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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Preliminary announcement of an 85 percent efficient reflector antenna

The aperture efficiency is defined in the usual way to include the spillovers, illumination, phase, and cross polarization efficiencies of the projected circular aperture. The antenna comprises a 157.1-lambda main reflector, a 47.1-lambda subreflector, and a 5.1-lambda aperture feedhorn. Both reflectors are shaped in such a way as to provide quasi-uniform aperture illumination with a degree of spillover control. By using an offset feed geometry, the configuration provides an unblocked main reflector. An analysis of expected performance yields a theoretical 86.5% aperture efficiency, whereas the measurement technique yields 84.9%. The estimated measurement tolerance is now + or - 3% (approximately + or - 0.15 dB) on a high confidence (approximately + or - 3sigma) basis.

Cha, A. G.↗
NASA NTRS1983

An offset dual shaped reflector with 84.5 percent efficiency

The theoretical and experimental study of a 1.5 m offset dual shaped reflector at 31.4 GHz is summarized. An efficiency of 84.5 percent, a possible record for reflector antennas of this size, was ascertained through careful measurements. For larger low noise reflector systems, a 2 to 3 dB improvement in gain over noise temperature ratio (G/T) performance over the state-of-the-art ultralow noise ground stations and 90 percent or better aperture efficiency now appear feasible.

Cha, A. G.↗
NASA NTRS1985

Segmented Trough Reflector

Segmented troughlike reflector for solar cells approach concentration effectiveness of true parabolic reflector yet simpler and less expensive. Walls of segmented reflector composed of reflective aluminized membrane. Lengthwise guide wire applies tension to each wall, thereby dividing each into two separate planes. Planes tend to focus Sunlight on solar cells at center of trough between walls. Segmented walls provide higher Sunlight concentration ratios than do simple walls.

Szmyd, W. R.↗
NASA NTRS1986

Controlled-Shape, Ultrasonic-Angle-Beam Standard Reflector

New ultrasonic angle-beam standard reflector uses impression of letter "l" steel-die stamp. NDE techniques and standard reflector apply to use of pulse-echo-type ultrasonic equipment for inspection of wrought metals including forgings and forging stock; rolled billet, bar or plate; and extruded bar, tube, and shapes. "l" reference standard reflector affords advantages of easy insertion in inspected item using common hand-tools and greatly reduced implementation time through elimination of machining operations.

Berry, J., Robertf.↗
NASA NTRS1985

Diffraction Analysis of Mesh Deployable Reflector Antennas

A formulation and many representative numerical results for mesh reflector antennas are presented. The reflection coefficient matrix for the prescribed mesh configuration was determined and the local coordinate system of the mesh cells at each point on the curved reflector surface was accentuated. A novel strip aperture model was used to formulate the transmission coefficient matrix for a variety of mesh cell configurations. Numerical data are tailored to the dimensions of a conceptually designed land mobile satellite system (LMSS) which employs a large mesh deployable offset parabolic antenna. Results are shown for an offset parabolic reflector with mesh surfaces similar to the mesh surface of tracking and data relay satellite system (TDRSS).

Rahmat-Samii, Y.↗
NASA NTRS1984

Control of jet shock associated noise by a reflector

Due to the discrete noise radiation from an imperfectly expanded supersonic jet, a reflecting surface can be used to impart changes to the jet. Powell's model of screech generation is used to analytically design the reflector size necessary to cause destruction of the feedback cycle inherent to screech production. This enabled experiments to be performed over a wide range of supersonic flow conditions for a sonic nozzle in the presence of a remotely controlled reflector. Results show that not only the screech amplitude can be controlled, but that marked changes in the jet structure and broadband noise production can also be effected by changing the position of the reflector.

Norum, T. D.↗
NASA NTRS1985

Secondary pattern computation of an arbitrarily shaped main reflector

The secondary pattern of a perfectly conducting offset main reflector being illuminated by a point feed at an arbitrary location is studied. The method of analysis is based upon the application of the Fast Fourier Transform (FFT) to the aperture fields obtained using geometrical optics (GO) and geometrical theory of diffraction (GTD). Key features of the present work are (1) the reflector surface is completely arbitrary, (2) the incident field from the feed is most general with arbitrary polarization and location, and (3) the edge diffraction is calculated by either UAT or by UTD. Comparison of this technique for an offset parabolic reflector with the Jacobi-Bessel and Fourier-Bessel techniques shows good agreement. Near field, far field, and scan data of a large refelctor are presented.

