Showing papers on "Radio wave published in 1977"

Interstellar Scattering and Scintillation of Radio Waves

Abstract: A review is presented of interstellar scattering and scintillation observations in the light of modern theory for extended, spatially homogeneous, power law inhomogenieties. Formulas are presented for the experimentally observable quantities and are compared with data. A general discussion of the model is given. (GHT)

Radio wave propagation along mixed paths in forest environments

Abstract: The propagation of radio waves is examined for communication paths that may be partly within a forest and partly in regions outside the vegetation. Analytic results are found for simple canonic geometries in which the fields can be described in terms of ray-trajectories. By viewing a realistic forest environment as a combination of such canonic cases, it is possible to evaluate radio losses in complex situations by using a ray-tracking approach. The pertinent fields can then be expressed in terms of relatively simple analytical expressions, which hold well for frequencies between 2 and 200 MHz.

Radio propagation for vehicular communications

Abstract: Radio propagation is affected by many factors including the frequency, distance, antenna heights, curvature of the earth, atmospheric conditions, and the presence of hills and buildings. The influence of each of these factors at frequencies above about 30 MHz is discussed with most of the quantitative data being presented in the series of nomograms. By means of three or four of these charts an estimate of the received power and the received field intensity for a given point-to-point radio transmission path ordinarily can be obtained in a minute or less. The theory of propagation over a smooth spherical earth is presented in a simplified form that is made possible by restricting the frequency range to above about 30 MHz where variations in the electrical constants of the earth have only a secondary effect. The empirical methods used in estimating the effects of hills and buildings and of atmospheric refraction are compared with experimental data on shadow losses and on fading ranges.

Modulation of the polar electrojet by powerful HF waves

Abstract: Powerful radio waves, as they are used in ionospheric modification experiments, can very efficiently heat the electron gas of the lower ionosphere. If the transmitter operates in a pulsed mode with the pulse repetition rate f, the electron temperature will oscillate with frequency f and thus lead to periodic changes of the electric conductivity. Thus if a dc current is present in the ionosphere, an ac component will be superimposed upon it. As a result the modified ionospheric region will radiate electromagnetic waves with frequency f. An estimate of the power of these very or extremely low frequency waves produced within the ionosphere is performed in this paper. For typical polar electrojet conditions and with the parameters of the modification experiment projected by Max-Planck-Institut (Lindau) a power of a few kilowatts appears possible at frequencies around 10 kHz. It is also found that at extremely low frequencies, around 10−2 Hz, pulsations of the magnetic field with an amplitude of a few 10−1 γ may be excited.

Radio wave loss deviation and shadow loss at 900 MHz

Abstract: Radio wave propagation between base and mobile stations is normally described as being Rayleigh distributed due to multipath radio wave combining. When the number of radio wave paths are limited the variation in received signal amplitude frequently follows a more general case given by a Weibull distribution. A significant portion of the Weibull distribution is defined as the transmission loss deviation. Other definitions have been included with the object of standardizing methods of measuring and reporting propagation data. Data will be presented showing loss deviation between 3 and 30 dB. Shadow loss over hills and around buildings are usually assumed to be knife-edge or rounded knife-edge. Shadow loss based on these assumptions is generally found to be less than the true measured value. Data will be presented comparing calculated shadow loss with measured value.

The Viking Solar Corona Experiment

Abstract: The 1976 Mars solar conjunction resulted in complete occultations of the Viking spacecraft by the sun at solar minimum. During the conjunction period, coherent 3.5- and 13-cm wavelength radio waves from the orbiters passed through the solar corona and were received with the 64-m antennas of the NASA Deep Space Network. Data were obtained within at least 0.3 and 0.8 R sub s of the photosphere at the 3.5- and 13-cm wavelengths, respectively. The data can be used to determine the plasma density integrated along the radio path, the velocity of density irregularities in the coronal plasma, and the spectrum of the density fluctuations in the plasma. Observations of integrated plasma density near the south pole of the sun generally agree with a model of the corona which has an 8:1 decrease in plasma density from the equator to the pole. Power spectra of the 3.5- and 13-cm signals at a heliocentric radial distance of about 2 R sub s have a 1/2 power width of several hundred hertz and vary sharply with proximate geometric miss distance. Spectral broadening indicates a marked progressive increase in plasma irregularities with decreasing ray altitude at scales between about 1 and 100 km.

