Showing papers on "Radio wave published in 1981"

Modeling attenuation and phase of radio waves in air at frequencies below 1000 GHz

Abstract: Moist air is characterized for the frequency range 1–1000 GHz as a nonturbulent propagation medium described by meteorological parameters An adequate spectroscopic data base for air consists of three terms: (1) resonance information for 29 H2O lines up to 1097 GHz and 44 O2 lines up to 834 GHz in the form of intensity coefficients and center frequency for each line; (2) an empirical water vapor continuum spectrum; and (3) a liquid water attenuation term for haze and cloud conditions This data base is the heart of two computer programs which calculate and plot attenuation rates (in decibels per kilometer), refractivity (in parts per million), and refractive dispersion (in parts per million) The first covers the troposphere and requires pressure, temperature, and relative humidity as input data The second addresses isolated line behavior in the mesosphere wherein the geomagnetic field strength H is an additional input parameter due to the Zeeman effect of the O2 molecules Each oxygen line splits proportionally with H into numerous sublines, which are juxtaposed to form Zeeman patterns spread over a megahertz scale Patterns of three main polarization cases are considered Various typical examples for a model atmosphere demonstrate the utility of the approach, provide new information, and underline the serious role that water vapor plays above 120 GHz

Radio propagation for space communications systems

Abstract: This paper presents a review of the most recent information on the effects of the earth's atmosphere on space communications systems. The design and reliable operation of satellite systems which provide the many applications in space and rely on the transmission of radio waves for communications and scientific purposes are dependent on the propagation characteristics of the transmission path. The presence of atmospheric gases, clouds, fog, precipitation, and turbulence cause uncontrolled variations in the signal characteristics which can result in a reduction of the quality and reliability of the transmitted information. Models and techniques used in the prediction of atmospheric effects as influenced by frequency, geography, elevation angle, and type of transmission are discussed. Recent data on performance characteristics obtained from direct measurements on satellite links operating to above 30 GHz are reviewed. Particular emphasis is placed on the effects of precipitation on the earth-space path, including rain attenuation, and rain and ice-particle depolarization. Sky noise, antenna gain degradation, scintillations, and bandwidth coherence are also discussed. The impact of the various propagation factors on communications system design criteria is presented. These criteria include link reliability, power margins, noise contributions, modulation and polarization factors, channel crosstalk, error-rate, and bandwidth limitations.

Clear weather meteorological effects on propagation at frequencies above 1 GHz

Abstract: This paper reviews the various effects the clear atmosphere may have on the propagation of radio waves above 1 GHz. The role of the transfer processes at the lower boundary of the atmosphere in modifying the refractive index structure of air masses is reviewed, and some results of modification of refractive index and its structure constant by the lower boundary are shown. The formation of various classes of elevated layers is discussed and examples are shown of subsidence layer formation. The formation of refractive structures capping the convective boundary layer is discussed and examples of monsoonal refractive layers are shown. Examples of refractive index gradients and structure constant distributions in the neighborhood of frontal interfaces are also shown and discussed.

The kilometric radio emission spectrum: Relationship to auroral acceleration processes

Abstract: Using current theories for the generation of auroral kilometric radiation, the possibilities for obtaining information on the auroral acceleration processes from the spectrum of this radiation are discussed. It is shown, for example, that under certain conditions the low-frequency cutoff provides a direct indication of the upper altitude limit of the acceleration region. What is more, certain discrete features in the kilometric radiation spectrum show a very close similarity to whistler-mode emissions, thereby suggesting that the basic instability mechanisms are quite similar. Certain of these discrete features have frequency drifts which can be interpreted in terms of the propagation of shock-like disturbances along the auroral field lines at velocities near the ion-acoustic speed.

Narrowband Electromagnetic Emissions from Jupiter's Magnetosphere.