Lee, S. W.↗
NASA NTRS1986

Electromagnetic backscattering by corner reflectors

The Geometrical Theory of Diffraction (GTD), which supplements Geometric Optics (GO), and the Physical Theory of Diffraction (PTD), which supplements Physical Optics (PO), are used to predict the backscatter cross sections of dihedral corner reflectors which have right, obtuse, or acute included angles. These theories allow individual backscattering mechanisms of the dihedral corner reflectors to be identified and provide good agreement with experimental results in the azimuthal plane. The advantages and disadvantages of the geometrical and physical theories are discussed in terms of their accuracy, usefulness, and complexity. Numerous comparisons of analytical results with experimental data are presented. While physical optics alone is more accurate and more useful than geometrical optics alone, the combination of geometrical optics and geometrical diffraction seems to out perform physical optics and physical diffraction when compared with experimental data, especially for acute angle dihedral corner reflectors.

Balanis, C. A.↗
NASA NTRS1987

Integrated structure electromagnetic optimization of large space antenna reflectors

The requirements for extremely precise and powerful large space antenna reflectors have motivated the development of a procedure for shape control of the reflector surface. A mathematical optimization procedure has been developed which improves antenna performance while minimizing necessary shape correction effort. In contrast to previous work which proposed controlling the rms distortion error of the surface thereby indirectly improving antenna performance, the current work includes electromagnetic (EM) performance calculations as an integral of the control procedure. The application of the procedure to a radiometer design with a tetrahedral truss backup structure demonstrates the potential for significant improvement. The results indicate the benefit of including EM performance calculations in procedures for shape control of large space antenna reflectors.

Padula, Sharon L.↗
NASA NTRS1987

User manual for semi-circular compact range reflector code: Version 2

A computer code has been developed at the Ohio State University ElectroScience Laboratory to analyze a semi-circular paraboloidal reflector with or without a rolled edge at the top and a skirt at the bottom. The code can be used to compute the total near field of the reflector or its individual components at a given distance from the center of the paraboloid. The code computes the fields along a radial, horizontal, vertical or axial cut at that distance. Thus, it is very effective in computing the size of the sweet spot for a semi-circular compact range reflector. This report describes the operation of the code. Various input and output statements are explained. Some results obtained using the computer code are presented to illustrate the code's capability as well as being samples of input/output sets.

Gupta, Inder J.↗
NASA NTRS1987

Integrated structural electromagnetic optimization of large space antenna reflectors

The requirements for extremely precise and powerful large space antenna reflectors have motivated the development of a procedure for shape control of the reflector surface. A mathematical optimization procedure has been developed which improves antenna performance while minimizing necessary shape correction effort. In contrast to previous work which proposed controlling the rms distortion error of the surface thereby indirectly improving antenna performance, the current work includes electromagnetic (EM) performance calculations as an integral part of the control procedure. The application of the procedure to a radiometer design with a tetrahedral truss backup structure demonstrates the potential for significant improvement. The results indicate the benefit of including EM performance calculations in procedures for shape control of large space antenna reflectors.

Padula, S. L.↗
NASA NTRS1988

Tool Removes Arc-Light Reflectors

New tool makes installation, adjustment, and removal of self-clamping arc-light reflectors on welding torches easy and simple. Consists of two arms reaching around from back of torch. Lip on each arm hooks into slot of arc-light reflector clamp. When handles squeezed together, slot spreads enabling easy installation, adjustment, or removal of reflector without disassembly of welding apparatus.

Gordon, Stephen S.↗
NASA NTRS1986

A new type of lamp and reflector for I.R. simulation

The lamps and reflectors used for infrared radiation simulation tests at ESTEC did not allow researchers to predict the intensity needed for test conditions with the desired accuracy. This was due to poor reproducibility of the polar diagrams, the unknown contribution of the radiation in the long wavelength range in vacuum, imperfections in the quartz bulbs, and misalignment of the lamp in the reflector. When using a 1000 W coiled spiral quartz lamp with a diffuse reflector, these shortcomings are overcome. Due to the good reproducibility, an overall accuracy within plus or minus 2 percent should be obtained.