The polarization of radio signals in the radio echo sounding of ice sheets

Abstract: The theory of the polarization and propagation of an electromagnetic wave in a birefringent medium is briefly reviewed. Particular attention is given to those aspects of the theory which relate to the radio echo sounding of ice sheets. Methods of determining the polarization state of a radar wave are discussed, and it is shown that in certain circumstances this may be achieved by observing the variation of the signal strength in a pair of rotating receiving antennae. The results of experiments performed on the Greenland ice sheet are presented. The radio echo is shown to be elliptically polarized, confirming the suggestion that the ice sheet is birefringent at radio frequencies. A figure is obtained for the anisotropy of the dielectric constant of the ice sheet.

Comments on the optical and radio detection of black hole explosions

Abstract: IT has recently been suggested1 that it may be easier to detect the explosions of miniholes by the radio and optical pulses produced by the interaction of the emitted particles with the interstellar magnetic fields, than by the direct detection of the high energy γ rays2.

Relationship between atmospheric electricity and microwave radio propagation

Abstract: ABRUPT changes in received polarisation coincident with lightning strokes have been seen on a 20 GHz satellite-to-earth radio link. These effects, and the surprisingly high levels of atmospheric cross polarisation observed on satellite links, have been tentatively attributed to high altitude ice crystals and their possible alignment by electric fields1,2. In this paper we shall add to the existing data by presenting a precise measurement of the time scale of the abrupt polarisation changes and an event showing a striking correlation between the electric potential gradient at the ground and the cross-polarisation of signals received from the satellite. This is believed to be the first time that direct measurements of atmospheric electric fields have been related to microwave propagation measurements.

Integrated refractive effects prediction system /IREPS/

Abstract: Non-standard variations in temperature and humidity in the lower part of the atmosphere often produce unexpected refraction of radio waves and possible waveguide-like ducts in which radio energy can be “trapped” and carried to great distances. At times the strength of such anomalous refractive effects can seriously alter the performance of electromagnetic systems. In the case of shipboard surveillance radars, low-flying targets can sometimes be detected at ranges far exceeding the normal radar horizon, while at the same time higher-flying targets can only be detected at less-than-normal ranges. Even though it is generally acknowledged that such refractive effects can create serious problems, there is presently no capability in the operational fleet to solve the problem. NELC’s Integrated Refractive Effects Prediction System (IREPS) is being developed to fill this need. A more detailed description of IREPS is given by Hitney and Richter (1976).

A refracting radio telescope

Abstract: Observations of extraterrestrial radio sources at the lower end of the radio frequency spectrum are limited by reflection of waves from the topside ionosphere and by the large size of antenna apertures necessary for the realization of narrow beamwidths. The use of the ionosphere as a lens is considered. The lens is formed by the release of chemicals such as H2 and H2O at the F2-layer peak. These chemicals promote dissociative recombination of O(+) in the ionosphere resulting in a local reduction in plasma density. Gradients in electron density in the vicinity of the gas release tend to focus rays propagating through the depleted region. Preliminary calculations indicate that a lens capable of focusing cosmic radio waves in the 1 to 10 MHz frequency range may be produced by the release of 100 kg of H2 at the peak of the nighttime F layer. The beamwidth of a refracting radio telescope using this lens may be less than 1/5 degree.

Temporal radio frequency spectra of multifrequency waves in a turbulent atmosphere characterized by a complex refractive index

Abstract: Ishimaru's formulation for the amplitude and phase spectra for radio waves propagating in a turbulent medium is extended to cover the case where the refractive index is complex and frequency dependent. New static limits for the amplitude and phase spectrum are derived. The temporal radio frequency spectra give an estimate of the short time variations (i.e., tenths to tens of seconds) that tend to occur when radio waves propagate in a turbulent medium. For example, the refractive index is complex in a dispersive medium with a Lorentzian line shape, such as the earth's atmosphere over the the range of 50 to 70 GHz, due to resonant frequencies of molecular oxygen. The results for the amplitude and phase spectrum corresponding to radio links on earth (altitudes less than 5 km) indicate that turbulence does not significantly alter the radio frequency spectrum even for radio frequencies separated by 3 GHz. By contrast, the impact of turbulence is to make the details of the spectrum due to an isolated oxygen line in the 60 GHz band time dependent and, therefore, not readily compensated for by an equalizer. An isolated line in the 60 GHz oxygen band does constitute the propagation medium for radio links at altitudes greater than 25 km.

