bluuebunny/crossref_metadata_2025 huggingface dataset:
Cutoff wave numbers of eccentric circular and concentric circular‐elliptic metallic wave guides
Authors: Roumeliotis J. A. , Siddique Hossain A. B. M. , Fikioris J. G.
Published: February 2008

The cutoff wave numbers knm and the field of two‐conductor, perfectly conducting wave guides are determined analytically. Three types of wave guides are considered: Eccentric circular conductors of radii R1, R2 and distance d between their axes, elliptic inner with circular outer conductor, and circular inner with elliptic outer conductor. The electromagnetic field is expressed in the first case in terms of circular cylindrical wave functions referred to both axes in combination with related addition theorems, while in the last two cases, both circular and elliptical cylindrical wave functions are used, which are further connected with one another by well‐known expansion formulas. When the solutions are specialized to small eccentricities, kd in the first case and h = ka/2 in the last two cases (where a is the interfocal distance of the elliptic conductor), exact, closed‐form expressions are obtained for the coefficients gnm in the resulting relations knm(d) = knm(0)[1 + gnm(knmd)2 + ···] and knm(h) = knm(0) [1 + gnmh2 + ···] for the cutoff wave numbers of the corresponding wave guides. Similar expressions are obtained for the field. Numerical results for all types of modes, comparisons, and certain generalizations are also included.
Reflection of waves by imperfectly periodic layered media
Authors: Hashimoto Masahiro
Published: February 2008

In order to facilitate the numerical evaluation of the grating effects of imperfectly periodic layered media, we introduce the extended local wave number concept, and we describe how the reflected wave is determined from the spatial behavior of the extended local wave number. A numerical method based on this concept is presented, which proves to be useful for structures with more than ten layers with the nonuniform period. Numerical comparisons with the invariant imbedding approach show the validity and utility of the method. Two specific structures are analyzed using this method.
Signal statistics of equatorial nighttime ionospheric scintillations
Authors: Raghava Reddi C. , Krishna Moorthy K. , Krishna Murthy B. V.
Published: February 2008

Cumulative probability distribution functions of equatorial nighttime ionospheric scintillations are studied and are found to agree well with the Nakagami m distribution. Relationship between percentage occurrence of scintillations and S4 index has been established on a monthly mean basis, facilitating the conversion of the scintillation data in analog form to fade margin data useful in transionospheric VHF communication systems design.
A probabilistic theory of scattering from a random rough surface
Authors: Nakayama J. , Ogura H. , Matsumoto B.
Published: February 2008

This paper describes a new theory of random surface scattering based on the group theoretic consideration of the stochastic homogeneity of the infinite random surface. To show the basic idea of the theory, we only discuss scalar wave scattering for one‐dimensional random surface that is described by a reactance boundary condition having a random reactance of a stationary random function. A form of the stochastic wave solution associated with the random boundary condition is determined by the group theoretic consideration. The wave solution is then written in terms of a stationary random function that is approximately solved as a stochastic functional of the random surface. The optical theorem, the angle dependence of the incoherent scattering, and the energy flow of the surface wave are calculated and are shown in the figures.
The eccentrically insulated circular loop antenna
Authors: An L. N. , Smith Glenn S.
Published: February 2008

The analysis for the circular loop antenna in a concentric insulating sphere is extended to the case where the loop is eccentrically located in the sphere, that is to say, coaxial with the sphere but not concentric. Both the transmitting and receiving antennas are treated. Parametric studies are used to determine the effect of the electrical size of the loop and sphere, the eccentricity, and the electrical properties of the media on the directivity of the antenna. The field pattern for the loop is found to be a vertical beam with approximately linear polarization when the electrical size of the loop is near that for resonance in the insulation, β1b ≈ 1.0, the ratio of the permittivity of the surrounding medium to that in the insulation εer2/εer1 is large, and when the eccentricity d/c is high. Measured driving point admittances and field patterns of model antennas are compared with theoretical calculations and found to be in good agreement. A limited experimental investigation indicates that the circular loop antenna with an eccentric cylindrical insulation will have directive properties similar to those obtained with a spherical insulation.
Excitation of lateral waves and the scattered radiation fields by rough surfaces of arbitrary slope
Authors: Bahar Ezekiel
Published: February 2008

The coupling between the lateral waves and the radiation field by rough surfaces is derived using a full‐wave approach. Both horizontally and vertically polarized waves are considered, and the solutions are shown to satisfy the reciprocity and duality relationships in electromagnetic theory. The slope of the rough surface is not restricted, shadowing effects are considered, and the solutions are also applied to random and periodic rough surfaces. Using stationary phase techniques, closed‐form solutions are also derived for high frequencies.
Theory of low‐frequency conductivity probing of roof structures in coal mines
Authors: Ralston Margarete A. , Wait James R.
Published: February 2008

Determining electrical ground constants of the earth is an important problem in geophysical probing [Keller, 1971; Wait, 1979] and underground communication system design [Aldredge, 1973]. Several different methods are used to determine the relative permittivity εr and resistivity p or conductivity ó of a stratified earth. As noted in the Institute of Electrical and Electronics Engineers (IEEE) [1974] Standard 356, techniques for determining the electrical constants of the earth are roughly divided into two categories: (1) methods where drill hole access into the earth does not exist or is not required, (2) techniques requiring drill hole access into the earth. Methods using drill holes are primarily for determining electrical ground constants in a local area.

