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  • 1
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    Inflection point caustic problems and solutions for high-gain dual-shaped reflectors (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The singular nature of the uniform geometrical theory of diffraction (UTD) subreflector scattered field at the vicinity of the main reflector edge (for a high-gain antenna design) is investigated. It is shown that the singularity in the UTD edge-diffracted and slope-diffracted fields is due to the reflection distance parameter approaching infinity in the transition functions. While the geometrical optics (GO) and UTD edge-diffracted fields exhibit singularities of the same order, the edge slope-diffracted field singularity is more significant and is substantial for greater subreflector edge tapers. The diffraction analysis of such a subreflector in the vicinity of the main reflector edge has been carried out efficiently and accurately by a stationary phase evaluation of the phi-integral, whereas the theta-integral is carried out numerically. Computational results from UTD and physical optics (PO) analysis of a 34-m ground station dual-shaped reflector confirm the analytical formulations for both circularly symmetric and offset asymmetric subreflectors. It is concluded that the proposed PO(theta)GO(phi) technique can be used to study the spillover or noise temperature characteristics of a high-gain reflector antenna efficiently and accurately.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); 38; 202-211
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    NASA TECHNICAL REPORTS
  • 2
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    Time domain version of the uniform GTD (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The uniform geometrical theory of diffraction (UTD) solutions can be inversely transformed analytically to obtain a time-domain version of the UTD. The time-domain solutions are valid in the early time period where an observation time t is close to the time after the arrival of the first diffracted wavefront. Comparisons with GTD (geometrical theory of diffraction) and also with available rigorous results reveal that the UTD solutions are accurate for substantial early time periods while the GTD results are valid for very early time periods.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); 38; 1757-176
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    NASA TECHNICAL REPORTS
  • 3
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    Low-loss offset feeds for electrically large symmetric dual-reflector antennas (1987)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Circularly symmetric, dual reflector, high gain antenna systems often require feeds placed off the system's axis because of the need for multiple feeds to use the reflector antenna. Also, the constraint requiring the hyperboloid or shaped subreflector to remain circularly symmetric is sometimes added. In a Cassegrainian system, the subreflector and feed may be rotated off axis around the paraboloid focus and retain main reflector focusing. However, substantial spillover results in considerable noise with a high gain/low noise temperature system. In a shaped system, the tilt of the shaped subreflector and feed together results in substantial defocusing as well as spillover noise. If the subreflector is tilted approximately one-half the angle of the feed tilt in either the Casegrainian or the dual shaped reflector antenna, it is found that spillover and noise are substantially reduced with tolerable defocusing. An extensive numerical analysis of these effects was conducted to determine the characteristics of a planned 70-meter, dual shaped reflector versus Cassegrainian antenna and to gain some understanding of the cause of the observed effects.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); AP-35; 745-755
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    NASA TECHNICAL REPORTS
  • 4
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    Interpolation methods for shaped reflector analysis (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The diffraction analysis of reflector surfaces which are described only at a discrete set of locations usually leads to the requirement of an interpolation to determine the surface characteristics over a continuum of locations. Two methods of interpolation, the global and the local methods, are presented. The global interpolation representation is a closed-form or series expression valid over the entire surface. The coefficients of a series expression are found by an integration of all of the raw data. Since the number of coefficients used to describe the surface is much smaller than the number of raw data points, the integration effectively provides a smoothing of the raw data. The local interpolation provides a closed-form expression for only a small area of the reflector surface. The subreflector is divided into sectors each of which has constant discretized data. Each area segment is then locally described by a two-dimensional quadratic surface. The second derivative data give the desired smoothed values.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); 36; 441-444
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    NASA TECHNICAL REPORTS
  • 5
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    Design considerations for beamwaveguide in the NASA Deep Space Network (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A generalized solution is found for retrofitting a large dual-shaped reflector antenna for a beamwaveguide. The design is termed as a bypass beamwaveguide. Both highpass design feed imaging and bandpass design feed imaging are considered. Each design was studied using geometrical optics, Gaussian wave analysis, and both low-frequency and high-frequency diffraction analysis. An important extension of the Mizusawa-Kitsuregawa criteria was discovered (Zusama and Kitsuregawa, 1973). The principle revealed shows how a two-reflector cell, although in itself distorting, may be combined with a second cell which compensates for the first and delivers an output beam which is a good image of the input beam.
    Keywords: SPACE COMMUNICATIONS, SPACECRAFT COMMUNICATIONS, COMMAND AND TRACKING
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); 36; 1779-178
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    NASA TECHNICAL REPORTS
  • 6
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    Caustic Singularities Of High-Gain, Dual-Shaped Reflectors (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Report presents study of some sources of error in analysis, by geometric theory of diffraction (GTD), of performance of high-gain, dual-shaped antenna reflector. Study probes into underlying analytic causes of singularity, with view toward devising and testing practical methods to avoid problems caused by singularity. Hybrid physical optics (PO) approach used to study near-field spillover or noise-temperature characteristics of high-gain relector antenna efficiently and accurately. Report illustrates this approach and underlying principles by presenting numerical results, for both offset and symmetrical reflector systems, computed by GTD, PO, and PO/GO methods.
    Keywords: ELECTRONIC COMPONENTS AND CIRCUITS
    Type: NPO-18046 , NASA Tech Briefs (ISSN 0145-319X); 15; 4; P. 38
    Format: text
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    NASA TECHNICAL REPORTS
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