bluuebunny/crossref_metadata_2025 huggingface dataset:
Solar cycle effects on the near‐Earth space environment
Authors: Gorney D. J.
Published: February 2008

Forecasts of the magnitude of solar activity in the 1990s (solar cycle 22) imply that the expected levels of activity might be some of the most extreme ever recorded, and almost certainly the levels of activity will be the highest experienced during the space age. Even as early as 1 year before the expected maximum of solar cycle 22 in 1990, unprecedented levels of solar activity (for example, the solar flares and solar particle events of August–October 1989) and geomagnetic activity (for example, the auroral events and geomagnetic storms of March 1989) have been observed. These solar and geophysical events have stirred scientific interest both in the long‐term behavior of solar activity and in the physics which couples the energy of solar events to the near‐Earth environment. Furthermore, the operational community (including those involved in satellite operations, telephone and radio communication, electric power distribution, aviation, and others) have experienced many adverse effects of these solar and geophysical events. Many more episodes of activity are expected throughout the upcoming 4–5 years. The purpose of this paper is to review the direct and indirect influences of solar activity on the near‐Earth environment and to describe some of the implications of the high levels of solar activity which are expected to occur in the 1990–1994 time period.
Cylindrical Antenna in Uniaxial Resonant Plasmas
Authors: Lee S. W.
Published: February 2008

The current distribution and the input admittance of a cylindrical antenna immersed in two types of uniaxial resonant plasmas are treated in this paper as boundary value problems. It is first indicated that the Hallen type integral equation approach is not suitable, since it does not describe the actual physical phenomenon closely. The method used here is based on the concept of multiple reflections of current waves between the two terminals of a solid antenna, and simple approximate solutions for thin antennas are obtained by using the newly developed extension of the Wiener‐Hopf technique.
Comparison Between Formulas for Ionospheric Radio Propagation and Atmospheric Wave Propagation
Authors: Davies Kenneth , Baker Donald M. , Chang Norman J. F.
Published: February 2008

Attention is drawn to similarities between certain magnetoionic formulas in ionospheric radio propagation and corresponding formulas for acoustic‐gravity waves. Acoustic waves are similar to radio waves with frequencies above the electron gyrofrequency, whereas gravity waves are similar to whistlers.
Electromagnetic Wave Propagation Over a Constant Impedance Plane
Authors: King R. J.
Published: February 2008

The classical Sommerfeld problem of electromagnetic wave propagation over a flat earth is reformulated by using the electromagnetic compensation theorem. The solution is carried through without recourse to the usual spectral representation of the potential function, and therefore complicated contour integrations are avoided in the subsequent evaluation. Using reciprocity and duality, the complete electromagnetic fields are calculated for elemental vertical or horizontal dipoles of both electric and magnetic types, which are elevated above an arbitrary but passive surface impedance such as that corresponding to homogeneous, horizontally stratified, or uniformly rough surfaces. The results are identical to the little known but more accurate work of Norton, and under certain approximations they reduce to the more well‐known results of Norton, Hufford, Bremmer, and others. The restrictions on the impedance properties of the surface are given, and certain direction finding errors are discussed. It is shown that the index of refraction contrast between the upper and lower medium need not be large.
Internal Guiding of Microwaves by an Elevated Tropospheric Layer
Authors: Wait James R. , Spies Kenneth P.
Published: February 2008

The waveguide modes guided by a smooth concave tropospheric layer are considered. The model is chosen so that the effective dielectric susceptibility is a linear segmented function of height. The calculations are made explicitly for a model that is linear with height except for a single inversion layer. It is shown that such a layer will support whispering‐gallery or earth‐detached modes with exceedingly small attenuation even when the refractive index contrast is four or five N units.
Survey of Polar and Auroral Region Effects on HF Propagation
Authors: Hunsucker R. D. , Bates H. F.
Published: February 2008

The effects of the auroral and polar zone ionosphere on forward propagation in the HF through low‐VHF (3 to ∼30 MHz) portion of the spectrum are reviewed. Phenomena that particularly affect the propagation of HF/VHF signals in this region include: the non‐great‐circle (NGC) mode, sporadic‐E ionization, F1‐layer effects, F‐region irregularities, D‐region absorption, and reflectivity properties of the polar ice caps. Specifically, we show that: (1) The NGC mode can cause deviations from the great‐circle path up to 90° and can carry the maximum frequency on a polar circuit. (2) Sporadic‐E ionization occurs primarily during the winter‐night period and is sometimes associated with the aurora. (3) F1‐layer effects are quite important and frequently determine the dominant propagation mode during summer mornings at sunspot minimum. (4) Auroral zone absorption is quite ‘patchy’ in spatial and temporal behavior and probably attenuates HF signals up to 60 db in severe cases. (5) Polar cap absorption, although a relatively rare event, can attenuate HF signals up to 100 db at times. (6) For multi‐hop modes, the relatively high attenuation for reflection on the ice caps should be considered in the transmission loss equation. Research performed during the decade ending in 1968 is emphasized, including some quite recent results of investigations conducted at the Geophysical Institute at the University of Alaska, not previously reported in the literature.
Noise Spectrum Received by an Antenna in a Plasma
Authors: Fejer J. A. , Kan J. R.
Published: February 2008

