bluuebunny/crossref_metadata_2025 huggingface dataset:
LF radio noise from the Earth's magnetosphere
Authors: Frankel Michael S.
Published: February 2008

Gyro‐synchrotron radio noise emitted by electrons trapped in the Earth's magnetosphere has been a subject of extensive research. Previous efforts, which considered frequencies greater than 1 MHz, have shown that this noise should not be detectable in the MF to HF range because its intensity is below the cosmic background noise level. This author, however, has investigated the LF range and has found that appreciable noise is generated at these frequencies. In fact, the theoretical results for this LF noise agree very well with experimental data obtained by the Goddard Space Flight Center radio astronomy experiment aboard the IMP 6 spacecraft. A comparison showed that the model predicted both variation in the observed noise intensity with Kp and the noise spectral characteristics. Consequently, it is concluded that detectable LF radio noise is emitted, by means of the cyclotron‐synchrotron mechanism, by electrons trapped in the Earth's magnetosphere, and that this noise is observable only for frequencies below about 300 kHz. For higher frequencies, the theoretical model and the experimental data reconfirm that this noise is below that of cosmic origin.
Surface waves on symmetrical, inhomogeneous dielectric layers
Authors: Casey K. F.
Published: February 2008

A method is proposed by means of which the propagation constants and field components may be obtained for the surface waves supported by symmetrically inhomogeneous dielectric layers. The specific profile shape of the permittivity across the layer is nearly arbitrary. The characteristic equations from which the propagation constants for the even and odd TE and TM modes are obtained are expressed directly in terms of the Fourier expansion coefficients of the permittivity variation, and the field components are expressed in terms of solutions to the Hill equation. Numerical results are presented to illustrate the analysis for a specific family of dielectric profiles.
High resolution D‐region measurements at Arecibo
Authors: Ioannidis G. , Farley D. T.
Published: February 2008

D‐region electron densities have been measured with a height resolution of less than 1 km using the incoherent scatter technique with coded pulses. Using a theoretical production model and the measured densities, recombination coefficients were computed for the altitude range from 80 to 95 km. The effective recombination coefficient decreases across the D‐region ledge from a value of about 2 × 10−5 cm3 sec−1 below the ledge to a value of the order of 1 to 5 × 10−7 above. The change is presumably due to a transition from water cluster ions to the molecular ions NO+ and O2+.
Seasonal variations in the thermosphere above Millstone Hill
Authors: Salah J. E. , Evans J. V. , Wand R. H.
Published: February 2008

Incoherent scatter radar measurements gathered since 1969 at Millstone Hill (42.6°N, 71.5°W) are used to study the seasonal variations of the neutral gas temperature, composition and wind velocity in the thermosphere. Between 110 and 120 km, the mean temperature is found to oscillate with a 20 K annual amplitude, 10 K semiannual amplitude, and maxima in July; the mean density oscillates with an annual amplitude of 13% of the mean and a semiannual amplitude of 6% of the mean with maxima in November. Similarly, the mean daily exospheric temperature possesses a 100 K amplitude variation with a maximum in summer; the semiannual component is only about 10 K. Moreover, a weak winter maximum characterizes the neutral density at 400 km, and the neutral wind at 300 km is found to blow northward for a longer period during the day in winter than in summer. The amplitude and phase of the daily semidiurnal tides in the lower thermosphere do not vary systematically with season but a large variability is observed from day to day, particularly in the phase of the oscillation during winter. In contrast, the semidiurnal harmonic of the exospheric temperature varies seasonally in amplitude and phase and reflects the controlling influence of the length of the day on the shape of the daily temperature oscillation. In the middle thermosphere, the neutral composition ratio O/N2 is larger in winter than summer, with a rapid transition at the equinox.
Tidal oscillations of the meridional neutral wind at midlatitudes
Authors: Amayenc P.
Published: February 2008

The meridional neutral wind values estimated over St. Santin (44°65′N, 2°19′E, geographic) from incoherent scatter observations, during the years 1971 and 1972, have been analyzed in terms of Fourier components in the whole thermospheric altitude range. Seasonal averages of the height structure of amplitude and phase for each harmonic term are presented and compared with relevant theoretical predictions. The high thermosphere results exhibit clear annual and semiannual effects and the steady meridional wind is found to depend on magnetic activity and solar flux. The discussion of the experimental results, in connection with predictions of the tidal theory, suggests future work in this field which is outlined in the conclusion.
Propagation integrals for estimation of ionospheric radar refraction
Authors: Fretwell L. J.
Published: February 2008

A systematic, thick refraction medium approach to calculating ionospheric radar refractive effects by means of propagation‐related integrals yields equations valid to second order in the tangent of the propagation angle with respect to the straight line between source and target. Significant differences are found from more intuitive developments currently used. When more than one propagation path exists between source and target, alternate propagation paths cannot be located reliably by the propagation integral method. However, inclusion of the higher‐order terms responsible for the alternate solutions does significantly improve the accuracy of the primary path solution when those terms can be estimated accurately in a strong refraction situation.
Satellite transmission studies of spread‐F produced by artificial heating of the ionosphere
Authors: Bowhill S. A.
Published: February 2008

