bluuebunny/crossref_metadata_2025 huggingface dataset:
On the irrotational component of the electric Green's dyadic
Authors: Johnson W. A. , Howard A. Q. , Dudley D. G.
Published: February 2008

It is shown that the dominant singularity of the electric Green's dyadic is proportional to the irrotational part of the unit delta dyadic. Contrary to statements in the literature, this term does not vanish identically away from the source. Therefore in the full three‐dimensional eigenf unction expansion of the Green's dyadic, one must account for the contribution due to the irrotational vector wave functions, even in a source free region. In the recent literature on this subject this point is in controversy. The apparent contradications are discussed and the differences resolved.
Ray theory of coupling between adjacent parallel plate wave guides
Authors: Driessen Peter F.
Published: February 2008

The coupling between two immediately adjacent semi‐infinite parallel plate wave guides is calculated by ray methods. A simple expression is derived for the coupling of the Nth mode in the driven guide into the nth mode in the parasitic guide. Numerical results are given for both TE and TM polarizations.
Crosstalk cancellation on linearly and circularly polarized communications satellite links
Authors: Overstreet W. P. , Bostian C. W.
Published: February 2008

Crosstalk caused by depolarization on a dual‐polarized communications satellite link can be reduced by using cancellation techniques. The phase of the cross‐polarized signal is crucially important to the design of a cancellation system, and if the phase exhibits only a small variation, a very simple cancellation network can be implemented.
Altair: an incoherent scatter radar for equatorial spread F studies
Authors: Tsunoda R. T. , Baron M. J. , Owen J. , Towle D. M.
Published: February 2008

We report preliminary results on 1‐m equatorial spread F irregularities obtained with a high‐power VHF backscatter radar located at equatorial latitudes. The field experiment was conducted at Kwajalein Atoll, Marshall Islands, during August 1977. The examples, which include both incoherent scatter measurements and backscatter from equatorial spread F irregularities, illustrate the usefulness of a fully steerable radar for equatorial spread F studies. The backscatter observations at 155.5 MHz are not consistent with the scattering model proposed by Woodman and Basu (1978), which includes a cutoff near the ion gyroradius. The results obtained thus far also indicate that the collisional Rayleigh‐Taylor instability is probably the principal mechanism producing equatorial spread F.
Near‐zone field‐strength meter for measurement of RF electric fields
Authors: Eggert S. , Goltz S. , Kupfer J.
Published: February 2008

The NFM‐1 near‐field‐strength meter developed in the German Democratic Republic utilizes two dipole probes designed for 60 kHz to 350 MHz. The measuring ranges of the probes overlap each other from 10 MHz to 30 MHz. The meter operates over the temperature range −10°C to +40°C. Field strength meters of 2 V m−1 to 2500 V m−1 sensitivity with a measuring error of ⩽ 20% must be calibrated by special equipment designed for the purpose. A calibrating stand based on the principle of a parallelplate capacitor is used in the frequency range from 60 kHz to 18 MHz, and a triplate‐stripline (TEM‐cell) is used in the frequency range from 10 MHz to 350 MHz, for calibrating the probes. Large‐scale measurements were made in 1976 by which the NFM‐1 meter was compared with the corresponding near‐field‐strength meters used in the USSR, Czechoslovakia, and Poland.
Part‐body and multibody effects on absorption of radio‐frequency electromagnetic energy
Authors: Gandhi O. P. , Hagmann M. J. , D'Andrea J. A.
Published: February 2008

Fine structure in the whole‐body resonant curve for radio‐frequency energy deposition in man can be attributed to part‐body resonances. As for head resonance, which occurs near 350 MHz in man, the absorptive cross section is nearly three times the physical cross section of the head. The arm has a prominent resonance at 150 MHz. Numerical solutions, antenna, theory, and experimental results on animals have shown that whole‐body energy deposition may be increased by 50 percent or more because of multiple bodies that are strategically located in the field. Empirical equations for SARs are also presented along with test data for several species of laboratory animals. Barbiturate anesthesia is sufficiently disruptive of thermoregulation that ΔTs of colonie temperature yield energy dose values in several mammals that compare quite favorably with those based on whole‐body calorimetry.
Numerical calculation of electromagnetic energy deposition in models of man with grounding and reflector effects
Authors: Hagmann M. J. , Gandhi O. P.
Published: February 2008

Image theory has been used to obtain moment‐method solutions of the deposition of electromagnetic energy in standard man as a function of grounding and reflector effects. The calculated values are in good agreement with experimental data. For the EIĽ orientation, the resonant frequency of standard man shifts from 77 MHz in free space to 47 MHz when standing on a ground plane. The dependence of reflector effects on spacing and frequency are in agreement with the gain enhancement calculated for dipoles by antenna theory.
Heating of spherical versus realistic models of human and infrahuman heads by electromagnetic waves
Authors: Rukspollmuang Sutus , Chen Kun‐Mu
Published: February 2008