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Low‐frequency scattering by a slit in a conducting plane
Authors: Hurd R. A. , Hayashi Y.
Published: February 2008

The problem of diffraction of an H polarized plane wave by a narrow slit in a perfectly conducting screen can be formulated as an integral equation with a Hankel function difference kernel. For narrow slits, we replace the Hankel function by the first three terms of its series expansion and solve exactly the resultant integral equation. The results are superior to those obtained by solving a succession of integral equations, each with a static kernel, which is the traditional approach. Also solved is the slightly more difficult problem of E polarization. We indicate that the method can be generalized to include more terms in the kernel expansion, to diffraction by multiple slits, with different media and with impedance boundary conditions on the screen.
Inversion of ground‐based radiometric data by Kalman filtering
Authors: Basili Patrizia , Ciotti Piero , Solimini Domenico
Published: February 2008

Retrieval of atmospheric vertical temperature profiles from ground‐based radiometric observations requires shrewdness and judicious choice of parameters to surmount the noxious effects of the ill‐posed nature of the inversion. Kalman linear estimation, already successfully used in satellite microwave sounding of atmospheric temperature, can be also applied to infer the thermal state of the lower troposphere from ground‐based infrared measurements. After a short survey of the relevant formalism the implementation of the Kalman filter is discussed with regard to the transition operator and to the error covariances. In particular, the measurement errors are considered in some detail, and their dependence on the atmospheric dynamics is pointed out. The attainable spatial resolution is compared with that of another commonly used inversion technique, and, finally, a set of temperature profiles estimated by the Kalman algorithm from a sequence of successive radiometric measurements is reported.
Estimation of the parameters of a turbulent drifting medium by a least squares analysis of spaced sensor measurements
Authors: Baker P. W.
Published: February 2008

A technique is outlined for determining the average structure and drift velocity of a turbulent medium by applying the least squares method to correlograms from two‐dimensional sensor measurements. A general spatiotemporal correlation function is defined from the average temporal autocorrelogram for all sensors. The technique could easily be extended to three dimensions. This approach avoids errors introduced by the choice of an analytical function and would seem to have significant advantages over other correlation analysis techniques, because it easily handles the case of a large number of sensors, and the statistical nature of the method makes it possible to determine accurately the confidence that can be placed on the estimated parameters. An analytical correlation function (Gaussian) and the present method were applied to spaced radio frequency sensor measurements of ionospherically reflected radio waves and the results compared. In a few cases, close agreement was obtained between the two methods, showing that in these cases the analytical correlation function was a good fit to the data. This shows that the present method can be readily used to test the goodness of fit of selected analytical functions. In many cases, formally acceptable solutions were not obtained, indicating that the spatial and temporal correlation functions of the medium were dissimilar.
Emission from an inhomogeneous layer with irregular interfaces
Authors: Fung A. K. , Chen M. F.
Published: February 2008

A theoretical model for microwave emission from an inhomogeneous layer has been developed using the radiative transfer method. The top and bottom boundaries of the layer are assumed to be randomly rough surfaces, and the scatter characteristics are assumed to be describable by the Kirchhoff scatter model. The presence of an irregular top boundary is found to cause a slower angular trend than the case of a plane layer and causes, in addition, a rise in temperature if the layer permittivity is large. On the other hand, if only the bottom boundary of the layer is rough, a rise in emission is observed relative to the plane layer. When both the top and the bottom boundaries are rough, the emissions at small nadir angles are closer to those of a layer with a rough bottom boundary, and the emissions at large nadir angles are closer to those of a layer with a rough top boundary. A comparison of the layer model with reported angular measurements from a snow layer at three different frequencies and polarizations shows promise for such a model. An additional comparison of emission versus the depth of a snow pile indicates that an irregular snow ground interface causes higher emissions at small depths. This fact is necessary to explain the observed measurements.
Scintillation index for a temporally partially coherent, spherical wave light source in weak turbulence
Authors: Baykal Y. , Ouyang C. F. , Plonus M. A.
Published: February 2008

The scintillation index due to a spherical wave light source that has a finite coherence bandwidth is formulated in weak turbulence. It is found that in the limit of narrowband approximation, the averaging effect of the bandwidth on the scintillation index is the same as that for a plane wave.
Impedance of a spherical probe immersed in a warm magnetized plasma
Authors: Fujita Takao , Adachi Saburo
Published: February 2008