A general expression for the noise spectrum received by an antenna in an isotropic plasma is derived by using a combination of plasma fluctuation theory and the reciprocity theorem. The special case of Maxwellian velocity distributions of the constituents is then considered. In the still more special case of thermal equilibrium Nyquist's theorem is shown to be satisfied. It is shown that a 'Maxwellian' energetic electron constituent carrying only a small fraction of the total thermal energy can enhance the spectral power density of antenna noise just above the plasma frequency. The maximum enhancement factor is equal to the square of the ratio of the 'thermal' velocities of the high‐ and low‐energy electrons. There is no such enhancement in the resistance of the antenna.
Measurement of Momentum and Heat Flux Variations in the Surface Boundary Layer
Authors: Kaimal J. C.
Published: February 2008

Recent measurements of momentum and heat flux between 5.66 and 22.6 meters on a tower over a flat uniform site indicate that both fluxes are constant with height to within ±20%. Some important aspects of instrument performance relevant to flux measurement, e.g., sonic anemometer array configuration, alignment and leveling of the axes, and errors in sonic thermometry, are discussed. The large variations observed in the vertical momentum flux gradient during unstable conditions appear to be the result of intermittent bursts in the momentum transport. Some characteristics of these bursts are described. Long averaging periods, of the order of an hour, are needed for meaningful estimates of flux gradients under such conditions.
On Richardson's Number as a Criterion for Laminar‐Turbulent‐Laminar Transition in the Ocean and Atmosphere
Authors: Woods J. D.
Published: February 2008

Dye‐tracer experiments in the seasonal oceanic theormocline have revealed a microstructure with a significant fluctuation of shear and density gradient within a vertical distance of 1 cm. It has proved possible to relate the occurrence of localized spots of turbulence with this structure and with internal waves traveling along especially (statically) stable ‘sheets’ within the thermocline. The thermocline microstructure has a predominant vertical scale of a few decimeters, the sheet waves have wavelengths in the range 5–10 meters, and the turbulence occurs at a Reynolds number in the range 200–1000. Similar features are found on a very much larger scale in the atmosphere, although the methods of investigation (mainly instrumented aircraft) used here are incapable of yielding the detailed information obtained by divers working in the thermocline. It seems useful, therefore, to treat the observations of microstructure, waves, and turbulence in the thermocline as a possible model for the corresponding features in the atmosphere. This leads to some predictions concerning the atmosphere microstructure and also to suggestions for the planning of experimental investigations. The available evidence from the atmosphere appears to support the predictions that (1) the critical gradient Richardson number for reverse transition occurs at about unity regardless of the Reynolds number of the turbulence, and (2) that most of the atmosphere (with the exception of clouds and the boundary layer) is in laminar flow.
Waves and Turbulence
Authors: Busch N. E. , Vinnichenko N. K. , Waterman A. T. , Beard C. , Stewart R. W. , Scotti R. S.
Published: February 2008

Observations on fluid flows indicate quite clearly that it may be feasible to distinguish between various classes of fluid motions with reasonably well‐defined properties.

Before we try to investigate the feasibility of distinguishing waves from turbulence, it is important to notice that these two types of fluid motion are not the only ones. If all realizable flow patterns are assumed to be solutions to the same set of equations, then in principle they all possess the same degrees of freedom or possible modes of motion. The spatial and temporal boundary conditions appropriate for a given flow field determine which modes can and will be excited.
Lunar Surface Mechanical Properties: Surveyor Results
Authors: Christensen E. M.
Published: February 2008

Lunar surface mechanical properties have been interpreted from the interactions of five Surveyor spacecraft with the lunar surface (exclusive of the surface sampler). The lunar soil at the one highland and the four lunar mare landing sites is remarkably similar, even though these sites are widely separated. The soil is predominantly fine‐grained and granular; it is estimated to have a cohesion value between 0.05 and 0.17 N/cm2. The static bearing strength of the soil increases with depth. It is estimated that, at the surface, the strength is less than 0.1 N/cm2; at a depth of 4 cm, for a 25‐cm‐diameter plate, the strength is somewhat greater than the originally estimated 4 to 6 N/cm2. The soil, at least in the upper few centimeters, is compressible.
Ray Theory for Lossy Media
Authors: Jones R. Michael
Published: February 2008