A numerical method based on a tensor integral equation has been employed to quantify the induced EM heating in realistic models of the human and infrahuman head. The head consists of a brain of realistic shape and eyes that are surrounded by a bony structure. The numerical method has also been used to determine the induced EM heating in spherical models of human and infrahuman heads and brains. The EM heating induced in the brain of the realistic model is lower than that induced in the brain of the spherical model. The bony stucture of the skull tends to attenuate heating of the brain, including the eyes.
The nonmetallic thermocouple: A differential‐temperature probe for use in microwave fields
Authors: Olsen Richard G. , Molina Efrain A.
Published: February 2008

A simple temperature probe is described that is suitable for use in microwave dosimetry. The sensor's tip is a thermocouple junction in which dissimilar nonmetallic conductors form the sensing element. The conductors themselves, although not entirely lossless, are readily available, carbon‐loaded materials such as conductive silicone or conductive fluorocarbon. Secondary thermocouple junctions are formed where metallic leads attach to the nonmetallic conductors and the voltage output from the device is, for certain temperature ranges, proportional to the temperature difference between the primary and secondary junctions. An instrumentation amplifier is described that boosts the output of the probe and eliminates much 60‐Hz interference. Tracking accuracy of the nonmetallic thermocouple (NMT) with the secondary junctions thermally insulated is within 0.1°C, and comparative dosimetric data are given to show that the NMT produces results that compare favorably with those obtained by other, much more expensive, probes. Use of the NMT in the laboratory environment requires the probe to be stationary to avoid noise pickup, a common problem associated with devices using high resistance leads.
Microwave‐induced auditory responses in guinea pigs: Relationship of threshold and microwave‐pulse duration
Authors: Chou Chung‐Kwang , Guy Arthur W.
Published: February 2008

Auditory brainstem‐evoked electrical responses of guinea pigs were used to determine microwave energy thresholds for perception of pulsed radiations. A klystron was used to generate 918‐MHz microwaves at pulse widths between 10 and 500 μs; the microwaves were fed into a circular waveguide that partly enclosed a guinea pig's head. At pulse widths less than 30 μs, thresholds were related to the density of absorbed energy (and to the density of incident energy). The minimal absorbed energy density per pulse is 5 mJ kg−1. As the width of the pulse increased, the threshold energy increased. For pulses longer than 70 μs, thresholds were related to the peak of the incident power density. The maximal power density of incident radiation is 90 mW cm−2 in the circular waveguide. The dependence of the evoked response on the width of the microwave pulse is in excellent agreement with predictions of thermal‐expansion theory. These results provide more evidence that the microwave auditory effect is caused by a thermal expansion in the exposed head.
Skilled visual‐motor performance by monkeys in a 1.2‐GHz microwave field
Authors: Scholl Dennis M. , Allen Stewart J.
Published: February 2008

Three young adult rhesus monkeys (Macaco mulatto) were exposed to radio‐frequency electromagnetic radiation (RFR) at average power densities of 10 and 20 mW cm−2, which resulted in respective SARs of 0.8 and 1.6 W Kg−1. Exposure sessions took place at two‐day intervals until completion of three 120‐minute exposures at each power level; 1.2‐GHz continuous‐wave (CW) irradiation was centered on a subject's head with the E‐field vector parallel to the horizontal plane of the cranium. The monkeys performed a compensatory visual‐tracking task with alternating periods of work and rest of 1.5‐minute duration before and during exposure to radiation. During a work period, the task required continuous visual vigilance and skilled motor performance. The animals were motivated by mild electrical stimulation to maintain a circular cursor within the center 15% of an oscilloscopic display. The cursor was driven off this safe on‐target area by a complex sinusoidal track that covered 40% of the display. Control data were used to establish 95% simultaneous confidence limits for the adjusted root mean square (ARMS) of tracking errors. The ARMS values from exposure sessions showed no decrement relative to control data. It is concluded that the exposures at 10 and 20 mW cm−2 did not have deleterious effects upon the skilled performance of a moderately large primate.
The response of pig skin to combined X‐irradiation and microwave heating
Authors: Hand J. W. , Field S. B. , Hopewell J. W. , Foster J. L.
Published: February 2008

A preclinical investigation has been made of the effects of microwave heating in enhancing the response of pig skin to X‐rays. Areas on the flanks of Large White English pigs were treated with X‐rays alone or with X‐rays followed by heating at 42°C for 1.5 hours. The treatment was given as a single dose or as three fractions over six days. Heating was produced with a microwave direct‐contact applicator operating at 2.45 GHz. The early skin reaction was scored quantitatively by an arbitrary numerical system. For all treatments, microwave heating was shown to have little effect in enhancing the response of pig skin to ionizing radiation.
Effects of microwave‐induced hyperthermia on the blood‐brain barrier of the rat
Authors: Sutton Carl H. , Carroll Frederick B.
Published: February 2008

