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At least 271 records · Page 15

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NASA NTRS1973

Permafrost - Relation between ice content and dielectric losses at 100 K

The dielectric response of permafrost at 100 K and vacuums of around 10 ntorr is analyzed, varying its percent ice content from 1 to 18.6. The distributions obtained correspond to dielectric relaxations of the Cole-Cole type, with maximum losses occurring in the 30- to 600-Hz frequency range. The logarithms of such maxima depend linearly on the permafrost ice content, two regions of linear variation being defined above and below 3.6% ice content. Such relations point out the feasibility of determining ice content in permafrost by electromagnetic means.

Alvarez, R.↗

NASA NTRS1973

Application of dielectric constant measurements to radar imagery interpretation

A ground-truth scheme is briefly outlined, specifically, the measurement of the dielectric constant. Two portable instruments were designed specifically for this purpose: a Q-meter for measurement of dielectric constant and loss tangent, and an instrument to measure electrical properties of the two operating frequencies of the imaging radar. Several general cases of radar-earth surfaces interaction are described; also, examples of radar imagery and some data on ice and snow are presented.

Bryan, M. L.↗

NASA NTRS1974

Analysis of strip antennas in the presence of a dielectric inhomogeneity

Moment method solutions are considered for treating the problem of wire antennas in the presence of an arbitrary dielectric inhomogeneity. In the first method, the current on the wire and the electric field intensity in the inhomogeneity are treated as independent unknowns, while in the second and third methods they are treated as dependent unknowns. The third method is applied to the problem of strip antennas in an electrically thin dielectric slab. Numerical results are presented, and are in good agreement with measurements and previous calculations.

Newman, E. H.↗

NASA NTRS1975

Electron radiation effects on time-dependent dielectric breakdown in SiO2 films

An experiment testing the effect of ionizing radiation on breakdown characteristics of SiO2 films is presented. Silicon wafers were oxidized and metallized, and a capacitor array was etched into a control sample while the rest were first irradiated with 1 MeV electrons and then etched. Time-dependent dielectric tests were made on all the capacitors, and the average characteristics of 96 capacitors are illustrated graphically. The curves are consistent with the model of holes trapped in the SiO2 film during irradiation leading to a retarding field for positive ion emission and drift toward the interface. It is shown how an externally applied field is reduced by the trapped charge, and that changes in the dielectric breakdown properties of the SiO2 film after irradiation depend on the positive trapped charge near the metal interface.

Li, S. P.↗

NASA NTRS1975

Radiative transfer in a plane stratified dielectric

A model is developed for calculating radiative transfer in a stratified dielectric. This model is used to show that the reflectivity of a stratified dielectric is primarily determined by gradients in the real part of the refractive index over distances on the order of 1/10 wavelength in the medium. The effective temperature of the medium is determined by the thermodynamic temperature profile over distances of the order delta T.

Wilheit, T. T., Jr.↗

NASA NTRS1975

Inherent time delay for dielectric breakdown in shock-loaded x-cut quartz

Dielectric breakdown in shock-loaded x-cut quartz is examined under conditions of both impact loading and Q-switched laser-irradiation loading. It is observed that breakdown is characterized by an inherent time delay which depends on the magnitude of the electric field. For pulse duration less than about 30 ns, dielectric breakdown is not observed.

Graham, R. A.↗

NASA NTRS1976

Dielectric coated wire antennas

An electrically thin dielectric insulating shell on an antenna composed of electrically thin circular cylindrical wires is examined. A moment method solution is obtained, and the insulating shell is modeled by equivalent volume polarization currents. These polarization currents are related in a simple manner to the surface charge density on the wire antenna. In this way the insulating shell causes no new unknowns to be introduced, and the size of the impedance matrix is the same as for the uninsulated wires. The insulation is accounted for entirely through a modification of the symmetric impedance matrix. This modification influences the current distribution, impedance, efficiency, field patterns, and scattering properties. The theory is compared with measurement for dielectric coated antennas in air.

Richmond, J. H.↗

NASA NTRS1978

Dielectric constants of soils at microwave frequencies-2

The dielectric constants of several soil samples were measured at frequencies of 5 and 19 GHz using the infinite transmission line method. The results of these measurements are presented and discussed with respect to soil types and texture structures. A comparison is made with other measurements at 1.4 GHz. At all three frequencies, the dependence of dielectric constant on soil moisture can be approximated by two straight lines. At low moisture, the slope is less than at high moisture level. The intersection of the two lines is believed to be a function of soil texture.

Wang, J.↗

NASA NTRS1978

Strip antennas in a dielectric slab

A method is proposed to analyze electrically thin strip antennas in an electrically thin dielectric slab. The method combines a moment method solution and a modification of the piecewise sinusoidal reaction formulation for thin wire radiators in a homogeneous medium. Strip antennas in both homogeneous and inhomogeneous media are considered. The analysis has the advantages that it is applicable to a wide variety of antenna geometries, that the slab may be of finite or infinite extent, and that the presence of the slab introduces no new unknowns in the moment method solution. Numerical calculations and verification by measurements are presented for the impedance of a strip dipole in a dielectric slab.