Standard ray‐tracing programs do not calculate satisfactorily the reflection of LF radio waves from the ionosphere because they do not take losses into account. In lossy media, requiring the ray path to have a minimum attenuation in addition to a minimum wave interference gives a more accurate approximation to the full‐wave solution. An extension of Fermat's principle, in which the complex phase refractive index is used instead of only the real part, expresses both of these criteria and leads to a corresponding extension of Snell's law or of Haselgrove's equations to calculate the ray path. Although such a path can have complex coordinates, only those with end points in real space are physically significant. An approximation, in which plane waves in the neighborhood of the receiver are assumed, solves the common ray‐tracing problem of homing in on the receiver, a problem that is worse for ray tracing in complex space. Applying ray tracing in complex space to a plane wave incident on a plane stratified medium gives a result that agrees exactly with the result obtained by the phase integral method and that agrees satisfactorily with full‐wave solutions above 30 kHz for all results shown.
Electron‐Density Distributions in the Lower Ionosphere with Associated Error Limits Derived From VLF and LF Sounder Data
Authors: Shellman C. H.
Published: February 2008

A method is given for determining electron‐density and collision‐frequency distributions in the lower ionosphere from VLF and LF sounder data. Error limits are placed on the distributions by mapping the standard errors of the data into error limits on the ionosphere parameters. Interpretation of the error limits is discussed, especially in regard to the relationship between the resolution of a distribution and its error band. Examples are given that illustrate the method.
Coupled Electroacoustic and Electromagnetic Waves in an Inhomogeneous, Compressible, and Lossy Plasma
Authors: Verma Y. P. , Raemer H. R.
Published: February 2008

Wave propagation in a one‐fluid compressible, inhomogeneous, and lossy plasma is studied. Coupling between the electroacoustic and electromagnetic waves is investigated analytically. The inhomogeneity of the medium is due to the variation of the electron density, collision frequency, and temperature along the vertical direction only (zdirection). AnEpolarized electromagnetic wave is assumed to be incident from a vacuum to a slowly varying, inhomogeneous, compressible, and lossy plasma occupying the half space defined byz> 0. Among the various possible boundary conditions atz= 0 that involve the electroacoustic fields, the vanishing of the pressure atz= 0 is chosen so that the numerical computation becomes relatively simple. The profiles of the electron density, the collision frequency, and the temperature are chosen so that they are consistent with this type of boundary condition. The numerical results presented here show the dependence of the coupling characteristics (i.e., the transfer of energy fromemwaves topwaves) on the various plasma parameters and other parameters. Conditions for the validity of the perturbation method are examined and are proved to be satisfied for the problem.
Observations of F‐region Vertical Velocities at Millstone Hill 3. Determination of Altitude Distribution of H+
Authors: Evans J. V. , Holt J.
Published: February 2008

Measurements of the vertical flux of O+ in the F region have been made by the incoherent scatter technique at Millstone Hill. In part 2 of this paper, evidence was presented for the existence of fluxes of ≥ 108 cm−2 sec−1 into the protonosphere during the day. In this paper we employ the altitude variation of the O+ flux to infer the magnitude of the proton flux in the charge‐transition region. Based on continuity considerations and the theory of minor ion diffusion in the topside of the F layer, we have attempted to determine the abundance of H+ in the altitude interval where it is a minor ion. The results indicate that the abundance of neutral hydrogen near 600 km must exceed about 105 cm−3. However, the absence of detectable amounts of H+ in the signal spectra taken near 1000 km indicates that the transition altitude must be above 1000 km and in turn the neutral‐hydrogen abundance cannot exceed this estimate by more than a factor of 3.
Orthogonality of VLF Height Gains in the Earth Ionosphere Waveguide
Authors: Pappert R. A. , Smith R. R.
Published: February 2008

Orthogonality conditions satisfied by the height gain functions governing VLF fields in the vertically inhomogeneous, anisotropic earth ionosphere waveguide are discussed. It is pointed out that these orthogonality conditions which in general involve both the height gains for the given waveguide and those of an appropriate ‘adjoint’ waveguide can serve as a sensitive test for numerical consistency in full wave calculations of waveguide parameters.
Sporadic‐E Formation by Wind Shear, Comparison Between Observation and Theory
Authors: Kato S. , Aso T. , Horiuchi T. , Nakamura J. , Matsuoka T.
Published: February 2008

A comparison is made between observation and theory of simultaneously observed wind shear and electron density in the lower ionosphere. The observation was done by K‐8–16 rocket flown from Kagoshima Space Center, Japan. The method of characteristics for time‐dependent solution of the equation of continuity, which is a partial differential equation of the first order, was used. The calculation shows a remarkable consistency between the observation and the theory even in details. The result strongly supports the wind‐shear theory for sporadic‐E formation. The solution suggests that the effective recombination rate is 10−8 cm3 sec−1 as found from other ionospheric observations.
Measurement of E‐Layer Effective Recombination Coefficient During Solar Flares
Authors: Wagner L. S. , Thome G. D.
Published: February 2008