In order to study the tolerance of the blood‐brain barrier to microwave irradiation at 2450 MHz, and to determine the upper limits of time and temperature for application of microwaves without excessive disruption of the barrier, an experimental model was developed. The rat's head was heated selectively in the near field by shielding the remainder of the body. Brain and body‐core temperatures were monitored via thermocouples. An enzymatic tracer protein, horseradish peroxidase, was administered intravenously (0.18 g kg−1) 30 minutes before an animal was euthanized. Biochemical quantitation of extravasated peroxidase in homogenized brains of animals, which had been perfused through the aorta with 5% PVP in saline at 4°C to purge intravascualr blood and tracer, was the index of barrier permeation. In initially normothermic (37°C) animals, barrier integrity was diminished after heating of brains for 10 minutes at 45°C, after 15 minutes at 42°C, and after 60 minutes at 40°C. In precooled (30°C) rats, mortality and barrier integrity were diminished after heating brains for 15 minutes at 45°C, after 30 minutes at 42°C, and after 180 minutes at 40°C. Microwave‐induced hyperthermia apparently increased the permeability of the blood‐brain barrier to protein, but less so when the lower body was cooled. The tolerance of the barrier to microwave irradiation is dose‐related and is dependent on the temperature to which the brain is heated and on the length of time during which that temperature is maintained. The barrier received significant protection from body‐core hypothermia, probably through cooling of endothelial cells by circulating blood.
Observations of stable layers in the troposphere and stratosphere using VHF radar
Authors: Green J. L. , Gage K. S.
Published: February 2008

Observations of the Sunset radar located near Boulder, Colorado, reveal order of magnitude enhancements of the signals received on the vertically directed antenna over the signals received nearly simultaneously on the antennas directed off the vertical. These enhancements are caused by Fresnel reflection and Fresnel scattering from thin stable layers. They are most pronounced from stable regions of the atmosphere in the troposphere and stratosphere and especially from the lower stratosphere just above the tropopause. The ability to detect stable layers from enhanced echoes on VHF radars is applied to the determination of the tropopause. It is concluded that the height of the tropopause can be determined within a few hundred meters.
Model computations of radio wave scintillation caused by equatorial ionospheric bubbles
Authors: Wernik A. W. , Liu C. H. , Yeh K. C.
Published: February 2008

In situ data measured on board AE satellites and rockets reveal spiky and wedgelike electron density structures inside the equatorial ionospheric bubbles. Two models are constructed to simulate the initial stage and fully developed stage of a bubble. Effects of radio propagation through such bubbles are simulated by solving the parabolic equation numerically. The results show that even though the amplitude scintillation at 136 MHz appears to be stationary, such is not the case at gigahertz frequencies. Instead, the amplitude at gigahertz frequencies shows outbursts with large excursions whenever the direct ray intersects the spicky ionization structure. Both the peak‐to‐peak excursion and the amplitude distribution cannot be predicted by the scintillation theory that assumes the medium to be random.
The effects of geophysical disturbance on the nighttime behavior of the electron content of the mid‐latitude plasmasphere, August–November 1974
Authors: Hargreaves J. K.
Published: February 2008

ATS‐6 radio beacon data obtained at Boulder, Colorado, between August and November 1974 are analyzed for effects related to the changing level of geophysical disturbance during four solar rotations. It is found that both the electron content at midnight and the decay pattern during the night vary strongly with the parameter ΣKp. Changes of ionization distribution may be inferred by using the effective longitudinal gyrofrequency, determined in the experiment, and the slab thickness. The consequence of increased activity is seen to be a preferred loss of protonospheric plasma. During the night, net upward shift in the distribution, owing to loss at the lower levels, is clearly evident. On magnetically quiet days a substantial electron content is present all night, presumably maintained by downward flow from a ‘reservoir’ in the distant protonosphere. Some comparisons are made with ionospheric models, but more sophisticated models will be needed to explain the observations in detail.
Ground wave attenuation function for a spherical earth with arbitrary surface impedance
Authors: Hill David A. , Wait James R.
Published: February 2008

Previous methods developed for computation of the attenuation function for a homogeneous spherical earth are generalized for an arbitrary surface impedance. The power series and small‐curvature formula developed for short distances can be used for such surfaces without modification. Care must be taken, however, to include properly the trapped surface wave that may arise in the formulation. In general, it is found that the inductive type of boundary enhances the total field but, if the resistive part is small, rather strong interference takes place between the trapped surface wave and the usual ground wave. These effects are illustrated for a surface‐based vertical electric dipole for frequencies from 1 to 30 MHz.
Cloud liquid measurement with a ground‐based microwave instrument
Authors: Snider J. B. , Burdick H. M. , Hogg D. C.
Published: February 2008

We describe the design of a new instrument developed to measure the integrated liquid content in clouds. The system simultaneously measures the absorption of a 28‐GHz coherent signal transmitted from the Comstar 3 synchronous satellite and the associated increase in brightness temperature radiated by the absorbing clouds by means of a bandwidth‐switched receiver. By combination of the two quantities the line integral of the cloud liquid is obtained. Examples of absorption events observed during the first six months of operation are presented.
Electromagnetic backscattering from a sparse distribution of lossy dielectric scatterers
Authors: Lang Roger H.
Published: February 2008