Experiments are described to study the morphology of artificial spread‐F (ASF) irregularities by measuring the scintillation of VHF and UHF signals transmitted through the disturbed region from geostationary and orbital satellites. The ASF irregularities are found to be approximately field aligned with correlation distances of about 100 m transverse to the field direction; a smaller structure less than 10 m is also seen. The fluctuations seem to be distributed through the entire F layer, and are less than one percent of the ambient electron density at most times.
Correlation functions of a wave in a random distribution of stationary and moving scatterers
Authors: Ishimaru Akira
Published: February 2008

In this paper, integral and differential equations are derived for the correlation function of a wave in a random distribution of discrete scatterers. For stationary (nonmoving) scatterers, relatively simple integral and differential equations for the correlation functions are derived from Twersky's general formulations and their relationships with the radiative transfer theory are clarified. Small‐angle approximations of these differential equations are shown to be identical to the parabolic equations for the mutual coherence function in a turbulent medium. For moving scatterers, Twersky's formulation is modified to take into account the particle motion and the variation of the field in time. General integral and differential equations for spatial as well as temporal correlation function are obtained which include the effects of the constant velocity as well as the fluctuating velocity. Differential equations governing the temporal frequency spectra of the field as well as the specific intensity are derived showing the Doppler shift and the spectrum broadening.
Differential angle of arrival: Theory, evaluation, and measurement feasibility
Authors: Fried David L.
Published: February 2008

The problem of measuring differential angle of arrival through two relatively small apertures of variable separation is considered. The motivation is to evaluate the practicality of a measurement program using such a quantity to observe atmospheric turbulence wave‐front distortion effects when one end of the link is moving and cannot be tracked precisely. (If precise tracking were possible, then measurement of the angular resolution with a single aperture could be used to characterize the propagation path.) Theoretical results for the mean‐square difference in angle of arrival are developed and numerical results are presented. Comparison of expected magnitude of effects with available measurement instrument precision indicates that the experiment should be possible, but will depend on our ability to achieve an rms single‐axis angle‐of‐arrival measurement precision of the order of 0.1 arc sec or better.
Analysis of nighttime E‐region winds and ionization production
Authors: Geller M. A. , Smith L. G. , Voss H. D.
Published: February 2008

Two sets of rocket measurements of nighttime E‐region electron densities at Wallops Island are analyzed. One is a series of five electron density profiles that were taken during the night of February 22, 1968, and the other is a collection of electron density profiles that were taken near midnight under varying conditions of geomagnetic activity. The method of analysis uses the steady‐state continuity equation for electrons and gives information on the height‐averaged ionization production rate and the vertical profile of the ion drift. Conclusions drawn from this analysis include the following: The more marked intermediate layer observed under geomagnetically quiet conditions is most likely a straightforward consequence of lesser ionization rates. Small values of vertical ion drift (about 10 m sec−1) are sufficient to give rise to a marked intermediate layer. Electron production in the E region is minimum near midnight. The production rate below 130 km (probably HI Lβ) increases sharply between 0130 and 0300 EST whereas the production rate in the region above remains relatively constant. The ionization rate in the nighttime E region varies strongly with κp, suggesting the importance of energetic electrons even under moderate and quiet conditions. The intermediate layer may be a result of electron redistribution by the tidal winds of the (2,4) mode of the solar semidiurnal tide.
Refractive attenuation formulas for an unstable critical ray in a circularly symmetric medium
Authors: Lipa Belinda J. , Croft Thomas A.
Published: February 2008

Through application of the basic equation of geometrical optics, the eikonal equation, we have derived an expression for the refractive defocusing of a critical ray in a medium which has circular symmetry. The equation of the ray itself is simply a circle, since by definition its radius of curvature is exactly the same as its distance from the center of symmetry. This mathematical development has a practical significance in that it permits for the first time the calculation of the strength of a signal which travels in the atmosphere of a planet at a fixed critical height. Our specific interest in this matter was due to the probable existence of such a critical ray about 33 km high in the atmosphere of Venus; at this altitude the pressure and temperature are reasonably Earth‐like and, as a result, some advanced space exploration studies have considered telecommunication by means of the critical ray (J. E. Blamont, personal communication, 1974). Through the application of the result given here, the strength of such a propagation mode can be determined. The result also has potential applications in geocentric studies and in other media which possess either spherical or cylindrical symmetry.
Theory of topside sounder transmission effects on antenna quasistatic sheath impedance
Authors: Laframboise J. G. , Rubinstein J. , Palmer F. H.
Published: February 2008