Simultaneous measurements of flare‐enhanced electron density in the E layer and of flare‐enhanced solar X‐ray flux were used to evaluate the ionospheric relaxation time constant at E‐layer heights. The method is an extension of Appleton's ionospheric “filter” idea to the case in which radiation flux and electron density fluctuations are simultaneously measured. Electron density measurements were made using the Thomson scatter technique at the National Astronomy and Ionosphere Center, Arecibo, Puerto Rico. X‐ray flux data in the 2‐ to 12‐Å band were obtained from instruments aboard the Explorer 33 satellite. Measured values of time constant were converted to an effective recombination coefficient using the familiar Appleton formula and known values of the ambient electron density. Values for the effective recombination coefficient of the order 10−7 cm3/sec were obtained at E‐layer heights.
Pattern Synthesis of Linear Arrays Using Fourier Coefficient Matching
Authors: Imbriale W. A. , Heller J. E. , Mittra R. , Cruz J. B.
Published: February 2008

This paper presents a computer technique for synthesizing an array pattern for the practical problem in which only the magnitude of the far‐field pattern is specified. The general design problem is one in which both the currents and the spacings of the elements are allowed to be variables. The synthesis is based on a procedure which minimizes the norm of the difference between the Fourier coefficients of the desired and actual patterns. This criterion is very useful from practical design considerations since it allows one to determine a priori the highest order Fourier coefficient that can be approximated if the maximum array size is specified. Also, if no array size is specified, the procedure gives the array size necessary to obtain a given degree of approximation in the pattern.
The Rayleigh hypothesis and a related least‐squares solution to scattering problems for periodic surfaces and other scatterers
Authors: Millar R. F.
Published: February 2008

The Rayleigh hypothesis is reviewed in relation to scattering by periodic surfaces, aperiodic surfaces, and bounded, two‐dimensional bodies. Conditions for its validity are described, and explicit results are quoted for a sinusoidal grating. Some methods to solve scattering problems for periodic surfaces are outlined. One particular procedure for periodic surfaces and bounded scatterers is examined in detail. This involves an expansion for the scattered field in terms of the same sets of elementary wavefunctions that occur in connection with the Rayleigh hypothesis. The coefficients are determined by satisfying the boundary condition in the least‐squares sense. It is shown that this solution converges uniformly to the scattered field at all points exterior to the boundary of the scatterer. Necessary completeness properties of the sets of wavefunctions are established in the appendices.
Ray‐tracing simulation of ionization trough effects upon radio waves
Authors: Helms Ward J. , Thompson Allan D.
Published: February 2008

The propagation of high frequency electromagnetic waves through an ionosphere perturbed by a gaussian electron density trough is investigated using a three‐dimensional ray‐tracing computer program. Ray trajectories are presented for a normalized model ionosphere with troughs of varying width, depth, and distance from the transmitter. Focussing and defocussing effects are demonstrated to occur under special circumstances. Particular emphasis is placed upon simulation of records from fixed‐ and swept‐frequency sounding equipment. In particular, a moving trough is found to have unique Doppler signature which may be useful for ground‐based observational work.
Perturbation of magnetic dipole fields by a perfectly conducting prolate spheroid
Authors: Hill David A. , Wait James R.
Published: February 2008

The effect of a perfectly conducting prolate spheroid on the surface fields of a buried vertical magnetic dipole is examined in the quasi‐static case. When the spheroid is in the vicinity of the source and near the surface, the errors introduced in source location can be significant. The results have possible application to rescue operations following coal mine disasters and in similar situations.
Measurement of three‐dimensional plasma velocities at the Arecibo Observatory
Authors: Hagfors T. , Behnke R. A.
Published: February 2008

An observation scheme is described in which it is possible to measure the three velocity components of plasma drift in the ionosphere by using a single monostatic incoherent scatter facility with a steerable antenna. The scheme involves a least‐mean‐squares fitting of the line‐of‐sight velocity observed in several different directions to what one would compute from a given set of three orthogonal velocity components. The method is applied to data gathered at the Arecibo Observatory and is shown to be capable of providing velocity information with rms errors of better than 10 m sec−1 for all three components with a time resolution of 20 min.
Estimation of raindrop size distribution using multiple wavelength radar systems
Authors: Goldhirsh Julius , Katz Isadore
Published: February 2008

The possible use of multiple wavelength radar systems to determine raindrop size distributions is examined. Pairs of radars at 1, 3, and 10 cm, respectively, are considered; the method involves measurements of rain reflectivities at two wavelength and at two range intervals separated by about one km. Assuming a general exponential form for the drop distribution, equations are derived relating the distribution parameters to the measurements. From these analytical forms the sensitivities of these parameters to radar inaccuracies are examined. The results show that one needs combined 2‐radar accuracies of 1 dB to get even marginal drop distribution accuracies. Such accuracies are considered impractical at present. Other impractical constraints implicit in the results are the assumed uniformity of reflectivity over distances of approximately 1 km, and the requirement to average as many as 104 independent power samples.
Currents, charges, and admittances of linear antennas in dissipative media
Authors: Mishra S. R. , Scott L. D. , King R. W. P.
Published: February 2008