Electromagnetic backscattering from a sparse distribution of lossy dielectric particles having random orientation and position is studied. The paper begins by using the Foldy approximation to find an equation for the mean field. From this equation, an effective permittivity for the scattering medium is obtained. The correlation of the scattered field is found by employing the distorted Born approximation, i.e., particles embedded in the effective medium are assumed to be single scatterers. The above method is then used to find the backscattering coefficients from a leaf canopy. The leaf canopy is modeled by a half space of dielectric discs that are small in comparison to a wavelength. Numerical results show that the depolarized cross section is a sensitive function of leaf inclination angle statistics.
Resonances and surface waves on conducting spheres with a dielectric coating
Authors: Moser P. J. , Murphy J. D. , Nagl Anton , Überall H.
Published: February 2008

We present a theory of the resonances that appear in the scattering cross sections of electromagnetic waves from a perfectly conducting sphere coated with a homogeneous dielectric of uniform thickness. The individual normal mode scattering amplitudes are separated into an infinite series of resonance amplitudes interfering with a nonresonant background. The resonances are seen to fall into families that recur in successive partial wave amplitudes at successively higher frequencies and are shown to constitute the manifestation of circumferential (surface or creeping) waves that generate the resonances by matching their phases in the course of their repeated circumnavigations of the scattering object. Dispersion curves of these surface waves are obtained here from a knowledge of the resonance frequencies.
An empirical path loss model for HF/VHF propagation in urban areas
Authors: Malaga Alfonso
Published: February 2008

A number of models have been proposed in the literature to predict the propagation path loss in built‐up areas (urban and suburban) at VHF and UHF. However, none of these models is applicable at frequencies below 80 MHz. Other models proposed for application at frequencies between 20 and 10,000 MHz are only accurate when propagation is over rural irregular terrain. This paper describes the development of a path loss model applicable to frequencies between 3 and 450 MHz and propagation in urban and suburban areas. The model has been developed from path loss measurements at 3.195, 4.44, 16.08, 26.63, 49.8, and 446 MHz, where little reported data is available. In addition, previously reported VHF and UHF data have also been used to complement these measurements in the development of the model. The measurements indicate that even at frequencies as low as 3 MHz the electromagnetic field in an urban area will exhibit random spatial variations caused by multipath interference. Thus the received signal is best described in terms of mean and median signal levels. The model describes the frequency and range dependence of the median path loss and should be useful in predicting the performance of point‐to‐point and mobile HF/VHF communications links located in built‐up areas and employing low‐elevation antennas.
Effect of the propagation path on lightning‐induced transient fields
Authors: Gardner Robert L.
Published: February 2008

Propagation over the earth's surface of electromagnetic transient waveforms radiated by lightning return strokes is examined. Radio propagation theory is applied to the particular case of the return stroke current modeled as a series of connected straight line segments. A known current pulse propagates along the line segments as in the discharge of a precharged transmission line. The effects of an imperfectly conducting ground and an anisotropic ionosphere also are considered by using an idealized waveguide model. Signals above about 10 kHz show significant attenuation due to the imperfectly conducting earth, as have been observed previously. There is qualitative agreement between the model predictions and observed transient data, but lack of detail of the earth model limits the applicability of the results.
Overmodulation of an upper hybrid electromagnetic beam in a plasma
Authors: Sharma R. C. , Sharma R. R.
Published: February 2008

Using the fluid equations for the nonlinear response of electrons, we have studied the self distortion of an amplitude‐modulated upper hybrid laser beam and a Gaussian laser pulse in a collisionless magnetoplasma. The beam possesses a Gaussian distribution of intensity in the transverse plane and gives rise to a ponderomotive force on the electrons. The parallel component of ponderomotive force causes a redistribution of electron density, which gets modulated at the modulation frequency of the beam, the profile of modified electron density results in the selffocusing of the beam. The high‐ and low‐amplitude portions of the beam are focused to different extents, and a severe distortion in the amplitude envelope of the beam results. For a typical 10.6‐μ, CO2 laser of 108 W the change in the index of modulation becomes ∼100% after propagating a distance ∼0.4 cm. Similarly, the intensity envelope of a Gaussian laser pulse also gets severely distorted as it propagates in the plasma.
Clear‐air propagation on line‐of‐sight radio paths: A review
Authors: Stephansen Erik T.
Published: February 2008