A quasistatic theory has been constructed for predicting low frequency sheath impedance changes produced on spacecraft sounding antennas as a result of intense RF emissions by them. The theory involves calculation of time‐independent sheath profiles around an infinite cylindrical antenna in a collisionless plasma, using a numerical scheme which includes the time‐averaged repelling force on electrons due to a nonlinear effect of the RF near field of the antenna. These sheath profiles then yield values of static sheath capacitance and conductance per unit antenna length. The theory includes a more accurate electron current calculation than in the earlier treatments by the authors. Results are presented for a range of parameters applicable to spacecraft sounding conditions. In most cases, the quasistatic sheath capacitance and conductance both decrease as RF amplitude increases. At large enough RF amplitudes, the capacitance becomes almost independent of antenna potential and Debye ratio.
Basic mechanisms of tunnel propagation
Authors: Delogne Paul
Published: February 2008

The basic mechanisms of tunnel propagation and of underground radio systems are briefly reviewed. The TEM approximation of the monofilar mode is compared with available rigorous solutions. Below the tunnel cutoff frequencies, the leaky feeder techniques can be described by the coupled line methods or, with a simpler physical picture, by the use of two eigenmodes. The only valid explanation of the leaky feeder radiation at higher frequencies is the random diffraction of the feeder leakage fields by the wall irregularities. To avoid the inefficiency of this process, the insertion of leaky stubs in a well‐shielded coaxial cable is proposed.
Calculated transmission loss for a leaky feeder communication system in a circular tunnel
Authors: Hill David A. , Wait James R.
Published: February 2008

The transmission loss for a leaky coaxial cable communication system in a circular tunnel is calculated for an idealized model. The transmitting and receiving antennas are electric dipoles that may be located anywhere within the tunnel. For typical cable parameters, the optimum frequency is found to be in the range from 2 to 10 MHz. Calculations reveal that the use of a sparse braid and a high velocity cable will strongly improve the cable‐dipole coupling with only a small penalty of a higher attenuation rate.
Radiation from a finite sinusoidally modulated reactance surface (SMRS)
Authors: Cho S. H. , King R. J.
Published: February 2008

An integral equation is used to study numerically the surface field and radiation from a finite planar structure which has a sinusoidally modulated reactance. The surface reactance is assumed to be uniform near a magnetic line source and modulated sinusoidally in the propagation direction beyond the uniform segment. On the modulated section of the surface, the magnetic field is essentially that for an infinitely long SMRS, provided that most of the energy is radiated through the fast waves before they reach the termination. By taking account of the radiation through the fast waves only, approximate expressions for the radiation pattern are also obtained. The approximated pattern compares well with the radiation pattern found numerically which includes all of the extraneous radiation due to the source, the transition between the uniform and the modulated reactance sections, and the termination insofar as the main beam and the largest sidelobe are concerned. The effects of the parameters such as the modulation amplitude, period, length of modulated section, and the average surface reactance are discussed.
The use of dispersion to eliminate multipath in planetary radio occultations
Authors: Croft Thomas A.
Published: February 2008

During planetary atmospheric and ionospheric study through radio occultation of spacecraft, multipath propagation often occurs at several altitudes in the medium. Although the signals are then simultaneous in time, they are separate in frequency. It is shown that dispersion can be used to separate such signals because they are inherently single‐valued with respect to a family of curves having dT/dF = λD/V2 (where F = frequency, T = time, V = velocity normal to the limb, λ = wavelength, and D = distance from the spacecraft to the limb plane). The concept underlying this corrective measure is much like that of an FM‐CW radar wherein frequency separation is converted to time separation. Application of the recommended dispersion may eliminate a major form of multipath and permit signal detection by receivers employing phase‐lock loops that depend for their action on the existence of only one frequency. This should facilitate the processing of open‐loop recorded data and it might permit the closed‐loop reception of multipath signals without loss of information, making possible the observation of radio occultations with signals from Earth to spacecraft. Heretofore, this approach has not shown promise in multipath situations. No specific processes or designs are given here, but there is a practical discussion concerning the degree of accuracy that must be provided in flight and during postflight analysis.
ULF/ELF magnetic fields generated at the sea surface by submerged magnetic dipoles
Authors: Fraser‐Smith A. C. , Bubenik D. M.
Published: February 2008

The complete expressions for the electromagnetic fields generated by buried harmonic dipole sources involve complex integrals that cannot ordinarily be solved in closed form. We present numerical solutions for the quasi‐static magnetic fields at the sea surface due to magnetic dipoles submerged from 25 m to 200 m in an infinitely deep sea, for frequencies in the interval 0.1 Hz to 100 Hz. We also present the ratios of the calculated fields to the fields which would be observed if the source were in unbounded free space. It is shown that the presence of the conducting sea water may enhance the field amplitudes in some cases. The results support the commonly proposed “up and over” propagation mechanism for the distant fields from a submerged dipole source.
ELF atmospheric noise excision by wideband clipping
Authors: Davis John R. , Meyers William D.
Published: February 2008