Two representations for the current on an isolated antenna in an arbitrary, homogeneous, isotropic medium have been obtained. These are the polynomial and the trigonometric forms, each with an exponential multiplier. The representations are used for solving coupled integral equations for two antennas in a dissipative medium. Measurements made over a wide range of the parameters of the medium, antenna lengths, and separations (in the case of coupled antennas) indicate a close agreement between the computed and observed values for the admittance and the current and charge distributions for isolated as well as coupled antennas.
Effects of multiple scattering on scintillation of transionospheric radio signals
Authors: Liu C. H. , Wernik A. W. , Yeh K. C. , Youakim M. Y.
Published: February 2008

Recent development in the optical scintillation theory has been adapted to the ionospheric geometry in order to study the ionospheric scintillation phenomenon in the presence of multiple scattering. Under approximations well satisfied in typical ionospheres for a frequency above about 20 MHz, the first through fourth moment equations have been derived and some analytic solutions given. The fourth moment equation has also been solved numerically. The numerical results show clearly the occurrence of focussing and saturation phenomena. In this paper the new multiple‐scatter effects are emphasized.
Radio frequency scattering from a heated ionospheric volume: 1, VHF/UHF field‐aligned and plasma‐line backscatter measurements
Authors: Minkoff J. , Kugelman P. , Weissman I.
Published: February 2008

It is observed that an ionospheric volume in the F layer subjected to high power HF illumination becomes an effective scattering medium for radio signals in the VHF/UHF frequency range. The experimental results are representative of a field‐aligned scattering geometry for which the first such observations of VHF/UHF scattering from a heated ionospheric volume are presented. Two distinct scattering modes are observed, center‐line and plasma‐line scattering. Center‐line scattering is observed at the transmitted radar frequency f; plasma‐line scattering is observed as a pair of sidebands at f ± fh where fh is the heater frequency. The two scattering modes are observed to have markedly dissimilar characteristics. Center‐line scattering is highly aspect sensitive with respect to the direction of the geomagnetic field, B; plasma‐line scattering is found to be much less aspect sensitive, if at all. The region of maximum backscatter for the center‐line mode is found from these measurements to consist of the locus of points within the heated volume over which perpendicularity between the radar line of sight and B is achieved, independent of the location of maximum heating. The backscattering region for the plasma‐line mode is found from these measurements to be determined by the altitude of maximum heating, independent of geometrical considerations involving B. The longitudinal coherence length, L, for center‐line scattering is found from these measurements to be greater than the maximum antenna diameter of 85 ft; no more exact estimate for L is possible. A striking reversal in frequency dependence is found between the center‐line and plasma‐line modes. The per‐unit‐volume center‐line backscatter cross section is found to be about 7 db greater at VHF than at UHF; the per‐unit‐volume plasma‐line backscatter cross section is found to be at least 11 db less at VHF than at UHF. Preliminary results concerning time‐dependent behavior are presented. For both modes the scattering cross section is found to be effectively turned on and off very rapidly in response to the heater excitation; the spectral width of the scattering for both modes is found to be quite narrow (about 10 Hz). The spatial configuration of the heated volume is investigated; significant differences are observed depending on whether fh/f0F2 is greater or less than unity.
On the probability distribution of line‐of‐sight fluctuations of optical signals
Authors: Strohbehn John W. , Wang Ting‐i , Speck James P.
Published: February 2008

In this paper some general problems concerning the probability distribution of an optical signal propagated over a line‐of‐sight path are discussed. The basic physical models for the log‐normal and Rice‐Nakagami probability distributions are given, and various theoretical methods for finding the distribution are discussed, including application of the central‐limit theorem, calculating moments, finding the characteristic functional, and trial and error. The not‐well‐known fact that the log‐normal distribution is not uniquely determined by its moments is pointed out. A summary of the present theoretical and experimental work is given. In addition, some different methods of experimental analysis are suggested, including the use of hypothesis testing and goodness‐of‐fit tests. These methods should permit better presentation of information on probability distributions.
The measurement of meteor winds over Atlanta (34°N, 84°W)
Authors: Roper R. G.
Published: February 2008

An all‐sky, continuous wave radio meteor wind facility has been installed in Atlanta by the Georgia Institute of Technology under National Science Foundation sponsorship. A double sideband suppressed carrier CW transmitter, operating on 32.5 MHz±360 Hz, with an rms output of 2 kw, has been installed on the Georgia Tech campus, and a receiving site established at Technology Park/Atlanta, 27 km northeast of the campus. Details of the equipment, together with height/time profiles of mean wind circulation and tides between 80 and 100 km, measured from August 9 to September 4, 1974, are presented.
Using the natural‐frequency concept in remote probing of the earth
Authors: Lytle R. J. , Lager D. L.
Published: February 2008

A technique is given for determining the thicknesses and the electrical constitutive parameters of a planar‐layered medium such as a coal seam in a mine environment. Time‐domain experimental data are analyzed with Prony's method to determine the natural frequencies of the layered medium. Explicit relations are given (for dielectric layers) for determining the thicknesses and dielectric constants from the experimentally determined natural frequency results. Explicit expressions are also given (for conductive layers) for calculating the electrical thickness from natural frequency results. The method is illustrated with sample numerical results.
On the strong phase screen theory of ionospheric scintillations
Authors: Taylor Leonard S. , Infosino Charles J.
Published: February 2008