The rf impedance characteristics of a spherical probe, which is immersed in a warm magnetized plasma, are investigated theoretically. Both grid and solid sphere models are treated. The fluid model is used to describe the magnetized electron plasma. The impedance contains a resistance resulting from the radiation of electrostatic plasma waves. The resonance frequency for the warm magnetized plasma, at which the reactance vanishes, is different from the electron cyclotron frequency, at which the resonance occurs in a cold magnetized plasma; it depends on both the radius of the sphere and the Debye length. Several previous theories, which have been studied, are unified in the present theory. It is also found that the surface charge density on the sphere is exactly uniform in the cold magnetized plasma.
Multimode optical fibers: Profile control and analysis
Authors: Presby Herman M.
Published: February 2008

The information carrying capacity of multimode optical fibers is mainly determined by the quality of the refractive index distributions of their cores. With ideal refractive index distributions, these fibers can, in principle, have signal band widths on the order of 10 GHz km. However, extremely careful control of the index profile is required to achieve this theoretical goal. Slight departures from the optimum index distribution lead to a dramatic decrease of the fiber bandwidth.
Lasers and LED's for optical communications using fibers
Authors: Kressel H.
Published: February 2008

Laser diodes and light‐emitting diodes suitable for optical communication systems have reached a stage of maturity which makes their practical use possible. Of the two spectral regions of interest, 0.8 to 0.9 μm and 1.1 to 1.7 μm, the first is the most widely used because the AlGaAs double heterojunction devices are the most developed. However, rapid progress is being made in the development of the longer wavelength InGaAsP/InP devices. Laser research and development is currently also focused on devices operating reliably in the fundamental spatial mode in order to insure kink‐free operation and a single beam far field. Several structures have been developed which meet the objective of fundamental spatial mode operation at power levels (cw) on the order of 10 mW.
Influence of electrode configuration on the efficiency of the electrooptic switch BOA
Authors: Favre F.
Published: February 2008

In this paper a theoretical analysis of the electrooptic switch BOA is presented. It is shown theoretically that electrode efficiency may be optimised by a partial overlay of the guiding stripe by the electrodes.
Radiation and reception characteristics of an oblate spheroidal antenna for electron plasma waves 1. Theory
Authors: Ohnuki Shigeo , Adachi Saburo
Published: February 2008

Unified theory on the radiation and reception characteristics of the oblate spheroidal antenna with an ion sheath or an absorptive surface for an electron plasma wave is presented. The analysis is carried out as a boundary‐value problem by using oblate spheroidal functions. The formulas for the radiation and reception characteristics such as the radiation impedance, electron charge distributions, radiated wave potential, directional properties, and receiving voltage of the oblate spheroidal antenna are analytically obtained. It is shown that the theory is applicable to monopole‐type plasma wave antennas, such as disk, sphere or thin rod with sheaths in general, depending upon the applied d.c. bias.
Foreword
Authors: Crane R. K.
Published: February 2008

Union Radio Scientifique Internationale (URSI) Commission F held an open symposium and workshop on the ‘Effects of the Lower Atmosphere on Radio Propagation at Frequencies above 1 GHz’ at Bishop's College in Lennoxville, Quebec, Canada, May 26‐30, 1980. It was one of several scientific symposia organized by Commission F during the interval between the 1978 and the 1981 URSI General Assemblies. The intent of the Lennoxville meeting was to assay recent advances in the experimental investigation and in the modeling of propagation phenomena which affect the design of communication systems operating at frequencies above 1 GHz. The workshop was charged with the preparation of a progress report to Study Group 5 (Propagation in Nonionized Media) of the International Radio Consultative Committee (CCIR).
Reflections from an inhomogeneous slab
Authors: Sivaprasad K. , Hubey H. M. , Vasudevan R.
Published: February 2008

The reflection and transmission functions of an inhomogeneous slab are calculated by labeling the wave by the number of reflections it has undergone in the medium. The Riccati equation, satisfied by the reflected amplitude, is decomposed into an infinite set of linear equations by taking into account the number of scatterings taking place inside the medium and is solved by a novel iterative approach. The order‐of‐scattering reflection functions method presented here and the coupled integral equation approach of the Bremmer solutions for the internal fluxes are set in a unified frame. The solutions of the Riccati equation are computed numerically for the reflection function using the order‐of‐scattering technique, and it is demonstrated that the method leads to better convergence and stability as compared to usual linearization methods.
Rain anisotropy prediction: Theory and experiment
Authors: Capsoni Carlo , Maggiori Dario , Matricciani Emilio , Paraboni Aldo
Published: February 2008