Because of the low transmitted power levels that are feasible in the extremely low frequency (ELF) band, it is essential to exploit such noise excision methods as are available to maximize communications signal‐to‐noise ratio (snr). A method that employs wideband clipping on ELF noise to match it to a linear matched filter has been evaluated with the use of noise data from northern latitude receiving sites. The analysis of month‐long samples of noise data from all seasons and nearly all times of day indicates that an improvement in snr of at least 10 dB can be realized nearly all the time with the use of one or two parallel, manually adjusted clippers whose clipping levels are set on the basis of seasonal mean noise level.
A partial reflection experiment using the FM‐CW technique
Authors: Rinnert K. , Schlegel K. , Kramm R.
Published: February 2008

The FM‐CW technique is used to perform a partial reflection experiment. This method seems to be the only possibility to overcome the enormous noise level in the frequency range 2–6 MHz in middle Europe. A frequency near 4 MHz is swept by 100 kHz in 1 second and a transmitter power of 50 W is used. The technique, the operation and the data reduction are described. The possibility of measuring the electron density and the collision frequency profiles simultaneously is discussed. Some examples of deduced collision frequency profiles and electron density distributions under normal and anomalous conditions are presented.
Case studies of beyond‐the‐horizon propagation in tropospheric ducting environments
Authors: Pappert R. A. , Goodhart C. L.
Published: February 2008

Several case studies of propagation in offshore San Diego tropospheric ducting environments are examined via waveguide formalism. Both ground‐based and earth‐detached ducts produced by elevated layers at about 600 and 2000 ft respectively are considered. The model assumes a trilinear refractivity profile and the layer to be stationary and horizontally homogeneous. The ground‐based duct study includes cases ranging from single‐mode propagation (65 MHz) to cases where more than a hundred modes have been evaluated (3300 MHz). The earth‐detached duct studies are for the single frequency of 3087.7 MHz with the transmitter at 68 ft. Approximate agreement has been found between measurements and calculations in the case of the ground‐based duct. Calculations for earth‐detached ducts dramatically overestimate the observed path losses for beyond the horizon fields. This discrepancy is probably due to layer fluctuations and/or inhomogeneity, along with the possible existence of an unaccounted for, persistent, ground‐based duct produced by a low level layer. The evaporation duct for which duct heights are generally less than 100 ft would be such a possibility.
Determining a subsurface electomagnetic profile from high‐frequency measurements by applying reconstruction‐technique algorithms
Authors: Lager Darrel L. , Jeffrey Lytle R.
Published: February 2008

We have applied the data‐inversion algorithms developed by investigators in the field of tomography to the problem of determining the structure of the ground between a pair of drill holes. The three most successful algorithms to date have been modified versions of the tomographer's ART (algebraic reconstruction technique), SIRT (simultaneous iterative reconstruction technique), and BPT (back projection technique). Projections were made by probing the ground with high‐frequency radio waves and recording the magnitude and phase of the received signal for various positions of transmit and receive antennas. The region between the holes was divided into rectangular zones (like a checkerboard). Ray‐optic approximations were made so that the equations describing the propagation between transmitter and receiver reduced to a linear system. The reconstruction technique algorithms were used to solve the system iteratively and assign appropriate values of skin depth to each zone. Simulations of a geological situation of interest were done to assess the sensitivity of the algorithms to noisy data. Then, measurements were taken in a coal field and the data were reduced using the ray‐optic assumptions and solution by the various algorithms.
On the energy flux and Poynting vector in a moving dispersive dielectric
Authors: Ko H. C. , Chuang C. W.
Published: February 2008

It is shown that in a moving, lossless, dispersive dielectric, the group velocity is distinct from the energy velocity defined as the ratio of the Poynting vector and the energy density. The total energy flux is equal to the product of the total energy density and the group velocity. It consists of the electromagnetic flux represented by the Poynting vector and the particle energy flux associated with the motion of particles in the medium. Only the group velocity is shown to obey Einstein's velocity addition theorem rigorously for a moving dispersive dielectric. When a dielectric which is only frequency dispersive in its own rest frame is set in motion, it is shown that the dielectric displays properties characteristic of spatial dispersion.
Lateral shifts of totally reflected gaussian beams
Authors: Shin S. Y. , Felsen L. B.
Published: February 2008

At an interface with incidence‐angle‐dependent reflection coefficient, a highly collimated totally reflected beam experiences a displacement of the beam axis and the phase center from the location predicted by geometrical optics. Studies in the literature of the lateral (longitudinal and transverse) beam shift have produced conflicting results. Here, the shift phenomenon for gaussian beams is analyzed by a rigorous method whereby the high‐frequency point source or line source Green's function solution as expressed in ray‐optical terms is converted into the beam solution by assigning a complex value to the source coordinates. Results obtained for two‐dimensional and three‐dimensional electromagnetic fields at plane isotropic and anisotropic interfaces are compared critically with those found elsewhere. Also included are the lateral shifts at a curved cylindrical boundary, which have not been studied heretofore. These planar and curved geometries are relevant, respectively, to beam coupling into slab and fiber waveguides. Our expressions for the beam shifts are consistent with those required for correct conversion of multiply‐reflected rays or beams in slabs and fibers into guided modes, thereby lending further support to the arguments presented.
On the matching of two optical waveguides
Authors: Rulf Benjamin
Published: February 2008