The problem of ionospheric scintillations is considered. It is shown that the result obtained using an ionospheric turbulence model with a single‐scale length parameter, predicting modulation indices much larger than unity, is not valid for a two‐parameter model of the Kolmogoroff type unless the rms phase variation for the wave exiting the ionosphere is larger than the ratio of the outer scale to the inner scale of turbulence.
Some highlights in reflector antenna development
Authors: Love A. W.
Published: February 2008

Reflector antennas have been used since the radio pioneering era of Lodge, Hertz, and Marconi, but it took the exigent demands of radar in World War II to stimulate a real development in the reflector art. Subsequent interest in the science of radio astronomy and the inception of microwave ground communication links were responsible for a burgeoning growth in the field, so that in the 1940s and 1950s the design principles and requirements for prime focus fed systems were well established. Cassegrain, or secondary focus systems, and horn reflectors came into prominence in the early 1960s with the advent of satellite tracking and communication networks. The desire to maximize the gain, or the gain‐temperature ratio, then led to development of sophisticated techniques for properly shaping the illumination over the reflector aperture in order to maximize efficiency and minimize spillover, among them being the shaping of the sub‐reflector in Cassegrain systems and the use of multimode and hybrid mode feed horns. Not all reflector antennas utilize paraboloidal surfaces. Some recent developments in line source feeds make the spherical reflector attractive for scanning applications and the conical reflector for deployable, space‐borne antennas. The large 1000‐foot diameter reflector at Arecibo is a well known example of the former. Although some extremely large spaceborne reflector antennas have been proposed and studied, the largest now in use appears to be the unfurlable 30‐foot reflector carried by ATS‐6. Finally, some gain comparisons are given for a few of the (electrically) largest reflectors that have been built both for radio astronomy and for space communications. If some milestones in reflector development have been overlooked it is due to the limitations inherent in a review paper.
Electromagnetic scattering by prolate spheroids for plane waves with arbitrary polarization and angle of incidence
Authors: Sinha Bateshwar P. , MacPhie Robert H.
Published: February 2008

Using modal expansions of electromagnetic fields in terms of prolate spheroidal wave functions, an exact formulation is presented for the scattering by a conducting prolate spheroid of a plane electromagnetic wave of arbitrary polarization and incidence angle. In this formulation the column vector of the series coefficients of the scattered field is obtained from the column vector of the series coefficients of the incident field by means of a matrix transformation. The matrix depends only on the scatterer; hence the scattered field for a new direction of incidence is obtained without repeatedly solving a new set of simultaneous equations. The exact curves numerically obtained for the monostatic radar cross section and relative phase of the scattered field as a function of aspect angle (angle of incidence) for both principal polarizations are given for a variety of prolate spheroids in the resonance region.
The modeling of channel current in the lightning return stroke
Authors: Price Gary H. , Pierce E. T.
Published: February 2008

The return‐stroke current in a lightning flash to ground is modeled as the response of an initially charged, uniform transmission line to the application of a terminating resistance at one end of the line. For a lossless line, the consequent current is constant between abrupt changes; this current differs substantially from that given by the common modeling procedure of injecting a double‐exponential current pulse into one end of a uniform line. The inclusion in the model of resistive losses along the line produces a current that retains the abrupt initial onset, but has a more realistic decay than the decay of the double‐exponential pulse. The fields associated with such a current in a vertical channel agree reasonably well with those observed.
Derivation of directional ocean‐wave spectra by integral inversion of second‐order radar echoes
Authors: Lipa Belinda
Published: February 2008

The derivation of sea‐state from a single decameter radio wave‐scatter observation requires the inversion of a two‐dimensional nonlinear integral equation which gives the second‐order radar cross section in terms of the ocean‐wave spectrum. The second‐order structure is sensitive to the directional properties of waves moving slower than the wind and the total nondirectional energy spectrum. We show how these properties, together with the rms waveheight and the wind direction, may be obtained by inverting the integral equation. The inversion method depends on recognition of physical properties of the wave‐wave interactions which allow linearization of the equation, followed by application of standard inversion methods. This method is tested by application to simulated Doppler spectra corresponding to known ocean wave spectra. For typical examples (radar frequency 30 MHz, Doppler resolution 0.01 Hz, with 20 range and 10 time cells averaged) the directional ocean‐wave spectrum was reproduced to within 10%, the true wind direction obtained to within 5 deg, and the rms waveheight predicted to within 5%.
Experimental and theoretical first approach to ƒH plasma resonance from a relaxation sounding rocket experiment
Authors: Higel B. , de Feraudy H.
Published: February 2008

