Engineering topics

Croswell, W. F.

Publications and source records attributed to Croswell, W. F..

At least 19 records

NASA NTRS1984

Hoop/column antenna RF verification model. Volume 2: Analysis and correlation

As part of the Large Space System Technology Program, the theoretical and experimental results of the RF characteristic of a hoop/column, quad aperture antenna using an RF verification model are presented. To satisfy the primary purposes of the model, experimental pattern data is provided for the quad aperture configuration at different reflector edge illumination levels, from which the geometry and edge effects can be assessed, and experimental data which can be compared with calculations using various theoretical reflector scattering formulae are provided. It also experimentally determines the effects upon secondary patterns of scale model quartz cables, as used in the hoop/column design, upon secondary patterns in order to assess the importance of developing a scattering theory to predict such effects. In addition, a comprehensive theoretical study and the experimental pattern results of quad aperture antenna feeds, a discussion of the fundamental affect of parasitic side lobes, their amplitude, and location in space.

Croswell, W. F.↗

NASA NTRS1984

Hoop/Column Antenna: RF Verification Model. Volume 1: Test Results

As part of the Large Space System Technology Program, this report, in two volumes, presents the theoretical and experimental results of the RF characteristic of a hoop/column, quad aperture antenna using an RF verification model. To satisfy the primary purposes of the model it provides experimental pattern data for the quad aperture configuration at different reflector edge illumination levels, from which the geometry and edge effects can be assessed, and provides experimental data which can be compared with calculations using various theoretical reflector scattering formulae. It also experimentally determines the effects upon secondary patterns of scale model quartz cables, as used in the hoop/column design, upon secondary patterns in order to assess the importance of developing a scattering theory to predict such effects. In addition, this report contains a comprehensive theoretical study and the experimental pattern results of quad aperture antenna feeds, a discussion of the fundamental affect of parasitic side lobes, their amplitude, and location in space.

Croswell, W. F.↗

NASA NTRS1983

Phased array-fed antenna configuration study: Technology assessment

Spacecraft array fed reflector antenna systems were assessed for particular application to a multiple fixed spot beam/multiple scanning spot beam system. Reflector optics systems are reviewed in addition to an investigation of the feasibility of the use of monolithic microwave integrated circuit power amplifiers and phase shifters in each element of the array feed.

Croswell, W. F.↗

NASA NTRS1983

Ocean experiments and remotely sensed images of chemically dispersed oil spills

A series of experiments was performed at sea where the effectiveness of dispersants applied from a helicopter was tested on fresh and weathered crude oils released from a surface research vessel. In conjunction with these experiments, remote sensing measurements using an array of airborne optical and microwave sensors were performed in order to aid in the interpretation of the dispersant effectiveness and to obtain quantitative images of oil on the sea under controlled conditions. Surface oil thickness and volume are inferred from airborne measurements using a dual-channel microwave imaging radiometer, aerial color photography, and an airborne oceanographic lidar. The remotely sensed measurements are compared with point sampled data obtained using a research vessel. The mass balance computations of surface versus subsurface oil volume using remotely sensed and point sampled data are consistent with each other and with the volumes of oil released. Data collected by the several techniques concur in indicating that, for the oils used and under the sea conditions encountered, the dispersant and application method are primarily useful when applied to fresh oil.

Croswell, W. F.↗

NASA NTRS1982

Characteristics of 13.9 GHz radar scattering from oil films on the sea surface

Aircraft microwave scatterometer measurements are presented, which were made in 1979 as part of a project to study the response of a number of active and passive microwave and optical remote sensors to an oil-covered sea surface conducted by NASA Langley Research Center. A 13.9-GHz Doppler scatterometer with a fan beam antenna and coherent detection was used to measure radar backscatter as a function of incidence angle. The radar scattering signature of the clear surface and signatures of the surface covered with various crude oil films are compared. Reductions in Ku band microwave backscatter up to 14 dB are observed for both treated and untreated LaRosa and Murban crude oil films deposited on the sea surface. Maximum Ku band sensitivity to the effects of the oil in terms of differential scatter is observed in the 25-35 deg incidence angle region.