The problem of matching end to end of two semi‐infinite, single or multimode optical dielectric waveguides is formulated. When the waveguides' properties are different, or their axes do not coincide, reflection and radiation occur from the point of discontinuity, resulting in transmission losses. The reflection and transmission matrices and the radiation coefficients are found to satisfy certain singular integral equations. Approximate (perturbation) solutions are obtained for cases of small discontinuities in the waveguides' properties or axial alignment.
On the validity of the thin wire approximation in analysis of wave propagation along a wire over a ground
Authors: Pogorzelski R. J. , Chang D. C.
Published: February 2008

An analysis of the propagation of electromagnetic waves along a perfectly conducting wire parallel to a plane interface between dissimilar materials is presented. Particular emphasis is placed on determination of the significance of the first‐order Fourier components of current on the wire compared with the zero‐order current which has no azimuthal dependence around the wire.
A comparison of nighttime propagation anomalies observed at two very low frequencies
Authors: Steele F. K. , Diede A. H.
Published: February 2008

Recent nighttime phase and amplitude anomalies have been observed on the 10.2 and 13.6 kHz signals from Trinidad, West Indies, as received at Boulder, Colorado. The anomalies, in some cases, are comparable in size to the normal diurnal variations of these signals and occur predominantly at 13.6 kHz. These observations, when taken with independent confirming observations, suggest localized changes in the ionosphere or in VLF mode structure that may be significant to radio navigation.
Descriptive summary: Radio‐frequency radiation dosimetry handbook
Authors: Johnson C. C. , Durney C H. , Barber P. W. , Massoudi H. , Allen S. J. , Mitchell J. C.
Published: February 2008

Considerable effort has been expended recently in biological experimentation and theoretical analyses of the effects of radio‐frequency electromagnetic radiation on experimental animals and human beings. An important part of this work is dosimetry, the determination of the quantities and time rates of energy absorbed by a biological body. To provide the link between biological effects that are observed in irradiated animals and corresponding effects that might occur in man, theoretical methods are needed to relate measures of absorbed energy in man and animals to the power density of incident radiation. A handbook of data has been compiled to fill this need and is described herein.
Microwave radiation alters peristaltic activity of isolated segments of rat gut
Authors: McArthur Gregory R. , Lords James L. , Durney Carl H.
Published: February 2008

Irradiation (960‐MHz) of isolated post‐pyloric segments of rat gut at a specific absorption rate (SAR) of 1.5–5.5 W/kg caused an increase in peristaltic activity. A pressure transducer was used to monitor waveforms of muscle movements in a canulated section of intestine. The preparation was suspended in isothermal, aerated, modified Ringer's solution. Microwave radiation was applied with a parallel‐plate capacitor‐type radiator. The SAR was calculated from temperature measurements made with a liquid‐crystal optical‐fiber (LCOF) temperature probe. As reported previously, similar radiation applied to isolated turtle and rat heart caused bradycardia. These effects, in both the heart and gut, are thought to be neurally mediated by the release of transmitter substances by the autonomic nervous system (ANS). Generally, these effects can be modified by the addition of drugs that are effective at synapses within the ANS. In both the heart and gut, sympathetic and parasympathetic effects can be separated by the application of drugs that selectively block one or the other of the two divisions of the ANS.
Detection of breast cancer by microwave radiometry
Authors: Barrett A. H. , Myers P. C. , Sadowsky N. L.
Published: February 2008

A microwave radiometer at 3.3 GHz was used to map thermal radiation from the human body. As is true of infrared thermography, regions of high or low brightness may be associated with pathological conditions, such as malignant tumors or vascular blocks. Microwave radiometry has coarser spatial resolution than infrared thermography (∼cm2 rather than ∼mm2 ), but greater sensitivity to deep tissue temperature (∼cm rather than ∼0.1 mm) because of the relative transparency of tissues at microwave frequencies. Temperature resolution is comparable (0.1 °C). Clinical application to detection of breast cancer was attempted with a dielectric‐filled waveguide antenna and a point‐by‐point mapping technique with over 2000 patients, including 26 with cancer. True‐positive and true‐negative rates of detection in excess of 70% were found. These rates are comparable with that obtained by infrared thermography but are inferior to xero‐mammographic rates of detection for the same set of patients. Combining the microwave and infrared thermographic data, the true‐positive detection rate becomes 96% based on 26 cases.
A formula for computing refraction effects on range measurements
Authors: Morduch Georg E.
Published: February 2008