The various plasma resonance phenomena observed in the ionosphere as relaxation signals near the characteristic frequencies ƒN, ƒT (plasma and upper hybrid frequencies), and n ƒH (electron gyrofrequency harmonics) have been correctly interpreted as oblique echoes. However, the ƒH case is not yet understood. The EIDI 3 relaxation sounding rocket experiment has provided original results regarding the plasma resonances. These results provide grounds to approach the ƒH problem. Indeed, the spectral features of the ƒH resonance signal look like pure sine‐wave ones; their frequency location is found to be very close to the gyrofrequency deduced from the earth's magnetic field model (relative frequency shift of the order of 10−4). The duration of the ƒH resonances is very short and their amplitude level very weak in the EIDI 3 ionospheric flight conditions. Moreover, a well‐defined dependence of this level on the orientation of the dipole antenna with respect to the earth's magnetic field has been pointed out, as well as an increase of the signal level when the antenna length is doubled. To interpret theoretically these experimental results, a brief review is made of the various modes of principal waves propagating close to ƒH. Their frequency range, their wave vector orientation, and the EM field polarization are examined with respect to the corresponding experimental features in order to discard the disagreeing kinds of waves. The ordinary wave mode gives good agreement with the EIDI 3 results. A first‐approach study of the interest of this mode for explaining the ƒH resonance phenomenon provides good grounds for its further study.
Hyperactivity and disruption of operant behavior in rats after multiple exposures to microwave radiation
Authors: Mitchell Daniel S. , Switzer Walter G. , Bronaugh Edwin L.
Published: February 2008

A multimode microwave (MW) exposure chamber was employed to investigate the effects of low‐intensity CW radiation on one innate and two learned behaviors of female Sprague‐Dawley rats. Relative to sham‐irradiated controls, exposure to 2.45‐GHz radiation at an averaged specific absorption rate (SAR) of 2.3 mW/g was associated with: (a) statistically significant increases in locomotor activity, (b) statistically reliable evidence of disrupted differential responding during tests of an appetitively reinforced operant behavior, but (c) no differential effects on the performance of a Sidman avoidance response. The observed effects emerged almost immediately following the onset of MW radiation and persisted throughout the course of a 22‐week sequence of exposures to radiation. Periodic sampling of rectal temperature revealed no indication of whole‐body elevation of temperature. The observed behavioral effects are consistent with a general activation hypothesis and, although the underlying mechanism remains unclear, suggest MW‐induced alterations in function of the central nervous system.
Computer simulation of ionospheric radio drift measurements, and their analysis by correlation methods
Authors: Wright J. W. , Pitteway M. L. V.
Published: February 2008

A computer model is used to simulate spaced‐antenna ionospheric drift experiments. It is shown that the random or “characteristic” velocity Vc observed by correlation analysis is about equal to the root‐mean‐square deviation of the velocity distributions used in the model. Correlation analysis with four fixed antennas is also shown to provide a reliable estimate of drift velocity and correlation scales over a range of wavelengths from about one third to three times the antenna spacing, with realistic error estimates on each parameter. “Dispersive motions” similar to those reported by other workers are observed even when the model contains no features which can be sensibly described by dispersion; it must therefore be presumed that the dispersion analysis of fading records is not necessarily a physically meaningful procedure.
Nonlinear interaction of magneto‐sound waves with whistler turbulence
Authors: Spatschek Karl H. , Shukla Padma K.
Published: February 2008

The damping of obliquely propagating magneto‐sound waves in the presence of weak turbulence consisting of an ensemble of random‐phased whistler wave packets is investigated. The damping rate is found to be strongly dependent on the turbulence level as well as the external magnetic field strength.
Electromagnetic wave propagation in a laterally nonuniform troposphere
Authors: Cho Se Hyun , Wait James R.
Published: February 2008

Electromagnetic wave propagation in a nonuniform troposphere is studied for a cylindrical model of the earth. The refractive index of the troposphere is assumed to vary both with height and range. The formulation is facilitated by the fact that the field within a piecewise (laterally) uniform section can be represented by a discrete sum of modes that are orthogonal in the transverse plane. Using these radial wave functions, mode conversion at a junction between two laterally uniform structures is analyzed for both TE and TM waves.
Numerical approximations of the Sommerfeld integral for fast convergence
Authors: Kuo W. C. , Mei K. K.
Published: February 2008

Several approximation techniques for the Hertz potential of an infinitesimal dipole in the presence of a conducting half space are presented. The formal solutions obtained by rigorous mathematical procedures are transformed, with the aid of some approximation, to fast convergent forms. The approximation techniques are different depending on the media where the observer and source are located. All results calculated are compared to those of the brute‐force integrations of the formal solutions. Restrictions of the validity of these approximations are discussed.
Effects of the inhomogeneous background on radiation propagating through turbulent planetary atmospheres
Authors: Haugstad Bjarne S.
Published: February 2008