Measurements of electromagnetic waves attenuation by intense rain at frequencies above 10 GHz can be used to estimate the rain anisotropy, which is an important factor in the prediction of depolarization (both in circular and linear polarization). This estimate, however, requires one to make some physical assumptions about some rain parameters such as the raindrop size distribution, the water temperature, the rain intensity profile along the radio path, and the distribution of the ellipsoidal drops orientation. This paper aims at estimating the uncertainties that arise from the lack of their knowledge. The study is based on experimental data that have been worked out in order to obtain the necessary physical information.
Geometrical data of dielectric objects from monostatic backscattering
Authors: Chaloupka Heinz
Published: February 2008

The first part deals with the scattering problem due to homogeneous dielectric bodies using eigensolutions for the polarization currents. These eigensolutions depend on the shape of the scatterer but are independent on its permittivity. The complex ratio of the electric field strength and current density belonging to the eigensolution is described as a generalized impedance in terms of an equivalent circuit representation. The second part is devoted to the discussion of the backscattered ramp response. An expression is given showing the dependency of the ramp response on the transient behavior of the equivalent circuits. For the case of a frequency‐independent permittivity the early part of the ramp response turns out to be approximately proportional to the ‘projected area function.’ Based on this result, algorithms well established for the shape reconstruction for metallic bodies can also be used for dielectric bodies. The last part is concerned with the Rayleigh scattering by bodies exhibiting variable permittivity (e.g., strongly dispersive dielectrics). It is shown that from the knowledge of the εr dependency of the copolar backscattering, the volume and slenderness of the body can be estimated.
Directivity optimization for large circular arrays with many parallel dipoles
Authors: Cheng David K. , Chen C. A.
Published: February 2008

In maximizing the directivity of large circular arrays with many parallel dipoles, it is necessary to consider the mutual coupling effects of the currents in the many dipoles. An application of the method of moments in the numerical solution will require the inversion of large matrices and lead to computational difficulties. In this paper these difficulties are alleviated by employing two techniques. First, the simultaneous integral equations are converted into matrix equations by the three‐term approximation. Dipole subdivisions are not required, thus greatly reducing the size of the matrices. Second, a rotational operator is defined, with which a unitary transformation is applied to the array matrix. The inversion of the array matrix can then be effected by a straightforward matrix‐multiplication process. Typical numerical results on maximum obtainable directivity and on the radiation pattern of an optimized circular array of 30 parallel dipoles are presented.
Theory of microstrip Yagi‐Uda arrays
Authors: Rana I. E. , Alexopoulos N. G. , Katehi P. L.
Published: February 2008

In this paper the theory of microstrip Yagi‐Uda arrays is considered. An integral equation is derived for the current distribution on the exciter, reflector, and parasitic elements of the array which is printed on a grounded dielectric substrate. The integrand involves improper Sommerfeld type integrals which are solved by analytic‐numeric techniques via a principal value integration along the real axis. In this manner the integral equations can be solved for the current distribution by the Galerkin method. Some results for the design of printed Yagi‐Uda arrays are discussed.
Numerical analysis of transient fields near thin‐wire antennas and scatterers
Authors: Landt J. A.
Published: February 2008

Under the premise that ‘accelerated charge radiates,’ one would expect radiation on wire structures to occur from driving points, ends of wires, bends in wires, or locations of lumped loading. Here, this premise is investigated in a series of numerical experiments. The numerical procedure is based on a moment‐method solution of a thin‐wire time‐domain electric‐field integral equation. The fields in the vicinity of wire structures are calculated for short impulsive‐type excitations, and are viewed in a series of time sequences or snapshots. For these excitations, the fields are spatially limited in the radial dimension, and expand in spheres centered about points of radiation. These centers of radiation coincide with the above list of possible source regions. Time retardation permits these observations to be made clearly in the time domain, similar to time‐range gating. In addition to providing insight into transient radiation processes, these studies show that the direction of energy flow is not always defined by Poynting's vector near wire structures.
Radiation from reflector antennas: Exact aperture and aperture‐like approaches
Authors: Bucci O. M. , Franceschetti G.
Published: February 2008