It is shown that models of the troposphere based on a constant temperature lapse rate lead to simple formulas for the computation of corrections to satellite range measurements. This is true for any lapse rate.
Oblique‐incidence sporadic‐E propagation and its ionospheric attenuation
Authors: Miya Kenichi , Shimizu Kohei , Kojima Toshiro
Published: February 2008

This paper describes an improved method for the calculation of sporadic‐E signal strength which was mainly developed through the work of the Interim Working Party 6/8 of the CCIR. In this method, the effect of atmospheric radio refraction on the sporadic‐E propagation path and of antenna height gain are given major consideration. Empirical formulas of sporadic‐E attenuation are also presented for use in computer calculation. Finally, some examples are shown for comparing the values predicted by using this method with measured ones. This study is believed to be applicable to the lower VHF band and upper HF band (e.g., 10 to 150 MHz).
Simulation of radiation from lightning return strokes: The effects of tortuosity
Authors: LeVine D. M. , Meneghini R.
Published: February 2008

A Monte Carlo simulation has been developed for the electromagnetic fields radiated from a tortuous lightning channel. This was done using a piecewise linear model for the channel and employing for each element the field radiated by a traveling wave on an arbitrarily oriented filament over a conducting plane. The simulation reproduces experimental data reasonably well and has been used to study the effects of tortuosity on the fields radiated by return strokes. Tortuosity can significantly modify the radiated waveform, tending to render it less representative of the current pulse and more nearly unipolar than one would expect based on the theory for a long straight channel. In the frequency domain the effect of tortuosity is an increase in high frequency energy as compared with an equivalent straight channel. The extent of this increase depends on the mean length of the elements comprising the channel and can be significant.
Approximate method for determining ELF eigenvalues in the earth‐ionosphere waveguide
Authors: Greifinger Carl , Greifinger Phyllis
Published: February 2008

A simple approximate expression is derived for the TEM eigenvalue for ELF propagation in the earth‐ionosphere wave‐guide under conditions where anisotropy due to the earth's magnetic field may be neglected. Eigenvalues obtained in this manner are in excellent agreement with full‐wave test calculations. It is shown that the eigenvalue is determined primarily by the location of two altitudes at which the conductivity attains certain values, and on the local conductivity scale heights at those altitudes.
Excitation of the Zenneck surface wave by a vertical aperture
Authors: Hill David A. , Wait James R.
Published: February 2008

Excitation of the Zenneck wave is examined for both finite and infinit vertical apertures over a homogeneous conducting flat ground. The infinit vertical aperture with a Zenneck wave variation is found to excite only the Zenneck wave with no radiation field. The finite vertical aperture excites a field which is similar to the Zenneck wave near the aperture but resembles the usual ground wave at large distances. Numerical results for various aperture heights are given for frequencies of 1 and 10 MHz. Both homogeneous ground and sea water paths are considered.
Spot size of the radar return from a radar‐acoustic sounding system (RASS) due to atmospheric refractive turbulence
Authors: Clifford S. F. , Wang Ting‐i , Priestley J. T.
Published: February 2008

We have calculated the intensity profile on the receiving plane of a radar‐acoustic sounding system (RASS) in the presence of varying amounts of atmospheric refractive turbulence. The results indicate that the refractive turbulence produces corrugations in the initially spherical acoustic wavefronts and these corrugations scatter electromagnetic energy over a large solid angle in the backward hemisphere. The net effect is to leave the scattered beamshape nearly unchanged from its form in the case of a homogeneous atmosphere. Of course the on‐axis intensity is reduced considerably but the energy is scattered into such a large solid angle that this produces only an increased background level or tails in the electromagnetic intensity profile.
Electromagnetic waves and antennas above and below the surface of the earth
Authors: King Ronold W. P.
Published: February 2008

The well‐known electromagnetic fields generated by infinitesimal dipoles in the presence of the earth are reviewed briefly with special reference to contributions by H. Bremmer. The fields due to currents in extended conductors can be expressed in terms of an integral over the occupied volume but its evaluation is possible only when the currents are known in their dependence on the properties of the earth. This is true of the horizontal‐wire antenna quite close to the earth both when driven by a localized emf and when acting as a scatterer in an incident field. It is also true of horizontal antennas in the earth not too close to the surface. Expressions for the most useful component of the radiated or scattered field are formulated for an end‐driven Beverage‐type antenna, for the horizontal wire as a scatterer in the presence of the earth, and for the embedded insulated traveling‐wave antenna.
The use of piecewise homogeneous ground models for computing sky‐wave mixed‐path loss and antenna elevation patterns
Authors: Arnbak J.
Published: February 2008

A vertical electrical dipole on a smooth multi‐section ground having piecewise constant electrical parameters is considered. This ground model corresponds to typical conductivity maps and lends itself to a systematic, non‐iterative solution of the Volterra integral equation for the vertical radiation pattern along the mixed path. The solution is constructed by summing individual wave contributions from each ground section, of the type identified by Bremmer 25 years ago.
Wire antennas coated with magnetically and electrically lossy material
Authors: Lee Jim P. Y. , Balmain Keith G.
Published: February 2008

