April 2005

After introducing the principles of thermoacoustic tomography and pure microwave imaging, we summarize the mechanisms attributable to the dielectrical properties of biological tissues in the microwave frequency range. Next, we introduce some theoretical results from studies of thermoacoustic tomography, including information about optimal microwave frequencies, effects of acoustical heterogeneity on breast thermoacoustic tomography, limited‐view thermoacoustic tomography, reconstruction algorithms, and spatial resolution. The experimental setups and the results of thermoacoustic tomography in planar and circular geometries are discussed. Finally, in vivo and in vitro results of both thermoacoustic tomography and pure microwave imaging are presented.
Cavity Resonators
Authors: Sharma Arvind K.
Published: April 2005

In this article, a brief description of the cavity resonators most commonly employed in various microwave components is presented. As far as possible, simple expressions have been provided for design applications. Basic parameters of microwave resonators are first presented because they describe a cavity. Then, various coaxial and waveguide resonators are described. Fabrication, coupling, measurements, and applications of cavity resonators are also included.
Chebyshev Filters
Authors: Carlos Antônio , de Queiroz M.
Published: April 2005

Direct Chebyshev filters are magnitude filters characterized by uniform ripple in the passband attenuation and monotonically increasing stopband attenuation. A variation are the inverse Chebyshev filters, characterized by maximally flat passband attenuation and uniform ripple in the stopband attenuation. The article discusses the characteristics of these approximations and the derivation of the poles and zeros of their transfer functions in Laplace transform, and presents explicit formulas for the element values of direct Chebyshev lowpassLCladder filters. Useful variations of the approximations are also discussed.
RF Circuit Noise
Authors: Marshall Leach W.
Published: April 2005

The
V
n
−
I
n
noise model of an amplifier is described. The model is used to define signal‐to‐noise ratio and noise factor. The optimum source impedance that minimizes the noise factor is derived. The effect of matching networks on noise factor is described. Two lossless matching network examples illustrate how the source impedance can be transformed to minimize the noise factor. The use of Smith chart noise circles and gain circles to evaluate amplifier performance is described and illustrated by an example. Measurement of noise factor and a method of determining amplifier noise parameters from noise factor measurements are described.
Circular Waveguides
Authors: Balanis Constantine A. , Holzman Eric
Published: April 2005

The electromagnetic features of circular waveguide are described, including the transverse and longitudinal fields, the propagation and attenuation constants, the cutoff frequencies, and wave impedances of all transverse electric and transverse magnetic modes. Practical aspects including mode excitation methods and applications are also reviewed.
Coaxial Line Discontinuities
Authors: Garg R.
Published: April 2005

Various types of discontinuities with their equivalent circuits are discussed. These include gaps in the center conductor, open‐ended configuration, open reflection standard, impedance step, and T (“tee”) junction. Design information for equivalent‐circuit parameters is provided. The effect of frequency on discontinuity parameters is also included.
Conformal Antennas
Authors: Sengupta Dipak L.
Published: April 2005

Specifically, this article describes the following:

The specific considerations that must be given to the performance of basic slot and microstrip or patch antenna elements when mounted on nonplanar conducting surfaces

A selected number of conformal antennas: for example, microstrip conformal antennas and dielectric‐filled edge slot (DFES) antennas

A class of wraparound antennas and antenna arrays

Cylindrical and spherical phased arrays used for omnidirectional and hemispherical coverage, respectively.
Conical Antennas
Authors: Adachi Saburo
Published: April 2005

This article describes characteristics such as input impedance,
radiation pattern, and directivity gain of conical antennas consisting of solid conducting cones, conducting conical plates, or their modifications. Conical conducting structures on which simple antenna elements such as dipole and a slot are mounted are also described.
Coplanar Waveguide (CPW) Transmission Lines
Authors: Zhu Lei
Published: April 2005

This article gives an overview of various coplanar waveguide (CPW) transmission lines developed to date for promising applications in monolithic microwave integrated circuits (MMICs). The article begins with a detailed description of the geometric schematics, transmission behaviors, and design formulas for the three basic CPW with infinite‐ and finite‐ground widths as well as backed conductor. Several improved CPW structures with metallic enclosure, vertical‐layer reallocation or varied conductor configurations are outlined, aiming at effectively reducing unexpected losses caused by a strip conductor, lossy substrate, or radiation in addition to problematic crosstalk effects. Coupled CPW transmission lines with different coupling architectures are briefly investigated in terms of coupling or interaction extent. Next, a variety of periodic CPW transmission lines with series inductive and/or shunt capacitive loading are presented and their slow‐wave characteristics are primarily applied for size miniaturization of CPW devices. In conclusion, guided‐wave propagation characteristics of periodic CPW are thoroughly studied in terms of dispersive and complex per unit length transmission parameters.
Dielectric Loaded Antennas
Authors: Shafai L.
Published: April 2005