A new computational approach is presented, which allows fast analysis of radiation from reflector antennas. The basic idea is the Fourier transform (FT) relationship between far‐field and aperture distributions. Accordingly, the far field can be exactly reconstructed from the knowledge of spatial samples at angular rate ka sin θ cosϕ = n π ka sin θ sin ϕ = m π, where a is the aperture radius (Shannon‐Whittaker theorem applied at Nyquist rate). The conclusion is that the aperture far field can be sampled at approximately one point per lobe. The finite reflector curvature introduces an extra term in the radiation integral which modifies the (finite) aperture/far‐field FT relationship. There are two ways to overcome this difficulty: (1) consider an infinite aperture, thereby saving the FT relationship, but losing the spatially band‐limited property and (2) reformulate the Shannon‐Whittaker theorem in such a way as to take into account the extra (curvature) term. The first approach requires truncation of the infinite aperture to an assigned degree of approximation in the far‐field computation. This states an equivalence between the field radiated by the reflector and the field radiated by an equivalent aperture of larger radius. Accordingly, the far field can still be reconstructed using the field radiated by the reflector, sampled at a higher rate (slightly higher for large dishes). These samples can be computed using physical optics or asymptotic techniques, whichever is appropriate. The second approach is based on a new sampling representation, which explicitly takes into account the extra term in the radiation integral. For a quadratic type of term, i.e., for parabolic reflectors, the sampling functions are the complementary error functions, and the far field can be exactly reconstructed in terms of the far field of an equivalent aperture. These samples can be computed using the fast Fourier transform, therefore further gaining in time saving. Computations showing the excellent performances of the two methods have been made, indicating the significant advantages provided in reflector antenna analysis.
Estimation of the radiation loss from microstrip antenna feed transitions
Authors: Henderson A. , James J. R.
Published: February 2008

The concept of creating thin planar microwave antennas from microstrip substrates is rapidly establishing itself as an important new antenna design technique, but the extraneous radiation that is emitted from the antenna/feed transition region is now recognized as a severe constraint on the achievable side‐lobe level performance of these antennas. Consideration is now given to the determination of the extent of this limitation and to how the extraneous radiation loss can be reduced. Two types of microwave feeds, the coax/microstrip and the triplate/microstrip, are analyzed using a variational method, and approximate field representations are introduced to obtain estimates of the radiation associated with these feeds. The computed results for polyguide and alumina substrates are examined in the light of the limited amount of independent experimental evidence available and the usefulness of the estimations endorsed.
Diffusion equation derived from the space‐time transport equation and light pulse propagation through thick clouds
Authors: Furutsu K. , Ito S.
Published: February 2008

The diffusion equation is derived from the ordinary space‐time transport equation and is shown to be given necessarily in the first order in time, as in cases of the diffusion equations derived from the Fokker‐Planck and Boltzmann equations. The systematic way of obtaining the higher‐order diffusion equations also has been shown elsewhere. The boundary equations on the boundary of free space are obtained and applied to the light pulse propagation through thick clouds. The explicit expression is obtained for the pulse broadening and is found to be considerably affected by a slight absorption of the medium, especially when the diffusion distances are large. With the experimental value of absorption for stratocumulus clouds, as suggested by Danielson et al., the theoretical values of pulse broadening are compared to those of experiments observed by Bucher and Lerner to show a good agreement.
Scattering from a perfectly reflecting arbitrary periodic surface: An exact theory
Authors: DeSanto J. A.
Published: February 2008

We extend our exact analysis for plane wave scattering from a sinusoidal surface to include the case of an arbitrary periodic surface. Both Dirichlet (transverse electric polarization) and Neumann (transverse magnetic polarization) boundary conditions are considered. The method uses Green's theorem and our previous idea of expanding the surface fields in Fourier series multiplied by the Kirchhoff or physical optics approximation. The expansion coefficients solve a set of linear equations. The field amplitudes of the upgoing scattered (or evanescent) waves (valid above the highest surface excursion) are expressed as a summation over these expansion coefficients multiplied by integrals over the surface function. The Rayleigh hypothesis is not invoked. Some examples are presented. For an analytic surface, steepest descent methods yield the asymptotic values of the amplitudes. Using this and other asymptotic results, the convergence of the scattered wave expansion is studied as it is analytically continued into the surface wells, and a simple and explicit confirmation of the conditions under which the Rayleigh hypothesis is valid is presented as well as new results for other examples. The periodic surface examples include a sinusoid, an echelette, a quadratic surface, a log cosine surface, a vortexlike surface, a cycloid, a trapezoid, a full‐wave rectified surface, and a surface of semicircular cylinders (bosses). The method is general and applies to a very broad class of physical problems.
Analysis of long‐term rain attenuation statistics
Authors: Kanellopoulos J. D. , Kefalas H. , Androulakakis N. I.
Published: February 2008

An experimental verification of Lin's prediction model for rain attenuation statistics, with data taken from a terrestrial radio link in Athens, is presented. It provides an encouraging indication that this model could be the appropriate one for radio paths in the Greek airspace.
Nonlinear ducting of whistlers in a plasma
Authors: Tewari D. P. , Tripathi V. K.
Published: February 2008

Experimental results of Sugai et al. on the self‐trapping of high‐power whistlers in a long plasma column are explained. It is suggested that the electrons of the plasma are heated nonuniformly and subsequently redistributed across the magnetic field. The density trough thus formed provides a duct for the propagation of the whistler in the frequency range ωc > ω > ωc/2. Thermal conduction across the magnetic field is seen to play a dominant role in the temperature, and the heating time agrees well with the experimental results.
Vectorial wave analysis of graded‐index fibers
Authors: lkuno Hiroyoshi
Published: February 2008