The effect of coating a thin dipole antenna with a thin layer of magnetically and electrically lossy material is considered with respect to the properties of input impedance, current distribution, far‐field pattern, bandwidth, radiation efficiency, and effective length. Moment method calculations show that the change in the antenna properties is a strong function of the location and the kind of lossy material applied. The calculated input impedance of a monopole wrapped both fully and partially with different types of lossy materials is shown to agree well with measurements. An approximate semigraphical procedure for impedance calculation is postulated and shown to be useful.
Clock comparison by laser in the nanosecond range
Authors: Benavente J. , Besson J. , Parcelier P.
Published: February 2008

A first time transfer experiment using an infrared laser link and flyover was performed in November 1977 between the Paris and San Fernando (Spain) observatories. This experiment follows from a cooperative effort of several French and Spanish organizations (Besson, 1970; Benavente et al., 1977). Equipment to be used in the operation was developed independently by the participating organizations, and a brief description of the system is given, along with a presentation of the organization of the whole operation. From a study of the data obtained, one can appreciate the feasibility and capabilities of the method employed. The final result is the comparison of the UTC scales of the Paris and San Fernando observatories.
Excitation of the HF surface wave by vertical and horizontal antennas
Authors: Wait James R. , Hill David A.
Published: February 2008

Using a spherical earth model, we develop a normal mode analysis for the excitation by an azimuthally symmetric source region. We then specialize the general formulation to deal with a vertical antenna with a prescribed current distribution. In particular, we consider the current distribution that would excite a pure Zenneck wave on the corresponding planar earth model. But we also consider excitation by vertical electric dipole and a uniform current distribution. Finally, we examine a horizontal antenna that is excited by a traveling current wave. Our general conclusion is that the Zenneck wave aperture illumination (i.e., current distribution) is a poor choice for enhancing the ground wave over a spherical earth. This point is illustrated for specific calculations at a frequency of 1 and 10 MHz over both land and sea.
Distortion and depolarization by the ionosphere of L‐band signals coded by phase reversals: Full wave solutions
Authors: Bahar E. , Agrawal B. S.
Published: February 2008

The distortion and depolarization of L‐band transient signals that propagate through an inhomogeneous anisotropic model of the ionosphere are investigated using a full wave approach. The dispersive and anisotropic properties of the ionosphere impede the decoding at the receiver of the two orthogonally polarized signals which are independently coded at the transmitter. Thus in addition to determining the signal delays and the phase shifts of the transmitted signals, features that distinguish between the depolarized and nondepolarized signals are also identified.
Digitizing the conventional ionosonde
Authors: Downing A. M.
Published: February 2008

This paper discusses the accuracy limitations of the conventional ionosonde and techniques which may be used to improve it and at the same time reduce analysis effort. Add‐on digital equipment is described which may easily be attached to any ionosonde to improve its performance at a low cost. Results are presented which illustrate the accuracy and advantages of the system.
Measurements of strengths of electric and magnetic fields near industrial radio‐frequency heaters
Authors: Hietanen M. , Kalliomäki K. , Kalliomäki P.‐L. , Lindfors P.
Published: February 2008

There are several hundred radio‐frequency (RF) heaters in industrial use in Finland. No exposure or performance standards have been established for this kind of device in our country. This paper describes measurements of strength of electric and magnetic fields near 35 plastic‐welding machines, all operating at 27 MHz at nominal power outputs that range from 0.5 to 35 kW. Approximately 70% of the heaters radiated RF energy at strengths that exceeded common standards (200 V m−1, 0.5 A m−1). The Finnish proposed safety standard (60 V m−1, 0.2 A m−1) was exceeded in 80% of the cases. The results indicate that both electric and magnetic fields should be measured independently.
Effects of repeated exposure to 148‐MHz radio waves on growth and hematology of mice
Authors: Lin James C. , Nelson John C. , Ekstrom Merlin E.
Published: February 2008

The effects of repeated exposure of mice to RF energy (148 MHz) was investigated. Mice were exposed at 0.5 mW cm−2 (63.25 V m−1) in a TEM exposure chamber. They were irradiated for one hour a day, five days a week, beginning on the 4th to the 7th day postpartum, for ten weeks. The mice were divided into two equal groups to serve as sham‐irradiated controls and irradiated subjects. The animals were weighed daily from the beginning of irradiation treatments for ten weeks, and weekly thereafter. The results indicate that the formed elements in the blood of the mouse are not affected by exposure to low‐level VHF fields. The means of body mass of the irradiated and control animals were comparable, which indicates that the animals remained in comparably good health during irradiation.
Treatment of experimental wounds in rabbits by radio‐frequency electromagnetic radiation
Authors: Photiades D. P. , Osamo N. O.
Published: February 2008