Velocity measuring system

Abstract: The relative transverse velocity between a local station and a zone of reference or observation is measured by providing first and second directional radiation receiving devices at the local station, such devices being aimed along parallel paths extending to the zone of reference. Such paths are spaced apart by a predetermined transverse distance. The first and second radiation receiving means include respective first and second converting means for converting the received radiation into first and second electrical signals. Any transversely moving radiation feature produces corresponding signal features in such first and second signals, such signal features being staggered with respect to time. The lapse of time between such staggered signal features is determined by time lapse measuring means. The velocity of such transverse movement is inversely proportional to such time lapse. The time lapse is preferably divided electronically into a constant which represents such transverse distance and a scale factor, to produce a quotient which is a direct measure of the velocity. The time lapse may be measured electronically with a high degree of accuracy. The received radiation may be either light focused by first and second optical systems, or radio waves received by first and second directional radio antennas connected to first and second radio receivers. The local station may be stationary, in which case the system may be employed to measure the velocity of objects or features in the zone of reference. Alternatively, the local station may be mounted on a movable vehicle, such as a helicopter, for example, in which case the system may be employed to measure the velocity of the vehicle relative to the zone of reference, which may be the terrain over which the helicopter is flying, for example. The velocity measuring system is adaptable to the measurement of the range and dimensions of objects in the zone of reference.

Selective Reception of Periodic Electromagnetic Waves with General Time Variation

Abstract: Several transmitters and receivers for periodic nonsinusoidal electromagnetic waves have been built during the last few years. Various applications of such waves, primarily in radar, have been recognized. The first practical equipment, an into-the-ground radar for construction surveying, has become commercially available. The efficient radiation of nonsinusoidal periodic waves has been discussed in the literature, but very little has been published on their selective reception. This paper describes the principle of selective receivers for periodic waves with general time variation, in analogy to the design of the usual receivers for waves with sinusoidal time variation.

Measurements of the solar wind using spacecraft radio scattering observations

Abstract: This paper reviews radio scattering measurements of the solar wind carried out with coherent, monochromatic, and point-source spacecraft signals. The observed phenomena which include spectral and angular broadening, and phase as well as intensity scintillations, have provided measurements of the solar wind previously not available from radio astronomical observations. These cover a wide range of heliocentric distances (as close as 1.7 solar radii), and large- as well as small-scale electron density fluctuations.

Magnetic field effects on electrons during ionospheric modification

Abstract: Ionospheric modification by intense radio waves is known to produce energetic tails on the electron velocity distribution function, resulting in enhanced airglow. We demonstrate that these energetic tails may be due to quasi-linear velocity space diffusion in the presence of the background magnetic field. The waves responsible for the diffusion are produced by the parametric decay instability. The magnetic quasilinear diffusion considered here may often be more important than the unmagnetized resonance-broadened diffusion considered by Weinstock; however, the two effects are additive, so that the predictions of either theory by itself are conservative.

Process for communication

Abstract: PURPOSE:To elevate the efficiency of radio wave use with protecting interference and communicating precisely and in a short time by using the specific signal mutually between transmission and reception.

Atmospheric waves in the ionosphere due to total solar eclipse

Abstract: CHIMONAS1 predicted that the ‘cooling spot’ of the lunar shadow of an eclipse, which travels at supersonic speeds through the atmosphere, is a continuous source of atmospheric gravity waves. These gravity waves show up by their interaction with the ionosphere and can be detected using standard radio techniques. Total electron content measurements2 in the USA at a distance of several thousand km from the path of totality, detected oscillations which were attributed to a solar eclipse in agreement with the theory of Chimonas3. Measurements made during the 1973 central African eclipse4 failed to detect any oscillations, although these results may have been obscured by the occurrence of a magnetic storm at the same time. We present here the results of measurements made on the angle of arrival of a high frequency radio wave inside the path of totality of an eclipse, which took place on 23 October 1976.

Electromagnetic Wave Interactions with Inhomogeneous Plasmas

Abstract: Experimental observations of electromagnetic wave propagation and interactions with a large inhomogeneous laboratory plasma are summarized. The dominant process is the conversion of electromagnetic waves into electrostatic waves whose field strengths are enhanced by several orders of magnitude in the vicinity of the resonant layer where the incident frequency matches the local plasma frequency. Strong local accelerations of electrons and ions by these localized resonant fields are described. Density cavities produced by the digging actions of these ponderomotive forces are called “cavitons” which coexist with the rf fields. Parametric decay instabilities are found to be a much weaker process compared with the caviton formation at the resonant layer. A double resonance technique of controlling ion dynamics at a specific location in an inhomogeneous plasma is demonstrated. The correlations between the present experiments and laser-plasma interactions, as well as radio wave interactions with the ionosphere, are discussed.