Abstract: Recent studies of wideband plasma wave data from Voyagers 1 and 2 have revealed the existence of narrowband radio emissions escaping from Jupiter's magnetosphere in the frequency range 1-12 kHz. These narrowband emissions are very similar to narrowband emissions previously discovered near earth and Saturn, and are believed to be produced by mode conversion from locally generated upper hybrid resonance waves at odd half-integral harmonics of the electron cyclotron frequency. This mode conversion process is believed to be one of the basic mechanisms for generating planetary radio emissions.

Depolarization of radio waves by atmospheric hydrometeors in earth‐space paths: A review

Abstract: Many measurements of earth-space radio wave depolarization from 10 to 30 GHz have been made in recent years. This review relates the results from these measurements within the framework of theoretical relationships derived from rain and ice depolarization models. The review includes measured statistical relationships between depolarization and rain attenuation and contains cumulative statistics showing the relative frequencies of occurrence of rain and ice depolarization. Depolarization is usually strongest for circular polarizations and for fixed linear polarizations oriented 45° from vertical and horizontal. Depolarization is usually minimized for fixed vertical and horizontal polarizations. For frequencies up to 30 GHz and elevation angles ranging from 10° to 50°, amplitude ratios of ice and rain depolarization are approximately proportional to frequency and to cos2 θ/sin θ, where θ is the path elevation angle.

Security system with infrared optical position detector

Abstract: A security device for determining the opening or closed condition of an access gate, such as a door or window hinged within an opening in a wall, has a detector unit mounted on the wall adjacent to the opening and linked to a remote control unit by a radio transmitter in the detection unit and a radio receiver in the control unit. The detection unit has an infrared generator and an infrared detector isolated from the generator, the detector and generator confronting a reflector mounted on the access gate when the access gate is closed. The reflector is translated from the field of the infrared generator when the access gate is open. The infrared generator is excited by a pulse generator to produce pulses of infrared radiation, and the scattered radiation from the reflector is detected by the infrared detector to produce electrical pulses coincident with the pulse generator. A coincidence circuit determines the presence of the detected pulse, and the absence of coincidence results in the transmission of a radio wave from the detector unit to the control unit to establish alarm conditions.

Radio wave absorber

Abstract: PROBLEM TO BE SOLVED: To exhibit a radio wave absorbing power in a high frequency range, including millimeter wave bands by fixing a radio wave absorptive material to a conductor plate which is composed of a powder of magnetoplumbite type hexagonal ferrite wherein part of Fe is substituted with one or more kinds of metals, or its mixture with grain holders or sintered material SOLUTION: Owing to the uniaxially anisotropic crystal magnetic anisotropy of a magnetoplumbite type hexagonal ferrite, a ferroelectric resonance phenomenon occurs at 40-60 GHz Powders of magnetoplumbite type hexagonal ferrite BaFe8 (Ti05 Mn )4 O19 and Z type hexagonal ferrite Ba3 Co2 Fe24 O41 and epoxy resin are mixed at a wt ratio 35:35:30 to form a composite material which is sheeted into a flat plate of 27 mm thick and backed with a conductor (metal) plate to form a radio wave absorber and an absorbing power of 20 dB or more at a frequency of 5-18 GHz in free space is obtd from its absorbing characteristic

The effect of plasma instabilities on the ionospherically reflected wave from a high‐power transmitter

Abstract: Observations of the reflected radio wave from a high-power transmitter show at times of not too high ionospheric absorption a drop of signal strength of typically almost 10db in the first few milliseconds after the arrival of the ionospheric echo following switching on a transmitter with 130 MW equivalent power. This decrease in signal strength is attributed to the excitation of the parametric decay instability. During about the next 5 s a further drop in signal strength by typically almost 10 db is observed. This decrease is attributed to the excitation of short-scale field-aligned irregularities which scatter the incident radio wave into Langmuir waves. These preliminary observations tend to confirm the theoretical threshold of the parametric decay instability. There are also indications that the formation of short-scale field-aligned irregularities inhibits the excitation of the parametric decay instability, as was previously inferred from the ‘overshoot’ effect in the Arecibo radar observations.