Twenty‐four adult rabbits of a white inbred strain were exposed ten times to sham radiation (controls) or to pulsed 14.2‐MHz electromagnetic radiation in the near field (200 μs pulses, 120 pulses s−1) for a 20‐minute period daily after surgical induction of a sterile wound along the dorsum of the ear. Radiation‐induced increments of ear temperature near the wound averaged 1.6°C. Median healing time of wounds in 12 controls was nine days; in 12 irradiated rabbits, six days. The augmented rate of healing is highly significant (P < 0.005).
Scattering of VHF and UHF radar signals from the turbulent air
Authors: Liu C. H. , Yeh K. C.
Published: February 2008

In the derivation of Booker‐Gordon scattering cross sections in a turbulent atmosphere, certain assumptions have been made so that the power scattered within the scattering volume will add coherently. This results in a scattered power that is proportional to the effective scattering volume. As the scattering volume increases without bound, it leads to a physically unreasonable result of infinite scattered power. In this paper we examine these assumptions. In particular, we study the scattering cross sections, taking into account the possible phase incoherence of the scattered fields within the scattering volume. In addition, the effect of a thin turbulent layer on the complex autocorrelation function is investigated. The turbulence is assumed to be locally frozen. Expressions for both the Doppler shift and the correlation time have been derived.
Radio wave scattering from the tropical mesosphere observed with the Jicamarca radar
Authors: Fukao S. , Sato T. , Harper R. M. , Kato S.
Published: February 2008

Using the VHF radar (49.92 MHz) at Jicamarca (12.0°S, 76.9°W), radio wave scattering from the tropical mesosphere was observed for more than 60 hours on November 14–16, 1977. The 60‐ to 90‐km region was probed at 2.5‐km intervals in two antenna beam directions: the vertical and 3.45° from the zenith to the west. Strong aspect sensitive scattering that is accompanied by a marked positive correlation between the temporal variation of the echo power and the signal correlation time is observed below about 75 km as in previous measurements. Above that altitude, the correlation becomes negative while the scattering is virtually isotropic. This difference appears to be directly related to the stability of the atmosphere, and enhanced reflectivity regions seem to be narrow structures that are horizontally stratified below 75 km. The rms turbulent velocity fluctuations generally increase above 75 km. Signal correlation time depends upon the period of data used to calculate power spectra in the period of 10–60 s, suggesting that there exist substantial velocity fluctuations on short time scales.
Multiple‐scattering effect on radiometric determination of rain attenuation at millimeter wavelengths
Authors: Ishimaru Akira , Cheung Rudolf L.‐T.
Published: February 2008

The multiple‐scattering effects in rain are negligibly small in comparison with the absorption effects when raindrop sizes are much smaller than a wavelength. This is often true for light to moderate rain below about 30 GHz, and the total atmospheric attenuation along the path may be estimated by the emission method using the usual radiometric formula. At higher frequencies and heavy rain the multiple‐scattering effects cannot be ignored. This paper describes a theoretical study of the correction due to the multiple‐scattering effect for the determination of the total atmospheric attenuation by the emission method. The observed temperatures at the ground both for vertical and horizontal polarizations and for various raindrop temperatures, precipitation rates, ground temperatures, ground albedos, and frequencies ranging from 30 GHz to 120 GHz are calculated on the basis of the equation of transfer, taking into account the polarizations and the Stokes parameters. The cross sections of rain droplets are calculated using the Mie solution with the Laws and Parsons drop size distribution and Saxton's formula for refractive index of water. These calculations show the differences between the total rain attenuation and the attenuation calculated from the sky temperature measurements and the assumed rain temperatures. The results are compared to some experimental data, showing a good agreement.
An algorithm for profile reconstruction in the presence of a known background slab
Authors: Raz S.
Published: February 2008

An iterative inversion algorithm is given which facilitates the reconstruction of a plane‐stratified profile from scattering data in the presence of a background slab. The inversion procedure, formulated within the bounds of the so‐called, ‘distorted wave Born’ model, is given for both an incident plane wave and a point source excitation. While an exact inversion is achieved for an incident plane wave, the point source configuration is inverted asymptotically, thus excluding wave ‘trapping’ effects.
The side‐looking radar image of time‐variant scenes
Authors: Harger Robert O.
Published: February 2008

The image formed by a side‐looking radar system of a scene characterized by a temporally and spatially varying reflectivity density effectively distributed over a plane is given. The implied spectral density transformation, with its intrinsic distortion of the scene's wave number spectral density, is noted. In the remainder it is assumed that the temporal and spatial behaviors are connected via a dispersion relation. It is shown that the image is a linear, space‐variant transformation of the scene at, say, t = 0, which can be described as the concantenation of three linear transformations: the first two are space‐invariant, one being the system transformation for a time‐invariant scene, and the third is space‐variant and gives a geometric distortion, most simply a scaling and skewing, which is present in any system using a side‐looking scan. Sufficient conditions are given for faithful imaging and spectral density mapping; those for the latter are more easily met.