Dielectric media affect the wave propagation by their reflective index. Their use in antennas can provide a number of diverse benefits. In applications such as high‐gain antennas, they influence the phase of the propagating waves, and thus influence the direction and gain of the transmitted waves. Lens antennas are typical examples that can be used alone, or in conjunction with other antennas, such as horn and microstrip antennas, to enhance their gain. In other applications like dielectric loaded horns and waveguides the use of dielectric inside them controls the excitation of various modes and thus the performance of the horns. Hybrid and multimode horns are examples that can improve the horn performance for reflector feed applications. Similarly, dielectric resonator antennas can control and improve the radiation performance by controlling the excitation and the phase of the wave propagating through them. Dielectrics being non‐conducting can also be used to insulate and, thus, isolate the antennas from living tissues, in medical treatment and studies. Large permittivity of some dielectrics reduces the signal wavelength and in areas of small antennas can be used to reduce the antenna dimensions. Finally, the dielectric permittivity also influences the wave impedance and can be used to match the impedance of antennas to the radiation media and enhance their gain. This article provides a brief review of some of these applications.
Dielectric Permittivity and Loss
Authors: Das-Gupta D. K.
Published: April 2005

The present article briefly reviews the electrostatic concepts that lead to time‐ and frequency‐dependent dielectric phenomena together with the models of dielectric relaxation behavior in various materials. It also includes some explanations for the dielectric aging of insulating materials under high fields in humid environments.
Dielectric Resonator Filters
Authors: Bila S. , Baillargeat D. , Verdeyme S. , Guillon P.
Published: April 2005

Dielectric resonators (DRs) are widely used in microwave circuitry since these components offer a higher integration than do classical empty waveguides and, at particular frequency bands, equivalent electrical performances. This article deals with the design of bandpass microwave filters employing DRs. The electrical characteristic parameters of these circuits are introduced in the first section. In the second section, different topologies of microwave filters using cylindrical DRs are described. The third section is a review of particular applications of DRs in microwave filtering: power handling, out‐of‐band rejection, and high frequency. Finally, the last section proposes an electromagnetic optimisation methodology for the design of DR filters.
Dielectric Resonator Oscillators
Authors: Fusco Vincent F. , Dearn Andrew
Published: April 2005

A dielectric resonator (DR) can be used to form the stabilizing element in an oscillator. Dielectric resonator oscillators (DROs) are characterized by the following properties: high‐frequency stability, high efficiency, and low manufacturing cost. They also can be made to provide good temperature stability.

Two‐ and three‐terminal active devices exhibit phase noise characteristics at microwave frequencies. Oscillators constructed using these devices can have their phase noise behavior improved by the addition of a DR element.
Directional Couplers
Authors: Rozzi Tullio , Morini Antonio
Published: April 2005

The directional coupler (DC) is the most useful four‐port microwave device. Substantially, it consists of a four‐port device, where a wave entering any port excites two other ports with prescribed amplitudes, whereas the remaining port is isolated. This property makes the DC an essential component when dealing with microwave devices, as it allows to distinguish reflected waves from incident ones. After a definition of the ideal DC, the typical parameters and the electrical characteristics of actual devices are illustrated, then, the main applications are considered and, finally, some common realizations of DC are shown, particularly focusing the attention on different coupling mechanisms and technologies.
Electromagnetic Field Measurement
Authors: Kanda Motohisa
Published: April 2005

Various electromagnetic field sensors for measuring radiofrequency (RF) electric and magnetic fields are discussed.

Throughout the discussion, the interplay between measured quantities and predicted (modeled) quantities is emphasized. The ability of measurements and the restrictions imposed by rigorous theoretical analysis of given models are discussed for the frequencies from 10 kHz to 40GHz and upward.
Electromagnetic Wave Scattering
Authors: Musselman Randall L.
Published: April 2005

Electromagnetic wave scattering is the reradiation of electromagnetic energy that results when an electromagnetic field encounters an abrupt change in electrical parameters. Typically, this occurs when an electromagnetic field is incident on a structure or scattering object. When the original electromagnetic field crosses the boundary between two regions of different materials, each with different electrical properties, the field will change as it enters the second region. Sources, including conduction, displacement, and polarization currents, will be induced at the discontinuity between the two regions. These currents act as sources of electromagnetic radiation, much like the sources of the original incident electromagnetic field. This reradiation is calledelectromagnetic scatteringbecause it scatters the incident electromagnetic field from its original propagation path.
Electromagnetic Waves in Ionosphere
Authors: Hunsucker Robert D.
Published: April 2005