Newman, E. H.↗

NASA NTRS1979

Electric fields in irradiated dielectrics

An existing model for quantitatively predicting electric field build-up in dielectrics is used to demonstrate the importance of material parameters. Results indicate that electron irradiation will produce 10 to the 6th power V/cm in important materials. Parameters which can alter this build-up are discussed. Comparison to known irradiation induced dielectric charging experiments is discussed.

Frederickson, A. R.↗

NASA NTRS1979

Stable dielectric charge distributions from field enhancement of secondary mission

The emission of secondary electrons from dielectrics subject to numerous effects of electric field which are experimentally difficult to control is discussed. Measurements are reported using pulse techniques such that local fields do not build to significant levels, but measurements with fields present are also of interest. A specific series of measurements under controlled conditions are described and their implications in terms of fields, magnitude and angle, near the dielectric surface were examined. The measurements were made for a charged fluorinated-ethylene-propylene surface near a grounded aluminum half-round resting on the surface. The geometry produced a stable surface-charge gradient being controlled by a strongly enhanced secondary emission for which a model is constructed. Observations of surface flashovers under various conditions confirm the predictions of some scaling exercises.

Robinson, J. W.↗

NASA NTRS1980

Dielectric-loaded waveguide circulator for cryogenically cooled and cascaded maser waveguide structures

A dielectrically loaded four port waveguide circulator is used with a reflected wave maser connected to a second port between first and third ports to form one of a plurality of cascaded maser waveguide structures. The fourth port is connected to a waveguide loaded with microwave energy absorbing material. The third (output signal) port of one maser waveguide structure is connected by a waveguide loaded with dielectric material to the first (input) port of an adjacent maser waveguide structure, and the second port is connected to a reflected wave maser by a matching transformer which passes the signal to be amplified into and out of the reflected wavemaser and blocks pumping energy in the reflected wave maser from entering the circulator. A number of cascaded maser waveguide structures are thus housed in a relatively small volume of conductive material placed within a cryogenically cooled magnet assembly.

Clauss, R. C.↗

NASA NTRS1980

Electromagnetic backscattering from a random distribution of lossy dielectric scatterers

Electromagnetic backscattering from a sparse distribution of discrete lossy dielectric scatterers occupying a region 5 was studied. The scatterers are assumed to have random position and orientation. Scattered fields are calculated by first finding the mean field and then by using it to define an equivalent medium within the volume 5. The scatterers are then viewed as being embedded in the equivalent medium; the distorted Born approximation is then used to find the scattered fields. This technique represents an improvement over the standard Born approximation since it takes into account the attenuation of the incident and scattered waves in the equivalent medium. The method is used to model a leaf canopy when the leaves are modeled by lossy dielectric discs.

Lang, R. H.↗

NASA NTRS1979

Some observations on the relationship between lubricant mechanical and dielectric transitions under pressure

Viscoelastic transition measurements of several lubricants by volume dilatometry to pressures of 1.75 GPa, dielectric transitions at atmospheric pressure on five fluids in a frequency range of 0.2 to 500 kHz and on two fluids in the same frequency range to pressures of 0.55 GPa are reported. Lines of constant rate dilatometry transition, constant rate dielectric transition and constant viscosity are shown to be essentially parallel on a temperature-pressure diagram.

Bair, S.↗

NASA NTRS1979

Microwave dielectric measurements of lunar soil with a coaxial line resonator method

A method is given for sensitive dielectric measurements at a series of microwave frequencies using a section of coaxial line. The line is used as a 1-port cavity resonator; it resonates when the electrical length of the center conductor equals 1, 2 . . . , N half-wave lengths. The dielectric properties of an Apollo 17 dried soil sample were measured in vacuum over a temperature range of 173 to 373 K. The relative permittivity and the loss tangent were determined and the frequency dependence was very small. The derivative with respect to temperature, per degree, was 0.00045 for the permittivity and 0.00002 for the loss tangent.

Bussey, H. E.↗

NASA NTRS1980

Liquid distribution and the dielectric constant of wet snow

The mixing theory of Polder and Van Santen is revised for application to three cases of wet snow. The dielectric constant is calculated for a range of liquid contents and porosities. These calculated values compare favorably with experimental data for the two cases in which data available. The application to a snow cover with a heterogeneous distribution of liquid is discussed. The possibility of applying this theory the calculate the imaginary part of the dielectric constant must be explored further.

Colbeck, S. C.↗

NASA NTRS1980

Measurement of dielectric properties and determination of microwave emissivity of polluted waters

The dielectric properties of polluted waters are measured with a reflection-type resonant cavity at 1.43 GHz. Very small water samples in quartz tubes of known volume are placed in the center of the maximum electric field. Measurement of the resonance-frequency variation and a change of the cavity's quality factor are used to determine the dielectric properties. The microwave emissivity of the polluted water is then calculated via the Fresnel equation and applied to data reductions of microwave radiometer measurements.

Blume, H.-J. C.↗

NASA NTRS1981

Electromagnetic backscattering from a sparse distribution of lossy dielectric scatterers

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.

Lang, R. H.↗