Phase locked loop amplifier for variable amplitude radio waves

Abstract: A modulated carrier signal, the amplitude variations of which are smaller than the amplitude of the unmodulated carrier, is not very suitable for transmission over radio links because the required use of non-linear components, such as class-C amplifiers, result in an unwanted widening of the transmitted spectrum. According to the invention an arrangement for amplifying such a signal comprises two oscillators (16, 17) the desired relative phase of which is adjusted in a loop including a control circuit 22. On combining the oscillator signals v'k and v'g, there is produced a resulting signal r which is a replica of the input signal v having the amplitude variations. As the oscillators 16 and 17 produce signals having a constant amplitude, these signals can be easily amplified, for example by class-C amplifiers.

Radio wave scattering observations of the solar corona First-order measurements of expansion velocity and turbulence spectrum using Viking and Mariner 10 spacecraft

Abstract: Radio wave scattering data were collected at 3.6 and 13 cm wavelengths by means of the radio link between the Viking orbiters and the earth during the Nov. 25, 1976 solar conjunction of Mars, which occurred near the beginning of solar cycle 21; Mariner 10 solar activity observations during 1974 are also used. It is found that the temporal frequency variance spectrum of amplitude fluctuations is useful for characterizing the bulk motion of the plasma, and the spectral index of electron density turbulence is obtained. The measurements of solar wind velocity and spectral index cover 78 days for Viking and 49 days for Mariner 10 and show the combined effects of changing heliocentric distance, solar latitude, and solar longitude as well as solar activity. It is concluded that the observational velocity profile differs significantly from the theoretical profiles in two ways: (1) the theoretical profile does not show the abrupt change in velocity at about 15 solar radii, and (2) the observational profile shows acceleration at larger radial distances than the model profiles. The observational profiles indicate velocities of less than about 150 km/sec out to 15 solar radii.

Coherence bandwidth loss in transionospheric radio propagation

Abstract: In this report a theoretical model is developed that predicts the single-point, two-frequency coherence function for transionospheric radio waves. The theoretical model is compared to measured complex frequency correlation coefficients using data from the seven equispaced, phase-coherent UHF signals transmitted by the Wideband satellite. The theory and data are in excellent agreement. The theory is critically dependent upon the power-law index, and the frequency coherence data clearly favor the comparatively small spectral indices that have been consistently measured from the wideband satellite phase data. A model for estimating the pulse delay jitter induced by the coherence bandwidth loss is also developed and compared with the actual delay jitter observed on synthesized pulses obtained from the Wideband UFH comb. The results are in good agreement with the theory. The results presented in this report, which are based on an asymptotic theory, are compared with the more commonly used quadratic theory. The model developed and validated in this report can be used to predict the effects of coherence bandwidth loss in disturbed nuclear environments. Simple formulas for the resultant pulse delay jitter are derived that can be used in predictive codes.

Nonthermal scattering of radio waves near 150 km above Jicamarca, Peru

Abstract: Ten hours of coherent scatter radar data were collected near 150-km altitude above Jicamarca, Peru, on July 27, 1977. The antenna system probed three separate volumes horizontally separated by approximately 9 km at 150 km. Measured vertical velocity varied between 10 and 20 m/s and is similar to the vertical ionization drift at these altitudes. However, the vertical velocity may change as much as 6 m/s over a 4-km height interval. Ten-minute amplitude modulations in the scattered power are seen throughout the observation period and are probably related to evanescent gravity waves. Several layers of enhanced scattering are observed at times. The half width of the measured spectra varies between 1 and 3 Hz with the vertically pointing antenna showing a consistently broader spectrum than the other two antennas. This narrow spectral width indicates a nonthermal scattering owing to electron density fluctuations.

Nonlinear Phenomena Arising from Radio Wave Heating of the Lower Ionosphere.