A vector wave analysis is proposed for analyzing the propagation characteristics of the guided modes of the graded‐index fibers. Two pairs of the vector wave equations with respect to the transverse components of the electromagnetic fields are derived and solved by the perturbation method in which we use the scalar wave equation as an unperturbed one. As an example, we calculate the eigenvalues and the eigenfunctions of the vector modes of a graded‐index fiber with an even polynomial‐profile core, and for the eigenvalues we obtain the analytic solutions. Moreover, the result coincides with that obtained by a more sophisticated method.
Single‐mode condition of optical fibers with axially symmetric refractive index distribution
Authors: Kokubun Yasuo , Iga Kenichi
Published: February 2008

The cutoff condition of HE, EH, TE, and TM modes is formulated in an integral equation by using a Sturm‐Liouville‐type boundary problem. A formula for calculating the cutoff frequencies of the TM01 and HE21 modes is derived in addition to the TE01 cutoff by calculating the eigenvalue of this integral equation. The cutoff frequencies of the TM01 and HE21 modes are calculated and compared with those of the TE01 mode for various index profiles. As a result, the cutoff frequency of the TM01 mode is dominant for the single‐mode limit over frequencies of the TE01, TM01, and HE21 modes for the α‐power law index profile. However, in the case of an index profile including a center dip, the cutoff frequency of the TE01 mode dominates the single‐mode condition.
Baseband frequency characteristics of the multimode fiber whose axis suffers from random bends
Authors: Itoh H. , Tatekura K. , Itoh K.
Published: February 2008

The effects of random bends on the baseband frequency response of the multimode optical fiber are very important in practical communication systems. But their theoretical investigation has not been fully carried out. Therefore in this paper we theoretically investigate the baseband frequency characteristics of the multimode fiber whose axis suffers from random bends with the help of the variational method. As a result, we obtain the phase characteristics as well as the amplitude characteristics and indicate that the phase distortion can be neglected within the bandwidth. Also, we confirm the existence of the quasi‐steady state, in which the transmission characteristics are fairly stabilized, although the bandwidth is not always proportional to the inverse square root of the fiber length.
Coupled multimode analysis of anisotropic heterostructure waveguides and its application to a light modulator
Authors: Nakajima Masamitsu , Onodera Hidetoshi , Ikenoue Jun‐ichi
Published: February 2008

A method of analysis for anisotropic planar optical waveguides is presented by taking the finite thickness of the substrate into account wherein the substrate radiation mode is quantized to have a discrete spectrum of propagation constants rather than a continuous spectrum. The analytical procedure is thereby systematized and makes it possible for a computer solution to obtain accurate results. As an example, this method is applied to the analysis of a light modulator in which the coupling of a guided mode to radiation modes is electrooptically controlled.
Statistical theory on optical directional couplers with random parameters
Authors: Araki Ken'ichi , Yoneyama Tsukasa , Nishida Shigeo
Published: February 2008

Statistical coupling characteristics of optical directional couplers subject to random irregularities are studied theoretically by means of coupled mode theory. A new method is used for solving coupled mode equations in which random components of coupling coefficients and phase constants are assumed to be Gaussian random processes with an arbitrary correlation length. Mean values, variances, and probability distributions of coupled powers in 0‐dB and 3‐dB directional couplers are presented. The influence of random parameters is made clear. It is seen that tolerance requirements can be relaxed by a proper choice of the average parameters. Discussions about the film waveguide couplers for integrated optics applications are presented.
Leaky waves supported by a square wire mesh with bonded junctions above a reactive surface
Authors: Barbosa A. M. , dos Santos A. F. , Figanier J.
Published: February 2008

The modal equation of a square bonded mesh of thin conducting wires over a reactive plane is derived and solved numerically by a technique that yields all the roots within a given region of the transverse wave number plane. Phase velocity and attenuation constant of the lowest order leaky wave mode is obtained as a function of mesh parameters, surface impedance of the reactive plane, frequency and direction of propagation. Some features of the radiation characteristics of this structure are mentioned.
Inversion of data from diffraction‐limited multiwavelength remote sensors: 3. Scanning multichannel microwave radiometer data
Authors: Rosenkranz P. W.
Published: February 2008

A spatial filtering algorithm is described, by means of which geophysical parameters are retrieved from measurements made over the ocean by the scanning multichannel microwave radiometer on the Nimbus 7 satellite. The parameters are near‐surface wind speed, sea surface temperature, rain rate, integrated atmospheric water vapor content, and integrated liquid water content (including rain). The capabilities and limitations of the algorithm are demonstrated with data obtained from a tropical cyclone which occurred in the Indian Ocean in November 1978 and with data from the North Atlantic. The major limiting factor, away from land, is the high opacity of the atmosphere in areas of heavy rain, particularly with respect to retrieval of surface temperature and water vapor. However, despite the presence of rain the wind speed retrieval locates the cyclone and provides a measure of its intensity.
An analysis of diffraction at edges illuminated by transition region fields
Authors: Tiberio R. , Kouyoumjian R. G.
Published: February 2008