Since the existence of the ionosphere was confirmed in 1925 using radio techniques, we have gained a good understanding of the basic physics and morphology of the solar–terrestrial environment. In this article we summarize the basic physics of the ionosphere and of the interaction of radiowaves with the ionosphere. The fundamental mechanisms of the various radio propagation modes and the effects of solar radiation and noise are described for specific radio services. The principal radio techniques for probing the ionosphere and “frontiers of ionospheric research” are summarized. Three of the principal journals that publish research in the subject of this article areRadio ScienceandJournal of Geophysical Research-Space Physics(published by the American Geophysical Union) andTransactions of the IEEE Group on Antennas and Propagation(published by the Institute of Electrical and Electronic Engineers).
Elliptic Filters
Authors: Noren Kenneth V. , Grimm Jerry Michael
Published: April 2005

This article provides an overview of elliptic filter synthesis for RF and microwave frequencies. The characteristics of an elliptic filter function are discussed and compared to other classical filter function responses. An overview of the mathematics of the lowpass elliptic filter function is discussed. Methods for transforming the low‐pass elliptic function to highpass, bandpass, and band‐reject functions are provide. The article concludes with a discussion of the realizations for RF and microwave elliptic filters and discusses issues related to RF and microwave elliptic filter synthesis.
Feedback Oscillators
Authors: Zhou Jianyi , Hong Wei
Published: April 2005

In this article, the basic theory of feedback oscillators related to balanced condition, startup condition, stability condition, frequency stability, and other characteristics phase noise, is summarized. Typical feedback oscillators, such asLCoscillators,RCoscillators, crystal oscillators, dielectric oscillators, ring oscillators, and voltage‐controlled oscillators, are illustrated.
Ferrite Isolators
Authors: Bayard B. , Sauviac B. , Vincent D.
Published: April 2005

Signal processing requires specialized functions, particularly in the microwave range, where wave propagation can easily be corrupted. The main applications of isolators as well as their characteristics are described. Some waveguide and planar microwave ferrite devices based on the Faraday effect, field displacement, or gyroresonance are then reviewed. For a better understanding of the functioning of such devices, microwave properties of ferrite materials are presented. Ferromagnetic resonance and the resulting nonreciprocal effects are also explained. Finally, more recent developments in the use of hard ferrites and in magnetic films technology are discussed.
Microwave Ferroelectric Devices
Authors: Patel D. P. , Pond J. M. , Rao J. B. L.
Published: April 2005

Control of reactance in microwave circuits, devices, and systems is a common method by which the response of a microwave circuit such as a filter, resonator, or phase shifter can be tuned. Devices based on a class of voltagedependent non‐linear dielectrics known as ferroelectrics provide an alternative to semiconductor varactor diodes and ferrimagnetic components, which are the most common devices of this type.

This article describes microwave devices that exploit the variation of the ferroelectric's permittivity with applied DC electric field. The main feature of these “tunable” microwave devices is the change of their capacitance, impedance, or phase velocity. We will describe varactors, oscillators, tunable filters, and phase shifting devices.
Ferroelectric Materials
Authors: Pandey R. K.
Published: April 2005

The interplay between the forces of electric field, mechanical stress, and temperature in a material can cause a host of effects and properties to evolve. Ferroelectricity is one of these fascinating phenomena, acquired by some dielectric materials, originating from the interactions between the three natural forces. For ferroelectricity to exist, the fundamental condition of noncentrosymmetry of a crystal unit cell must be satisfied. For this and some other reasons, ferroelectricity without exception coexists in materials that are simultaneously piezoelectrics and pyroelectrics. Thus all ferroelectrics are good examples of multifunctional materials. Their multifunctional nature makes them extremely attractive for a host of applications ranging from classical to ultramodern. This article attempts to explain the complex but very interesting properties of ferroelectric materials and their counterparts such as piezoelectrics and pyroelectrics. Phenomena such aspyrooptic effectandmultiferroicmaterials, which are rarely covered in a discussion of ferroelectricity, are also briefly discussed. Processing of ferroelectric materials and their applications have also been included in the discussion with the hope that they will prepare interested readers for more advanced studies of this vast subject.
Filter Theory
Authors: Görür Adnan
Published: April 2005