Recent progress in theoretical and experimental work on clear air propagation on line‐of‐sight paths is reviewed. Descriptions of the physical mechanisms involved, illustrations of the variability in time and in space of the phenomena, and descriptions of the modeling of the propagation are addressed. Absorption by atmospheric gases is dealt with briefly. Refractive index structures are mentioned as background information. Theoretical and experimental studies of the physical modeling of multipath propagation conditions are dealt with in a detailed form. Various examples of multipath parameters modeling measured propagation conditions are presented. Aspects of statistical descriptions of multipath characteristics are also treated. Impacts on radio communications systems are mentioned. Attention is called to the difference between physical models of the propagation behavior and simplified models relating statistical characteristics. Duct theory and duct propagation characteristics are described. Examples of valuable statistical descriptions of radiometeorological data are mentioned. Other clear air effects are mentioned briefly. Theoretical and experimental progress in the description of low elevation angle propagation is also included. Finally, a number of areas for further work are pointed out. It should be noted that the aspects of cross polarization are not included, as they are dealt with elsewhere.
Scattering from hydrometeors: A survey
Authors: Oguchi Tomohiro
Published: February 2008

Investigation of the scattering properties of hydrometeors, especially those of nonspherical hydrometeors, has become increasingly important in connection with the estimation of depolarization due to hydrometeors in terrestrial and earth‐space microwave communication systems. Scattering from nonspherical hydrometeors is also important in remote sensing of precipitation parameters. This paper first reviews various analytical‐numerical approaches for the calculation of single‐scattering properties of nonspherical hydrometeors. Approaches for the analysis of the practical problems, such as depolarization phenomena and rain scatter interference, are outlined in order to see how the single‐scattering calculations are related to these problems. The present knowledge of incoherent scattering effects on rain attenuation, cross polarization, and channel transfer characteristics at the microwave and millimeter wave region is also reviewed.
A phenomenological theory for gratings: Perfect blazing for polarized light in nonzero deviation mounting
Authors: Maystre D. , Cadilhac M.
Published: February 2008

A rigorous phenomenological theory allows us to define the fundamental parameters of a perfectly conducting grating, at least in the case where only two diffracted orders propagate. We give some remarkable properties of these parameters. In particular, the behavior of the efficiency curves of sinusoidal gratings when the groove depth increases is explained from a topological point of view. Finally, we describe a new and surprising phenomenon: a perfect blazing can be obtained for polarized light in a nonzero deviation mount.
A new technique for the analysis of the dispersion characteristics of planar waveguides and its application to microstrips with tuning septums
Authors: Schulz Uwe , Pregla Reinhold
Published: February 2008

An efficient method for calculating the dispersion characteristics of planar waveguide structures is presented, of which the principle is known as the ‘method of lines’ in mathematical literature. The wave equation is discretized in only one direction, and the resulting differential‐difference equation can be solved analytically. As an example of application the phase constant and the characteristic impedance of microstrips with tuning septums are calculated.
Real Hamilton equations of geometric optics for media with moderate absorption
Authors: Suchy Kurt
Published: February 2008

For lossless media, Hamilton's equations of geometrical optics can be derived from the dispersion equation either by the method of characteristics or by its combination with Sommerfeld‐Runge's refraction law, Whitham's conservation law, and the expression ∂ω / ∂ k for the group velocity. The formal generalization to media with absorption leads to characteristics with complex space‐time coordinates due to the now complex coefficients of the dispersion equation. For media with moderate absorption a real‐valued generalization of Hamilton's equations is proposed. It is based on a dispersion equation with complex coefficients, on Sommerfeld‐Runge's and Whitham's laws for the real parts of k, ω, on Connor and Felsen's condition for the imaginary part of k, and on the expression Re (∂ ω/∂k) for the velocity of a wave packet. These real‐valued equations have been shown to hold in homogeneous media with moderate absorption.
Electromagnetic coupling between apertures and biological structures
Authors: Bozzetti M. , Corsi F. , De Leo R.
Published: February 2008

The coupling between a radiating aperture and a biological body standing in its near field region is analyzed, with particular reference to the evaluation of (1) em field distribution inside the biological body, including, in particular, the hot spots at the interface between two different biological media and (2) the effects of the body upon the aperture radiation characteristics (total radiated power and aperture transmission cross sections). The solution is obtained by applying the moment method technique to the aperture, once the em field distribution on the aperture has been approximated by a set of triangular functions and the biological body has been subdivided into homogeneous cubes. The numerical program has been written for use on a desk top computer and partitioned into subprograms in order to minimize the central memory requirements, by storing the partial results on a floppy disk. As an example the coupling between a λ/2 × λ/20 aperture excited by a plane wave and a cylindrical biological body is examined.
‘Peninsula impedance measurements’ in the Marlborough Sounds, New Zealand
Authors: Barr R.
Published: February 2008

Measurements have been made of the input impedance, at VLF, of a series of transmission lines connected across a peninsula in the Marlborough Sounds, New Zealand. No resonances in the input impedance have been found which cannot be explained in terms of basic earth‐return transmission line theory, the surrounding sea serving only to provide low impedance line terminations. In contrast to the assertions of Morgan (1980), the phase velocity of propagation along the earth‐return transmission lines has been found to decrease markedly as the cables are lowered to the earth's surface. It is concluded that the ‘peninsula’ and ‘island’ resonances observed near 10 kHz by Gould (1961) and Morgan (1979) were simply cable resonances, the phase velocity along the cables having been reduced by their proximity to the ground.
The current induced in a resistive half plane
Authors: Senior Thomas B. A.
Published: February 2008