Johnson, J. W.↗

NASA NTRS1981

Microwave remote sensing measurements of oil pollution on the ocean

Microwave and optical remote sensors were flown over fresh and weathered crude oil released from a surface research vessel and also over a slick formed on the sea by frozen oleyl alcohol cubes released from a helicopter. For the crude oil experiments, microwave radiometric measurements at 1.43, 2.65, 22, and 31 GHz are reported, along with the variable incidence angle scattering measurements at 13.9 GHz. For these experiments, unusual depressions in the L-band brightness temperature were observed, possibly related to dispersants applied to the crude oil. Similar depressions, but with much larger values, were observed over the oleyl alcohol monomolecular slicks. Images obtained at 31 and 22 GHz were used to infer oil volume, yielding values which bound the known amounts spilled. Ku band measurements obtained in repeated passes over crude oil slicks are also discussed.

Croswell, W. F.↗

NASA NTRS1979

Calculated scan characteristics of a large spherical reflector antenna

A previously published numerical method to calculate the radiation properties of parabolic reflectors has been modified to also include very large spherical reflectors. The method has been verified by comparing the calculated and the measured results for a 120-wavelength spherical reflector.

Agrawal, P. K.↗

NASA NTRS1978

On space deployable antennas and electronics: Plans and programs

Technology plans are presented for developing deployable reflectors having diameters of 30 to 300 meters and surface accuracies of several millimeters for L and X band applications, as well as for the electronic subsystems required for such large structures. An electromagnetic analysis method for predicting the radio frequency performance of large reflectors involves projecting field points onto an aperture plane. The data points in the aperture plane are quantized to produce contours of constant phase and amplitude. Far field patterns are then calculated using reduced computer storage.

Campbell, T. G.↗

NASA NTRS1976

Analysis of the radiation patterns of reflector antennas

The development and application of a numerical technique for rapid calculation of the far-field radiation patterns of a reflector antenna from either a measured or computed feed pattern are reported. The reflector is defined by the intersection of a cone with any surface of revolution or an offset sector of any surface of revolution. The feed is assumed to be linearly polarized and can have an arbitrary location. Both the copolarized and the cross-polarized reflector radiation patterns are computed. Calculations using the technique compare closely with measured radiation patterns of a waveguide-fed offset parabolic reflector. Unique features of this technique are freedom from restrictive feed assumptions and the numerical methods used in preparing the aperture-plane electric-field data for integration.

Kauffman, J. F.↗

NASA NTRS1975

Radiation from a homogeneous sphere mounted on a waveguide aperture

The results of an experimental and theoretical study of the effects of placing a dielectric sphere over the aperture of a circular waveguide are presented. A dielectric sphere excited by a Huygens' source is used as the theoretical model to interpret the experimental results. Calculations based on this model show good overall agreement with experimental patterns and directivity measurements. It is concluded that dielectric loaded antennas such as the dielectric sphere excited by a waveguide can produce directivities in excess of that obtained by a uniformly illuminated aperture of the same cross section, particularly for dielectric objects with dimensions of two wavelengths or less. Measurements of the coupling between these antennas show that they, in general, are not as closely coupled as plain apertures without dielectric object loading.

Croswell, W. F.↗

NASA NTRS1972

Waveguide excited dielectric spheres as feeds.

The results of experiments with dielectric obstacles of various geometries placed directly upon the apertures of circular and rectangular waveguides are presented. It is found that dielectric spheres, and in some instances dielectric cubes, two to four wavelengths in dimension produce directive patterns with low sidelobe levels. For some cross sections, the measured gain of these antennas is greater (6 dB in some cases) than that produced by optimum horns having apertures of identical cross section. The linearity of the polarization of these antennas is essentially the same as that of an open-end waveguide antenna.

Chatterjee, J. S.↗

NASA NTRS1972

Computations of the aperture admittance of an axial slot on a dielectric coated cylinder.

An extensive set of curves for the aperture admittance of a waveguide-fed axial slot on a coated metal cylinder is presented for both an underdense plasma and a Teflon coating. Examination of these curves shows that a slot on a cylinder with a Ka of 2 to 3 has about the same aperture admittance as the identical slot opening onto a flat ground plane.

Croswell, W. F.↗

NASA NTRS1972

Dielectric loaded aperture antennas.

This paper describes a primarily experimental study of the effect of placing dielectric obstacles having various geometries directly upon the aperture of circular and rectangular waveguide antennas. It is found that plexiglas spheres, and in some cases plexiglas cubes, one to four wavelengths in dimension, produce directive patterns with low sidelobes. The measured gain in some cases is 6 dB greater than the gain of an optimum horn having an equal aperture size.

Croswell, W. F.↗

NASA NTRS1971