The main categories of filters are defined in terms of the general response types of lowpass, bandpass, highpass, and bandstop. The basic concepts of filters including transfer function, insertion loss, return loss, phase delay and group delay, are described. Filter characteristics such as maximally flat or Butterworth, equally ripple or Chebyshev, elliptic function, generalized Chebyshev or pseudoelliptic, and maximally flat time delay are discussed in terms of the poles of transfer function. Two‐port lumped‐element lowpass prototype networks and their element values and filter transformations are given. Richards’ transformation and Kuroda's transformations (or identities), impedance, and admittance inverters are also investigated, which are required for transforming lumped elements to transmission‐line sections, for separating physically filter elements and changing impractical elements into more realizable elements in a filter network.
Fractal Shaped Antennas: A Review
Authors: Anguera Jaume , Puente Carles , Borja Carmen , Soler Jordi
Published: April 2005

The combination of fractal geometry with electromagnetic theory opens an interesting window for research to find new antenna designs and applications. Fractal technology applications can be divided into six main groups: (1) multifrequency antenna elements, frequency‐selective surfaces, and arrays; (2) electrically small antennas; (3) high‐directivity radiating elements; (4) low‐sidelobe arrays; (5) undersampled arrays; and (6) fast computational methods. This article reviews most of the work done in the fractal technology field covering the last six fields.
Frequency Modulation
Authors: Fertig L. B. , Mc Clellan J. H.
Published: April 2005

In this article, concepts relating to instantaneous frequency, along with algorithms for its computation, are reviewed. First, several means by which instantaneous frequency is commonly defined are discussed, and the relationships between instantaneous frequency and time–frequency distributions are explored. Next, several measures of performance commonly used to evaluate instantaneous frequency algorithms, such as the Cramer–Rao lower bound, are examined. Finally, a number of algorithms that have been suggested for IFE are summarized. Despite the relative maturity of frequency modulation in the field of radiocommunications, the field of IFE is a growing one, and one in which research is still quite active.
Frequency Selective Surfaces
Authors: Wu Te-Kao
Published: April 2005

A detailed description of frequency‐selective surface (FSS) is provided. Subjects covered include various kinds of FSS and characteristics, bandpass and bandstop FSS, aperture and patch element FSS, thin/thick‐screen FSS, FSS analysis techniques and design tools, dielectric loading effects, grating lobe and Woods anomaly, as well as FSS applications. An FSS bibliography is also provided listing all the references necessary for further research on the subject of FSS.
Frequency Stability
Authors: Howe David A.
Published: April 2005

Radio navigation, digital signal processing, photonics, high‐speed communications, and numerous other important systems require a high degree of frequency stability in a variety of oscillators used to “clock” or time the electronic circuits. This article reviews definitions of frequency stability and methods of measuring the frequency stability of oscillators. Physical interpretations of common noise processes are discussed. The equipment and setup for stability measurements are outlined. Examples and typical stability results are presented.
Frequency Synthesizers
Authors: Goldberg Bar-Giora
Published: April 2005

Traditionally, frequency synthesis (FS) generated a single tone with variable frequency or amplitude. There are new applications, especially for testing and simulation, that require multitone generation and even arbitrary wave generation, which relate to digital frequency synthesis. Since the late 1970s, FS has evolved from mainly analog to a mix of analog, radiofrequency (RF), digital, and digital signal processing (DSP) technologies.
Friis Free‐Space Transmission Formula
Authors: Levis Curt A.
Published: April 2005

The Friis transformation formula predicts the power available at the terminals of a receiving antenna due to the power applied to a transmitting antenna when the two are well separated. It is often given in logarithmic form, using decibels. It is assumed that the transmission occurs in free space, that the separation is sufficient to satisfy the far‐field criterion for antennas, and that the antennas are matched in polarization (i.e., the polarization of the receiving antenna is optimum for the polarization of the transmitted wave).

The formula, first published by Harald T. Eriis in 1946, is easily derivable from basic physical principles.
Generalized Scattering Matrix Technique
Authors: Monorchio Agostino , Mittra Raj , Manara Giuliano
Published: April 2005

The generalized scattering matrix (GSM) technique, which is a convenient method for analyzing complex microwave devices, consisting of separable and longitudinally uniform waveguiding structures, is presented in this article. GSM differs from the conventional scattering matrix used in circuit theory or microwave network theory in that it is extended to consider evanescent as well as propagating modes in each waveguide. Some numerical examples are provided to illustrate the importance of the inclusion of higher‐order modes for generating accurate solutions of waveguide discontinuity problems.