For a resistive half plane illuminated by an E‐polarized plane wave at edge‐on incidence, an exact expression is available for the total induced current as a function of the electrical distance kx from the edge. The expression is cast in a form that is amenable to computation for any complex ‘resistivity.’ The results of such a computation are presented, and it is shown how the data can be used to predict the backscattered field of a strip.
Analysis of short‐term statistical variations in rain depolarization
Authors: Kanellopoulos J. D.
Published: February 2008

Observed electromagnetic wave depolarization in forward propagation due to precipitation in a short‐term period tends to distribute statistically about some mean level of depolarization. Using a special Rayleigh depolarization model, we have analyzed the statistical scatter of the cross‐polarization discrimination (XPD) of the received signal and found an approximate normal model with a standard deviation of approximately 4.34 dB about the mean, independent of all the parameters of the radio link.
Fresnel zone structure in planetary occultations
Authors: Croft Thomas A.
Published: February 2008

In many planetary and satellite occultations, both at light and at radio wavelengths, there may exist two propagation paths around an occulting body. The shorter of these two paths traverses the nearest portion of the limb, supports a stronger signal, and accounts for all radio occultation measurements to date. If the atmosphere of the occulting body is sufficiently deep and the path beyond the body is sufficiently long, a weaker signal will propagate via the far limb. Both paths satisfy Format's principle; the short path lies at a minimum of delay, and the long path is shown to be at a stationary point. Others have pointed out that the Fresnel zone for the near limb is a compressed circle much like an ellipse. I show here that the Fresnel zone at the far limb is shaped somewhat like the letter X. In the immediate vicinity of the zone, the phase path variation with respect to lateral and vertical displacement may be represented in the form of a hyperbolic paraboloid resembling a saddle. Farther away, at least in the lateral direction, the variation can be represented by a tilted torus for which the general formula is derived. By use of parameters representative of Venus and Jupiter, the formula is evaluated and the zones are drawn for circumstances of one special, limiting case. This peculiar X‐shaped zone structure may influence the prediction and interpretation of farside occultation signal strength and scintillation.
Asymptotic vector modes of inhomogeneous circular waveguides
Authors: Hashimoto Masahiro
Published: February 2008

An asymptotic theory is developed for vector modes of inhomogeneous circular waveguides, by means of which closed form expressions for the propagation characteristics of modes can be obtained. Basically, the theory is an extension of WKB's asymptotic theory based on scalar wave or geometric optical considerations to the coupled equations describing the basic properties of vector waves, and, therefore, the leading terms are the well‐known lowest‐order WKB solutions. The major part of the paper is offered on the theoretical foundations of the method. However, the formulas derived here are shown to be applicable for general classes of refractive index profile.
The horizontal circular loop antenna near a planar interface
Authors: An Lam N. , Smith Glenn S.
Published: February 2008

The horizontal circular loop antenna near a planar interface between two materials is analyzed theoretically for both the transmitting and the receiving cases. For the transmitting antenna the distribution of current, input admittance, and electric field in both media are obtained by applying a limiting process to the previously obtained results for the eccentrically insulated circular loop antenna. The receiving formulation is for an electromagnetic plane wave incident at an arbitrary angle from either side of the interface, i.e., incident from either the region that contains or the region that does not contain the loop. Parametric studies are used to investigate the effect of the interface on the input admittance of the loop and the directive properties for the electric field of the loop in both media. Measured input admittances and electric field patterns of model loop antennas above and below an interface between air and fresh water are in good agreement with the theoretical calculations.
Lateral waves: new formulas for E1ф and E1z
Authors: Wu T. T. , King R. W. P.
Published: February 2008

New simple formulas are derived as an approximation for the exact general integrals for the transverse and vertical components of the electric field generated by a horizontal electric dipole in a half‐space of water or earth near its boundary with air. These formulas supplement the earlier derivation of the more important radial component.
Radar waveform synthesis for exciting single‐mode backscatters from a sphere and application for target discrimination
Authors: Chen Kun‐Mu , Westmoreland Doug
Published: February 2008

A new scheme for radar detection and discrimination, the radar waveform synthesis method, is investigated for a spherical target. This scheme consists of synthesizing a waveform for the incident radar signal which excites the target in such a way that the return signal from the target consists of only a single natural mode of that target in the late‐time period. When the incident waveform synthesized to excite a particular natural mode of a known, preselected target is applied to a wrong target, the return signal will be significantly different from that of the expected natural mode; thus the wrong target can be sensitively discriminated. An approximate impulse response of a sphere is obtained by summing up an infinite number of natural modes of the sphere based on the singularity expansion method. Required incident signals for exciting various single‐mode backscatters from the sphere are obtained. Numerical examples are given to demonstrate target discrimination sensitivity based on this method.
Compensation techniques for rain depolarization in satellite communications
Authors: Yamada Matsuichi , Yuki Hironori , Inagaki Kazunori , Endo Shizuo , Matsunaka Naoto
Published: February 2008