Abstract: : This document describes a theoretical and experimental study of the interaction of high power, high frequency radio waves with the lower ionosphere. The theoretical calculations presented here show that the electron temperature of the ionospheric plasma can be greatly enhanced when the plasma is irradiated by a powerful groundbased HF transmitter with an effective radiated power of the order of 100 MW. If this plasma heating is maintained for times exceeding a few seconds, the composition of the plasma can also be altered. These temperature and composition modifications cause significant changes in the plasma conductivity and wave absorption in the medium. Two experiments were conducted in order to test for the predicted absorption and conductivity modifications: a vertical incidence plus absorption experiment and a nonlinear demodulation experiment. Data from the absorption experiment clearly show a large (9 dB) increase in wave absorption at 2.4 MHz due to a high power (60 MW ERP) HF heating of the ionosphere. The nonlinear demodulation experiment generated strong VLF radiation when the ionosphere was irradiated by a powerful modulated HF wave. These VLF signals are believed to be due to HF heating induced conductivity modulation of the dynamo current system. (Author)

Sea state frequency features observed by ground wave HF Doppler radar

Abstract: This paper investigates the relationship between the frequency features of the directional spectrum of ocean waves (cutoff frequency and peak frequency of wind waves, frequency of quasi-monochromatic swell) and the properties of the spectrum of the sea echo obtained by HF Doppler radar. A theoretical analysis is performed to check the validity of a simple result that states that at upper HF, a long-wave spectral feature at frequency F0 affects the radar echo at frequencies ±ƒB ± F0 on either side of the Bragg lines at ±ƒB corresponding to resonant backscattering of radio waves with a given wavelength by half-wavelength ocean waves. An experiment using both radar and standard in situ measurements for several weeks at different times of the year shows that this relationship can be used to formulate a method for estimating the frequency features of the sea with suitable accuracy.

Solar radio blips and X-ray kernels

Abstract: Hudson1 proposed that the primary flare energy release goes into fast electrons. Others2,3 found the flare soft X-ray emission to consist of bright, small kernels (knots and loops) embedded in a more diffuse halo. These kernels with a typical size of 5′ arc s and a temperature of 6–12 × 106K emit in radio waves at 3.7 and 11 cm (ref. 4). The discovery of a radio brightness temperature equal to the temperature derived from the X rays and the radio size larger than in X rays shows that the observed radio emission is thermal (free–free) and the source optically thick at λ > 3.7 cm. Therefore, Kundu et al.4 did not observe the fast electrons expected to heat the kernels. Here we present high time resolution measurements at 2.8 cm from the Effelsberg 100-m telescope and at decimetric and metric wavelengths with the ETH Zurich spectrometers. Both instruments have detected signatures of non-thermal particles.

Contribution of water vapor monomer resonances to fluctuations of refraction and absorption for submillimeter through centimeter wavelengths

Abstract: Atmospheric fluctuations in temperature, humidity, and total pressure cause fluctuations in the absorption and refraction of electromagnetic waves We consider only the absorption and refraction fluctuations caused by variations in the width and strength of water monomer resonances We differentiate the absorption coefficient and refractive index with respect to temperature, humidity, and total pressure in order to obtain the coefficients that relate the fluctuations in absorption and refraction to these atmospheric parameters The resulting coefficients as well as the absorption coefficient and the refractivity are calculated by summing over all water vapor infrared resonances and are displayed graphically In the limit of low frequencies, these calculated coefficients are in good agreement with the formula for radio wave refractive index The effects of turbulent fluctuations in total pressure are usually negligible

Radio acoustic measurement of fog-capping thermal inversions

Abstract: The radio acoustic sounding system (RASS) is a remote technique for measuring temperature profiles in the lower troposphere1,2 In essence, it consists of a powerful acoustic source beaming a short burst of sinusoidal waves towards the zenith The speed of this pulse in its upwards path is proportional at every height to the square root of the local temperature This speed is continuously measured from the ground by means of a Doppler radar The radar echo is due to the change in refractive index of air compressed by the acoustic wave The faint echo is enhanced by choosing an acoustic wave in Bragg resonance with the radio wave The record of the measured sound speed displayed as a function of delay from the start of the acoustic beam leads to the acquisition of the temperature vertical profile We now describe the use of such a system at metre-wavelengths in the study of fog-capping thermal inversions at a site in the Po Valley

