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Results for “DIPOLE ANTENNA”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 109 records · Page 6
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NASA NTRS1972

New measurements of the galactic radiation spectrum between 200 KHz and 2.6 MHz

The Goddard Space Flight Center radio astronomy experiment aboard the IMP-6 satellite has provided new data on the absolute spectrum of the galactic background radiation at frequencies between 0.2 and 2.6 MHz. The measurements have been obtained both in the terrestrial magnetosphere and in interplanetary space. One of the primary investigations was the measurement of the low frequency component of the galactic radiation. The main sources of error are considered to be the rms noise fluctuations of the receiver, the measurement of the dipole antenna impedance, and in the determination of the noise source used in the preflight calibration of the receiver system. Interstellar models are planned considering the ambient plasma.

Brown, L. W.↗
NASA NTRS1972

Interstellar medium model

A model of the ionized part of the interstellar medium was developed, based on the low frequency observations by the Radio Astronomy Explorer Satellite with a background of nonthermal radiation. This nonthermal background radiation is caused by synchrotron emission from cosmic ray electrons, and at low frequencies this emission is heavily absorbed by free-free absorption from the residual thermal electrons in the interstellar medium. By an appropriate model, parameters relevant to both the thermal and nonthermal components of the interstellar medium are shown. The observations were taken with the 100 deg dipole antenna and separated into galactic and extragalactic components. This model was developed using only the separated galactic component.

Novaco, J. C.↗
NASA NTRS1972

Electrostatic waves in the magnetosphere.

Electric dipole antennas on magnetospheric spacecraft measure E field components of many kinds of electromagnetic waves. In addition, lower hybrid resonance emissions are frequently observed well above the ionosphere. The Ogo 5 plasma wave experiment has also detected new forms of electrostatic emissions that appear to interact very strongly with the local plasma particles. Greatly enhanced wave amplitudes have been found during the expansion phases of substorms, and analysis indicates that these emissions produce strong pitch angle diffusion. Intense broadband electrostatic turbulence is also detected at current layers containing steep magnetic field gradients. This current-driven instability is operative at the bow shock and also at field null regions just within the magnetosheath, and at the magnetopause near the dayside polar cusp. The plasma turbulence appears to involve ion acoustic waves, and the wave particle scattering provides an important collisionless dissipation mechanism for field merging.

Scarf, F. L.↗
NASA NTRS1973

The galactic radio spectrum between 130 and 2600 kHz.

The IMP-6 radio astronomy experiment has provided new measurements of the galactic background spectrum at 22 frequencies between 130 and 2600 kHz. A highly accurate spectrum is presented which corresponds to the minimum galactic radiation observed with a short dipole antenna. The estimated maximum spectrum is presented also. The data demonstrate the need to include the effect of the ambient plasma on synchrotron emission in low-frequency galactic models.

Brown, L. W.↗
NASA NTRS1973

Surface electrical properties experiment

The surface electrical properties (SEP) experiment was used to explore the subsurface material of the Apollo 17 landing site by means of electromagnetic radiation. The experiment was designed to detect electrical layering, discrete scattering bodies, and the possible presence of water. From the analysis of the data, it was expected that values of the electrical properties (dielectric constant and loss tangent) of lunar material in situ would be obtained. The basic principle of the SEP experiment is interferometry. This principle involves only the interference of two or more waves to produce an interference pattern. The inversion of the interference pattern in terms of the spatial distribution of the electrical properties is the basic aim of the experiment (fig. 15-1). The experiment is most easily understood in terms of a single dipole antenna for radiating electromagnetic energy and a loop receiver for measuring the magnitudes of the fields.

Simmons, G.↗
NASA NTRS1974

Observations of interplanetary plasma waves, spacecraft noise, and sheath phenomena on Imp 7

The Imp 7 plasma wave instrument measures electric and magnetic wave components of plasma oscillations over the frequency range from 10 Hz to 100 kHz. The instrumentation and relevant external characteristics of the spacecraft that appear to be responsible for some in-flight disturbance effects are briefly described. It is shown that as each one of the 16 solar panel flats rotates into shadow or sunlight, the array transients produce fluctuating magnetic fields that are detected on the magnetic loop mounted 3.4 m from the spacecraft. These transients occur 16 times per spin period, and the corresponding magnetic noise has a high frequency on the rapidly spinning Imp 7 spacecraft. The analysis suggests that some Imp magnetic threshold levels measured 3-4 m from the spacecraft are determined by the solar array current transient effects associated with the discrete 16-sided geometry of the spacecraft. The geometry also influences the response of the electric dipole antenna by modulating the sheath.

Scarf, F. L.↗
NASA NTRS1974

Geophysical subsurface probing with radio-frequency interferometry

The radio-frequency interferometry method can be used to probe interiors of celestial bodies and terrestrial areas with low conductivity. In order to interpret the interference patterns, a theoretical study is made of the electromagnetic fields due to a dipole antenna on the surface of a horizontally stratified n-layered medium. Three approaches are used to calculate the interference patterns: direct numerical integration, asymptotic evaluation by the saddle point method, and a residue series approach. The asymptotic approach leads to the geometrical-optics interpretation. The residue approach leads to modal analysis. The validity of the formulation is checked by comparisons with analog model tank experiments and actual field data obtained from glaciers.

Kong, J. A.↗