A solution is obtained for the high‐frequency scattering by a pair of parallel wedges where the edge of one wedge is illuminated by the shadow boundary field of the other. A generalized spectral extension of the uniform GTD is used to determine the contribution from the doubly diffracted ray. Expressions for the diffracted field are given for plane, cylindrical, and spherical wave illuminations. Incidence oblique to the edges is also considered. These expressions reduce to a closed form at the shadow boundaries and outside the overlapping transition regions where the results coincide with those obtained by the uniform GTD augmented by slope diffraction. The solutions to the scalar problems are then used to derive a dyadic diffraction coefficient for the doubly diffracted field, in the ray‐fixed coordinate system. The general expressions presented here directly apply to the diffraction by half planes, strips, and joined wedges (thick edges) where the number of terms involved significantly reduces.
A deconvolution method for pulsed coherent radars
Authors: Lehmann Paul
Published: February 2008

The effects of a necessarily limited transmission and reception bandwidth upon a pulsed coherent radar experiment can be partially overcome by the use of the deconvolution method described. Unlike ‘conventional’ techniques, an accurate knowledge of the radar system pulse response is not required to estimate the propagation delays of reflected pulses (echoes). However, a knowledge of the complex pulse response facilitates the simulation (Monte Carlo) of a ‘generalized’ echo structure model from which a deconvolution analysis can yield 95% confidence limit estimates for the calculated echo delays. Simulation results are presented showing dependence between the errors in delay estimates, the form of pulse response, and deconvolution control parameters.
Localized ELF nocturnal propagation anomalies
Authors: Bannister Peter R.
Published: February 2008

On numerous occasions during the last decade, extremely low frequency nighttime field strength measured at various sites in the northeastern United States has displayed rapid decreases of from 4 to 8 dB in several hours. In this article we present further examples of localized ELF nighttime field strength variations and show that these variations are not restricted to measurement locations in the northeastern United States. Particular attention will be focused on simultaneous Connecticut and North Atlantic area measurements where nighttime propagation anomalies occurred 2–4 hours apart. A candidate for the cause of these anomalies is a moving nocturnal sporadic E layer.
Using an S band radio interferometer for measuring the phase shift of radio waves coherently reflected from the sea surface
Authors: Glazman Roman E.
Published: February 2008

An analytical study of the interference pattern formed over a sea surface by a coherent source of S band radiation is presented. Results of the theory of wave scattering by rough surfaces are employed to account for the effect of sea state on the interference pattern. The phase of the reflection coefficient for the coherent part of the reflected field is shown to cause a vertical shift of the interference pattern. This shift is utilized to develop interferometric techniques of reflection coefficient phase measurement based on a bistatic transmitter‐receiver scheme. Also, a procedure for retrieving sea level information from interferometric measurements is proposed that is based on incorporating the phase shift measurements into a previously developed sea level measurement technique. The measured phase shift precision is estimated to be ±1°, corresponding to a ± 1‐cm accuracy in sea level measurements.
Low‐frequency scattering by a perfectly conducting body
Authors: Senior T. B. A.
Published: February 2008

For a plane electromagnetic wave incident on a perfectly conducting body, the first two sets of terms in the low‐frequency expansion of the far zone scattered field are derived. The first set consists of dipole contributions which can be expressed in terms of the electrostatic and magnetostatic potentials, and the next set can be obtained from these same potentials plus one other.
Scattering by Pruppacher‐Pitter raindrops at 30 GHz
Authors: Yeh C. , Woo R. , Armstrong J. W. , Ishimaru A.
Published: February 2008

Optimum design of modern ground‐satellite communication systems requires the knowledge of rain‐induced differential attenuation, differential phase shift, and cross‐polarization factors. After a comprehensive assessment of different available analytical techniques, an efficient technique is chosen to yield the desired scattering results. Tabulation of the scattered fields for Pruppacher‐Pitter raindrops with sizes ranging from 0.25 mm to 3.5 mm at 20°C and at 30 GHz for several incidence angles is given.