Intense daytime radio wave scintillations and sporadic E layer near the dip equator

Abstract: The paper describes two cases of very intense scintillation activity (amplitude exceeding 10 dB) recorded at Huancayo during the daytime on radio beacons on 257 MHz from Marisat satellite and ATS 3 (137 MHz) and LES 6 (254 MHz) signals. Both these events are shown to be associated with an intense blanketing equatorial sporadic E layer at Huancayo. It is suggested that the bursts of strong daytime scintillations of VHF radio waves are due to densely ionized thin sheets of ionization over the equator and only normally weak (≈ 1 dB) daytime scintillations may be due to instabilities associated with the q type of equatorial Es.

Method and apparatus for remote measurement of wind direction and speed in the atmosphere

Abstract: An acoustic wave source and a radio wave source are installed close to each other on the ground. When an acoustic wave pulse is transmitted vertically into the atmosphere by the acoustic wave source, spherical wavefronts formed in the atmosphere by the acoustic wave are propagated upwardly at the velocity of sound. When a continuous radio wave is transmitted from the radio wave source toward the spherical wavefronts, it is reflected by the wavefronts and the reflected radio waves are converged to form a focusing spot on the ground. The position where the spot due to the convergence of the reflected radio waves is formed with the maximum intensity is detected by means of an antenna network formed of a multiplicity of receiving antennas laid out in the pattern of a lattice. The time-course change of such positions of the spots is traced to realize remote measurement of the height distribution of wind direction and speed in the atmosphere under surveillance.

Ray tracing in the Io plasma torus: Application to the PRA observations during Voyager 1's closest approach

Abstract: An attempt is made to simulate propagation of the low-frequency radio waves in the vicinity of the Io plasma torus near Jupiter. In absence of a definitive and detailed electron density model for this torus, a number of minimal hypotheses were used. Nevertheless, the anomalous radio dynamic spectrum observed during the day of Voyager 1's closest approach could agree with an emission source situated around the planet in the southern hemisphere and related to the local gyrofrequency. The major conclusion is that the entire low frequency (up to a few megahertz) dynamic radio spectrum of Jupiter could be modified by refraction inside and around the Io torus. The magnitude of the effects depends on the location of the source and on the plasma parameters distribution inside the torus.

Fine structure in the ionospheric D-region

Abstract: Assuming that their observations of the weak echoes of high frequency radio waves scattered from the lower ionosphere were caused by Fresnel reflections from discontinuities in the refractive index, Gardner and Pawsey1 developed a method for determining the height distribution Ne(h) of the D-region electron densities from observations of the ratio Ax/Ao (h) of the back-scattered amplitudes of the extraordinary and ordinary magneto-ionic components. This ‘partial reflection’ method turned out to be a useful tool for studies of the ionospheric D-region2–5. We compare here observations of weak radio echoes from the ionospheric D-region, with model computations of radio wave scattering, based on in situ measurements of ionospheric irregularities. The results show excellent agreement, indicating that an important source of the weak partial reflections has been identified.

Polarization of radio waves in ice sheets

Abstract: Summary. The polarization behaviour of radio waves which have been transmitted and reflected at or near normal incidence in an ice sheet can be explained if the ice is assumed to be a uniaxial birefringent material. Equations are derived for describing the change in polarization for the experimental conditions usually encountered when sounding ice sheets. One of the initial unknowns which can be found is the direction of the effective optic axis which determines the basic reference frame. Other parameters which can be found and related to the physical nature of the ice and reflecting surface are the total phase shift between the ordinary and extraordinary rays and their relative amplitude. The operation of several different types of polarimeter is discussed in order to compare the amount and the quality of information whch can be obtained for various degrees of complexity in equipment and technique.

Radiating coaxial cable having apertures spaced at a distance considerably larger than a wavelength

Abstract: High-frequency radio waves are transmitted through a coaxial cable having apertures spaced at a distance considerably larger than the wavelength of the signals; the apertures are feed points for surface waves on the cable.