Weak scattering, wave optics results are given for the first‐order log‐amplitude and phase, and for the second‐order phase and angle of arrival, of radiation propagating through a turbulent planetary atmosphere. The analytical results include the effects of coupling to the inhomogeneous background upon which the turbulence is superimposed. Such coupling significantly affects signal characteristics in stellar and spacecraft occultation experiments, in contrast to situations involving much shorter paths, as in terrestrial propagation experiments.
Incoherent backscatter from rough surfaces: The two‐scale model reexamined
Authors: Leader J. Carl
Published: February 2008

An expression for the incoherent backscatter cross section per unit area of a two‐scale rough surface is derived that includes the effects of local polarization tilt angles, thus providing a first‐order cross‐polarized scattering component. A rigorous numerical method of evaluating the derived expression is developed and two approximate solutions are also derived. The approximate solutions clarify the two‐scale interaction for the cases of diffuse and quasi‐specular small‐scale scattering. Calculated scattering distributions are compared with experimental data at radar and optical frequencies using both perturbation theory and physical optics polarization coefficients. Good agreement with copolarized radar data is obtained using perturbation theory polarization coefficients while physical optics polarization predictions are observed at optical frequencies. Ambiguities in the comparisons between experimental data and model predictions are discussed.
Rain‐rate estimates for an attenuating radar
Authors: Meneghini Robert
Published: February 2008

A number of estimates are available for the extraction of rain rate from the measurements of an attenuating frequency radar. The estimates, which arise from various approximations for the attenuation, include at one extreme the estimate in which no correction for attenuation is made. An infinite number of higher‐order estimates exist which are shown in the limit to approach the Hitschfeld‐Bordan solution. An error analysis is carried out for four of the estimates, taking into account the various "system" errors which include the randomness of the radar return power and the k‐Z, Z‐R relations as well as the offsets in the radar calibration constant. The results indicate that as these errors are more narrowly bracketed, progressively higher‐order estimates can be beneficially employed. However, since the behavior of the estimates is strongly dependent on the system errors, the choice of the best estimate may be difficult without an accurate specification of these errors. Furthermore, in order for any of the estimates to provide a reliable means of rain‐rate prediction in the presence of realistic errors, the attenuation must be kept small. This is usually insured for antenna pointing angles away from the horizontal at radar frequencies at the lower end of the X‐band.
Laboratory experiment on the excitation of electron cyclotron harmonic waves observed in the topside RF sounder experiment
Authors: Sasaki Susumu , Kaneko Osamu , Kawashima Nobuki
Published: February 2008

An experiment is performed to study the interaction of a high power microwave pulse with a magnetized dense plasma, in relation to topside RF sounder experiments in the ionosphere. Electron cyclotron harmonic waves similar to the emissions observed in the ionospheric experiment are excited after the complete termination of a pumping microwave pulse when the frequency of the pumping wave is equal to harmonics of the electron cyclotron frequency and is near the plasma frequency. The excitation of the wave is explained by an anisotropic velocity distribution of hot electrons produced by the high power microwave pulse. The experimental results imply that some of the VLF emissions observed in the magnetosphere are excited by the electrostatic instabilities.
Diffraction by a flanged parallel‐plate waveguide and a slit in a thick screen
Authors: Henke H. , Früchting H. , Winz R.
Published: February 2008

The diffraction of an H‐polarized plane wave is investigated when the wave is obliquely incident on a flanged parallel‐plate waveguide or on a slit in a thick screen. Using discrete spectra of periodic functions inside the waveguide and of Mathieu functions in a half‐space, the solution can be obtained by matching the different field representations in the aperture plane. The technique has the advantage that it operates in a wide frequency range, ka > 0 through ka ≈ 20, and yields rapid convergent results of excellent accuracy (k is the free‐space wavenumber, 2a is the aperture width). The calculated fields satisfy the edge condition. Investigations into a proper ratio between series truncation numbers to avoid relative convergence appear to be unnecessary. A number of numerical results are presented. The transmission coefficient is critically dependent on the cutoff frequencies of the waveguide modes and on the thickness of the screen, which causes strong resonances, whereas the near‐ and far‐field patterns are influenced by the thickness, especially for oblique incidence.
Remote sensing of atmospheric water vapor, liquid water, and wind speed at the ocean surface by passive microwave techniques from the Nimbus 5 satellite
Authors: Chang A. T. C. , Wilheit T. T.
Published: February 2008

The microwave brightness temperature measurements for Nimbus 5 electrically scanned microwave radiometer (ESMR) and Nimbus‐E microwave spectrometer (NEMS) are used to retrieve the atmospheric water vapor, liquid water, and wind speed by a quasi‐statistical retrieval technique. It is shown that the brightness temperature can be utilized to yield these parameters under various weather conditions. Observations at 19.35, 22.235, and 31.4 GHz were input to the regression equations. The retrieved values of these parameters for portions of two Nimbus 5 orbits are presented. Then comparison between the retrieved parameters and the available observations on the total water vapor content and the surface wind speed are made. The estimated errors for retrieval are approximately ±0.15 g/cm2 for water vapor content, ±6.5 mg/cm2 for liquid water content, and ±6.6 m/s for surface wind speed.