Saenger, G.↗
NASA NTRS1987

New main reflector, subreflector and dual chamber concepts for compact range applications

A compact range is a facility used for the measurement of antenna radiation and target scattering problems. Most presently available parabolic reflectors do not produce ideal uniform plane waves in the target zone. Design improvements are suggested to reduce the amplitude taper, ripple and cross polarization errors. The ripple caused by diffractions from the reflector edges can be reduced by adding blended rolled edges and shaping the edge contour. Since the reflected edge continues smoothly from the parabola onto the rolled surface, rather than being abruptly terminated, the discontinuity in the reflected field is reduced which results in weaker diffracted fields. This is done by blending the rolled edges from the parabola into an ellipse. An algorithm which enables one to design optimum blended rolled edges was developed that is based on an analysis of the continuity of the surface radius of curvature and its derivatives across the junction. Futhermore, a concave edge contour results in a divergent diffracted ray pattern and hence less stray energy in the target zone. Design equations for three-dimensional reflectors are given. Various examples were analyzed using a new physical optics method which eliminates the effects of the false scattering centers on the incident shadow boundaries. A Gregorian subreflector system, in which both the subreflector and feed axes are tilted, results in a substantial reduction in the amplitude taper and cross polarization errors. A dual chamber configuration is proposed to eliminate the effects of diffraction from the subreflector and spillover from the feed. A computationally efficient technique, based on ray tracing and aperture integration, was developed to analyze the scattering from a lossy dielectric slab with a wedge termination.

Pistorius, C. W. I.↗
NASA NTRS1987

The design of blended rolled edges for compact range main reflectors

In a compact range (which employs a parabolic main reflector to convert the spherical wave originating from a point source to a reflected plane wave) the edge-generated diffractive fields degrade the uniformity of the plane wave. To reduce the energy diffracted from the edge surfaces, terminations can be added to the parabola, such that the slope of the surface is continuous at the junctions. This paper discusses the problem of designing an optimized rolled edge. Two examples of reflectors with rolled edges are presented, one with elliptic rolled edges and the other with blended rolled edges, both adhering to the same constraints on minimum operating frequency and size. It was found that the reflector with the cosine-squared blended rolled edges has a much larger usable target zone than the one with the elliptic rolled edges.

Pistorius, C. W. I.↗
NASA NTRS1989

Optical Receivers With Rough Reflectors

Receiver for optical communications uses rough reflector instead of diffraction-limited reflector customarily thought necessary for such systems. Rough reflector collects and focuses optical signal. Other receiver components include narrow-passband optical filter to reject out-of-band background radiation, spatial filter to limit receiver field of view, optical-detector array (typically two concentric detectors), and postdetection processor to reconstruct transmitted message.

Vilnrotter, Victor A.↗
NASA NTRS1989

Adaptive feed array compensation system for reflector antenna surface distortion

The feasibility of a closed loop adaptive feed array system for compensating reflector surface deformations has been investigated. The performance characteristics (gain, sidelobe level, pointing, etc.) of large communication antenna systems degrade as the reflector surface distorts mainly due to thermal effects from a varying solar flux. The compensating systems described in this report can be used to maintain the design performance characteristics independent of thermal effects on the reflector surface. The proposed compensating system employs the concept of conjugate field matching to adjust the feed array complex excitation coefficients.

Acosta, Roberto J.↗
NASA NTRS1989

Feasibility study of a synthesis procedure for array feeds to improve radiation performance of large distorted reflector antennas

Surface errors on parabolic reflector antennas degrade the overall performance of the antenna. The errors are in the form of roughness on the surface, distortions in the shape, or structural design details. They cause amplitude and phase errors in the aperture field which lower the gain, raise the sidelobes, and fill in the nulls. These are major problems in large space reflector antennas. Planned mobile satellite communications systems having limited signal margin need high gain from the space reflectors. Future multiple beam antenna systems requiring spatial isolation to allow frequency reuse could be rendered useless if high sidelobes are present. High sidelobes are also responsible for noise. Ways of compensating for surface errors by pattern synthesis using an array of feed antennas are examined. Pattern corrections are directed specifically toward portions of the pattern requiring improvements. The pattern synthesis does not require knowledge of the surface errors. Both the amplitude and phase of the high side lobes caused by the distortion are required.

Stutzman, W. L.↗