Table 1 is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N. W., Washington, D. C. 20009. Document S82‐001; $5.00. Payment must accompany order.
Analysis of rain clutter data from a frequency agile radar
Authors: Gordon William B.
Published: February 2008

Measurements of rain clutter were taken with a frequency agile S band radar for the purpose of studying the Doppler power spectra and the variation of backscattered power with space, time, and frequency. The clutter was found to be nonhomogeneously distributed in both space and time, but the mean backscattered power versus frequency relation seemed to involve only a single parameter having a spatial and temporal variation. Although the radar was looking into the surface wind, a significant portion of the backscattered signal energy was always found to lie in the negative frequency range, and the Doppler power spectra were sometimes found to have multiple peaks at both positive and negative Doppler frequencies. The simplest explanation for such spectra is that of a wind shear operating on a vertically stratified distribution of clutter, and assuming this to be the case, it can be estimated that the wind shear was of the order of 8 m s−1 km−1 and that the peaks were produced by horizontal bands or clumps of clutter whose vertical extent was of the order of 200 m.
VLF mode conversion at terminator taking into account earth's curvature
Authors: Ledinegg E.
Published: February 2008

As an extension of earlier work in which VLF mode conversion at the solar terminator has been treated with a plane model of the earth‐ionosphere waveguide, here a spherical model is used. This is done with some simplifications. First, the real three‐layer problem (ionosphere‐atmosphere‐earth) is reduced to a boundary value problem involving fictitious surface impedances along the interfaces to the ionosphere and to the ground, chosen such that they describe in an asymptotically equivalent way the influence of these layers. Second, the thin shell approximation is employed, which permits one to reduce the problem to a quasi‐plane model still containing the spherical characteristics, as, for example, the characteristic values of the spherical modes. A system of coupled integral equations is obtained which is solved rigorously by an iterative procedure.
The scattering of electromagnetic waves by single hydrometeors
Authors: Holt A. R.
Published: February 2008

This paper is concerned with the state of the art in the calculation of electromagnetic wave scattering by hydrometeors. Four methods currently being used are briefly described (point‐matching, T matrix, unimoment, and Fredholm integral equation), and aspects of their computational implementation are reviewed and compared. The ranges of validity of three types of approximation are discussed—those with respect to size, orientation, and frequency. Finally, scattering patterns from various sizes of spheroidal raindrop at 11.7 GHz are briefly discussed.
Brillouin revisited: Signal velocity definition for puise propagation in a medium with resonant anomalous dispersion
Authors: Trizna Dennis B. , Weber Thomas A.
Published: February 2008

The theoretical problem of wave propagation and the definition of the signal velocity in a medium exhibiting resonant, absorptive, anomalous dispersion is investigated. The special case of a dilute medium is considered, such as the upper atmosphere with dispersion due to resonant line structure (e.g., the water vapor or oxygen lines in the microwave region), as opposed to plasma dispersion, which is not resonant. Signal velocity definitions analogous to those first made by Brillouin were derived in another work and are called to question as a result of an exact solution of the problem derived here, using a Bessel function series, which allows exact wave forms to be calculated and displayed. When one represents the signal development by using the simulations, it is found that one cannot realistically separate signal from precursor. The pulse envelopes do have some interesting ringing characteristics which are shown to depend upon the density of absorbers and the resonance linewidth. Qualitative experimental results are presented that verify the simulations. The measurement of this ringing appears to be a more sensitive technique than does differential absorption for determining constituent number densities.
Oscillatory integral theory for uniform representation of wave functions
Authors: Arnold J. M.
Published: February 2008

This paper describes and evaluates a method of constructing asymptotic representations of wave functions, using only local geometrical properties of ray trajectories, which are uniform on and through caustics, shadow boundaries, or any other singularities which may appear in the conventional ray optical description of a wave field. This method is similar in philosophy to the geometrical methods of Maslov and Duistermäat, but can be carried out by means of elementary geometry on local ray trajectories, without recourse to the complicated topological structures employed by mathematicians. Thus it is possible to consider realistic applications of uniform methods to quite complicated problems, unobscured by the difficult language of differential topology favored by mathematicians as a framework for uniform geometrical asymptotics. The method is based on the fundamental concept of an angular spectrum of reference waves which may, but need not, be plane waves. The reference wave spectrum is used only as an intermediate device in the construction of the uniform representation and does not feature in the final form of the wave function. An application to cusped caustics in inhomogeneous media is considered.
Tilted refractive surfaces at Eglin Air Force Base, Florida
Authors: Schleher Jeffrey S.
Published: February 2008

Radar ducting layers along the Gulf coast of Florida are examined for a 9‐day period in May of 1976. Tilted refractive surfaces are shown to exist with enough of a slope to significantly increase the chance of radar ducting at small look angles. Radiosonde observations are found to agree with average refractometer measurements but do not detect ducting layers with small vertical depths.
Generalization of the Booker‐Gordon formula to include multiple scattering
Authors: Fante Ronald L.
Published: February 2008

A general expression, which includes multiple scattering, has been obtained for the radiation scattered from a random medium. This result is evaluated analytically and then compared with results calculated using the Booker‐Gordon formula, which includes only the effects of single scattering.