Compensation techniques for rain depolarization in satellite links are discussed from the viewpoint of system configuration, performance, and technical feasibility. The compensator using two rotatable polarizers has sufficient performance at microwave region. Two types of compensators, as combination of 90° and 180° polarizers and a combination of two 90° polarizers, were developed and tested. Satisfactory results were obtained at 4‐GHz band by using the INTELSAT satellite. The compensation system for an operational use is also presented in this paper.
An improved model for earth‐space microwave attenuation distribution prediction
Authors: Dutton E. J. , Kobayashi H. K. , Dougherty H. T.
Published: February 2008

The Dutton‐Dougherty (DD) model for predicting microwave attenuation distribution on an annual basis on earth‐space telecommunications links was first developed at the Institute for Telecommunication Sciences in 1973. The version for making predictions throughout the United States, with year‐to‐year variability allowance, was developed in 1977 and had been essentially unmodified until the present improvement efforts were undertaken. The refinements of the Dutton‐Dougherty model discussed in this paper include an extension of the attenuation distribution prediction range to 0.001% of a year, whereas it had previously extended only to 0.01% of a year. This extension is accomplished in two alternative ways: empirically and analytically. Both extension procedures give nearly identical prediction results. An analysis is conducted of the pertinency of ‘effective path length’ models for evaluating rain attenuation at microwave frequencies to the DD model. It is determined that whereas effective path length methodologies may be useful in their own right, it would be difficult and unwieldy to incorporate this concept into the DD model, at present.
A two‐component rain model for the prediction of attenuation statistics
Authors: Crane Robert K.
Published: February 2008

A new model is presented for the calculation of the probability of exceeding specified attenuation values due to rain on single earth‐satellite or terrestrial propagation paths. The model was formulated to explore a prediction procedure that differs significantly from most of the accepted models for attenuation prediction but holds considerable promise for extension to modeling the joint statistics required for space diversity system design, the statistics of interference due to rain scatter at attenuating frequencies, and the duration statistics for attenuation events. The new procedure calculates the probability of occurrence of a convective (volume) cell or widespread debris which could cause the specified value of attenuation. The performance of the new model was evaluated by comparison with observed attenuation statistics and by comparison with the performance of the recently adopted International Radio Consultative Committee (CCIR) model and the earlier global model. The results of the comparison show that the new, two‐component model performs as well as the other models for prediction on earth‐satellite paths. It is the promise of the application of the new model to the unsolved problems of diversity improvement and interference prediction, however, that justifies the consideration of yet another model.
A random‐motion model of fluctuations in a nearly transparent medium
Authors: de Wolf D. A.
Published: February 2008

Nearly transparent media are characterized by propagation constants k2(r) = k20
(r)[1 + δε(r)] where
(r) is slowly varying and close to unity, whereas δε(r) ≪ 1. The statistics of wave amplitudes that are governed by those of δε itself is invoked; a spectral density Φ(K) follows from arguments about the form of the autocorrelation function of δε. In this work we provide a simple model of randomly moving circular eddies for δε. The connection between Φ(K) and the model is worked out, and the familiar example of Kolmogorov turbulence is analyzed in terms of the model parameters.
A test of the cosine relationship using three‐radar velocity measurements
Authors: Nielsen E. , Whitehead J. D. , Hedberg L. A. , Jones T. B.
Published: February 2008

Three coherent radars have been used to obtain simultaneous radial Doppler velocity measurements in three directions from a common volume in the E layer of the high‐latitude ionosphere. The cosine relationship, which was derived in the fluid approximation of the plasma instabilities associated with radar aurora, provides a consistent framework in which to interpret radial Doppler velocity observations. However, a systematic discrepancy between the observed radial velocities and the cosine relationship is found and discussed. For observations in the eastward electrojet this discrepancy is qualitatively in line with expectations based on the kinetic theory of the instabilities. It appears that the possible phase velocities in the plasma are limited to lower values than previously thought. This implies that an estimate of the electron drift velocity obtained using the fluid approximation at times may represent an underestimate of the actual electron drift velocity.
Heater‐generated intermediate‐scale irregularities: Spatial distribution and spectral characteristics
Authors: Livingston Robert C.
Published: February 2008

The first extensive phase scintillation measurements during both overdense and underdense heating at Platteville, Colorado, are described. The data were collected from an aircraft making repeated scans in the vicinity of the heated volume, to establish the overall spatial distribution of intermediate‐scale irregularities. During overdense heating, the irregularities maximize in strength near the HF reflection height, and map downward at least 100 km with only moderate weakening. From the relative frequency shifts of the phase spectra measured for different directions of aircraft motion, the irregularity anisotropy and drift have also been estimated. The change in phase spectral shape between 6.2‐MHz and 9.9‐MHz overdense heating is consistent with thermal self‐focusing irregularity generation. Underdense heating produces a different phase spectral signature than is theoretically predicted, although enhanced energy does occur at scintillation‐producing scale sizes.
Analysis and design of frequency scanned transmission gratings
Authors: Shmoys J. , Hessel A.
Published: February 2008

A transmission grating can be used as a frequency scanned antenna, provided most of the incident power is transferred to the diffracted mode, i.e., the grating is blazed. A design procedure for blazing a rectangular bar transmission grating is presented. The performance of a number of gratings is presented. Efficiencies in excess of 99% can be obtained at a single frequency. A scan range of 55° between 3‐dB points can be obtained.
Wave propagation on a strip grating over an impedance plane: Solution by reduction to a modified Riemann‐Hilbert problem
Authors: Barbosa A. M. , dos Santos A. F. , Figanier J.
Published: February 2008

The TM electromagnetic wave propagation along a strip grating above an impedance plane is studied. The problem is formulated in terms of a modified Riemann‐Hilbert problem. The modal equation of the structure is derived and solved numerically. Surface wave and leaky wave modes are identified. The phase velocity and attenuation constant of the dominant leaky wave modes are presented, and their dependence on frequency and geometrical parameters is discussed. The possibility of applying this structure to the design of a leaky‐wave antenna is suggested.
Analytic solution for the two‐frequency mutual coherence function for spherical wave propagation
Authors: Knepp Dennis L.
Published: February 2008

The quadratic approximation for the phase structure function is used to obtain the two‐frequency mutual coherence function Γ(Δρ, Δω) for spherical wave propagation through a finite slab with transmitter and receiver located in free space on opposite sides of the slab. General analytic solutions are derived for two cases. In the first case the random slab is represented by a one‐dimensional power spectrum of electron density fluctuations corresponding to propagation through elongated irregularities as would occur for an equatorial satellite link to a ground station. In the second case the random slab consists of isotropic ionization irregularities. Both cases taken together represent the extremes of the range of results to be expected for propagation through ionospheric fluctuations, solar wind irregularities, or ionization irregularities caused by barium cloud instabilities. For both cases the complex general analytic results are simplified by use of the thin phase screen approximation to obtain useful analytic expressions for Γ as well as the resulting impulse response function. It is shown that the impulse response to a transmitted power delta function reduces to an exponential form in the limit of strong diffraction and to a Gaussian form in the geometrical optics limit. The Gaussian form corresponds to pulse wander while the exponential form corresponds to diffractive spreading produced by multipath effects. The relationship between the generalized power spectrum and the impulse response function is given, and results are presented for the mean time delay and time delay jitter. The accuracy of the thin phase screen calculation of these quantities is investigated in detail.
Further observations on resonance cones in non‐Maxwellian plasmas
Authors: Thiemann Heinz , Singh Nagendra
Published: February 2008

Resonance cones in a hot magnetized non‐Maxwellian plasma are investigated. The non‐Maxwellian nature of the plasma is described by two electron populations: a background population and an electron beam with variable beam parameters. It is shown that the Cerenkov cone occurs at angles much smaller than the group velocity cone angle θc for the lower branch of the oblique resonance cone irrespective of the magnitude of the biasing magnetic field. When the beam velocity is sufficiently large with respect to the thermal velocity of the background electrons, a characteristic wave pattern appears outside the resonance cones. It is shown that this pattern occurs because of a propagating wave for θc ≲ θ ≲ 180°‐θc. When the beam density is low (nB ≈ 1%), the upstream resonance cone structure remains nearly unmodified, and it can be used for plasma diagnostics to deduce electron temperature and density using theoretical results based on a Maxwellian plasma.
Ionospheric cavitons and related nonlinear phenomena
Authors: Wong A. Y. , Santoru J. , Darrow C. , Wang L. , Roederer J. G.
Published: February 2008

Experimental evidence and theoretical results are presented for a physical model of ionospheric cavitons. The relationship of cavitons to other nonlinear phenomena and their role in ionospheric modification experiments are discussed. The recently completed HIPAS HF Transmitting Facility is described.
High‐latitude exospheric temperature observed over a solar cycle
Authors: Kelly J. D. , Heinselman C. J. , Petričeks J.
Published: February 2008

The exospheric temperature is an important parameter for neutral atmospheric models. This parameter varies diurnally, seasonally, and over longer periods because of solar‐cycle effects. Observations made with the Chatanika radar over the past 11 years have resulted in high‐latitude exospheric temperature data over these periods. These data are now available. This very large data base indicates that the high‐latitude exospheric temperature varies dirunally and seasonally as expected and by 500°K over this solar cycle.
Observations of thermospheric dynamics at high latitudes from ground and space
Authors: Meriwether John W.
Published: February 2008

Comprehensive experimental investigations of the phenomenology of the F region neutral wind and ion convection at high latitudes combined with improved theoretical modeling efforts have resulted in major advances in our current understanding of thermospheric dynamics of this region. We review the recent progress in this field.