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  • 1
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    Evaluation of frequency-selective reflector antenna systems (1993)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: The performance of a four-frequency (S/X/Ku/Ku bands) frequency-selective surface (FSS) with double-ring elements (the type of geometry particularly well suited for the circular polarization requirement of the NASA's Cassini project) is evaluated for a planar and a curved FSS subreflector in a dual reflector antenna system. Good agreement is obtained between the calculated and measured data for the planar FSS model. The FSS effects in a four-frequency Cassegrain reflector antenna were accurately evaluated by taking into account the surface curvature and the FSS subreflector's transmitted/reflected field variation as functions of the polarization and the incident angles with respect to the local coordinates.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Microwave and Optical Technology Letters (ISSN 0895-2477); 6; 3; p. 175-179.
    Format: text
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  • 2
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    Directivity versus element spacing for a scanning array (1993)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: The results of the present examination of the relationship of directivity with element spacings of linear, rectangular, and hexagonal arrays indicate that element spacings for optimal directivity vary with scan angles; this is especially true in the case of changes in aperture sizes and the appearance of grating lobes in the visible space. Grating lobe effects are more pronounced for isotropic than for directive element arrays. Directivity decreases as the beam scans off from boresight, and rapidly diminishes for scans over 30 deg. The hexagonal array has higher directivity for scans of less than 20 deg, and the linear array for those over 30 deg.
    Keywords: COMMUNICATIONS AND RADAR
    Type: Microwave and Optical Technology Letters (ISSN 0895-2477); 6; 6; p. 361-363.
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  • 3
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    A comparison of reflector antenna designs for wide-angle scanning (1989)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: Conventional reflector antennas are typically designed for up to + or - 20 beamwidths scan. An attempt was made to stretch this scan range to some + or - 300 beamwidths. Six single and dual reflector antennas were compared. It is found that a symmetrical parabolic reflector with f/D = 2 and a single circular waveguide feed has the minimum scan loss (only 0.6 dB at Theta sub 0 = 8 deg, or a 114 beamwidths scan). The scan is achieved by tilting the parabolic reflector by an angle equal to the half-scan angle. The f/D may be shortened if a cluster 7 to 19 elements instead of one element is used for the feed. The cluster excitation is adjusted for each new beam scan direction to compensate for the imperfect field distribution over the reflector aperture. The antenna can be folded into a Cassegrain configuration except that, due to spillover and blockage considerations, the amount of folding achievable is small.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA, Langley Research Center, Earth Science Geostationary Platform Technology; p 217-240
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  • 4
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    Method to determine the optimal parameters of the Arecibo 46.8-MHz antenna system (1986)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: The spherical reflector at the Arecibo Observatory (AO) offers great advantages for the design of simple and inexpensive high performance steerable antennas at VHF. Light and small feeds have the added benefit that they can be quickly installed in the Arecibo platform. It is important to evaluate the performance of any given feed including the effects of the spherical reflector. The optimization is emphasized of two parameters, namely, the distance below the focal point of the reflector and the beam width of a point feed. For the design of the feed at 46.8 MHz at the AO there were other requirements independent of MST (mesosphere stratosphere troposphere) work. The design of the primary array is discussed along with its performance with the AO spherical reflector.
    Keywords: COMMUNICATIONS AND RADAR
    Type: International Council of Scientific Unions, Middle Atmosphere Program. Handbook for MAP, Vol. 20; 5 p
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  • 5
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    A 640-MHz 32-megachannel real-time polyphase-FFT spectrum analyzer (1991)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A polyphase fast Fourier transform (FFT) spectrum analyzer being designed for NASA's Search for Extraterrestrial Intelligence (SETI) Sky Survey at the Jet Propulsion Laboratory is described. By replacing the time domain multiplicative window preprocessing with polyphase filter processing, much of the processing loss of windowed FFTs can be eliminated. Polyphase coefficient memory costs are minimized by effective use of run length compression. Finite word length effects are analyzed, producing a balanced system with 8 bit inputs, 16 bit fixed point polyphase arithmetic, and 24 bit fixed point FFT arithmetic. Fixed point renormalization midway through the computation is seen to be naturally accommodated by the matrix FFT algorithm proposed. Simulation results validate the finite word length arithmetic analysis and the renormalization technique.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommunications and Data Acquisition Report; p 132-140
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  • 6
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    Integrated reflector antenna design and analysis (1993)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Reflector antenna design is a mature field and most aspects were studied. However, of that most previous work is distinguished by the fact that it is narrow in scope, analyzing only a particular problem under certain conditions. Methods of analysis of this type are not useful for working on real-life problems since they can not handle the many and various types of perturbations of basic antenna design. The idea of an integrated design and analysis is proposed. By broadening the scope of the analysis, it becomes possible to deal with the intricacies attendant with modem reflector antenna design problems. The concept of integrated reflector antenna design is put forward. A number of electromagnetic problems related to reflector antenna design are investigated. Some of these show how tools for reflector antenna design are created. In particular, a method for estimating spillover loss for open-ended waveguide feeds is examined. The problem of calculating and optimizing beam efficiency (an important figure of merit in radiometry applications) is also solved. Other chapters deal with applications of this general analysis. The wide angle scan abilities of reflector antennas is examined and a design is proposed for the ATDRSS triband reflector antenna. The development of a general phased-array pattern computation program is discussed and how the concept of integrated design can be extended to other types of antennas is shown. The conclusions are contained in the final chapter.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA-CR-195164 , NAS 1.26:195164 , TR-94-4
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  • 7
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    Spur-reduced digital sinusoid synthesis (1993)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: This article presents and analyzes a technique for reducing the spurious signal content in digital sinusoid synthesis. Spurious-harmonic (spur) reduction is accomplished through dithering both amplitude and phase values prior to word-length reduction. The analytical approach developed for analog quantization is used to produce new bounds on spur performance in these dithered systems. Amplitude dithering allows output word-length reduction without introducing additional spurs. Effects of periodic dither similar to those produced by a pseudonoise (PN) generator are analyzed. This phase-dithering method provides a spur reduction of 6(M plus one) dB per phase bit when the dither consists of M uniform variates. While the spur reduction is at the expense of an increase in system noise, the noise power can be made white, making the power spectral density small. This technique permits the use of a smaller number of phase bits addressing sinusoid lookup tables, resulting in an exponential decrease in system complexity. Amplitude dithering allows the use of less complicated multipliers and narrower data paths in purely digital applications, as well as the use of coarse resolution, highly linear digital to analog converters (DAC's) to obtain spur performance limited by the DAC linearity rather than its resolution.
    Keywords: COMMUNICATIONS AND RADAR
    Type: The Telecommunications and Data Acquisition Report; p 91-104
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  • 8
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    A segmented mirror antenna for radiometers (1989)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: An antenna is designed for the radiometer application of the planned NASA Earth Science Geostationary Platforms in the 1990's. The antenna consists of two parts: a regular parabolic dish of 5 meters in diameter which converts the radiation from feeds into a collimated beam, and a movable mirror that redirects the beam to a prescribed scan direction. The mirror is composed of 28 segmented planar conducting plates, mostly one square meter in size. The secondary pattern of the antenna was analyzed based on a physical optics analysis. For frequencies between 50 and 230 GHz, and for a scan range of + or -8 deg (270 beamwidths scan at 230 GHz), the worst calculated beam efficiency is 95 percent. To cover such a wide frequency and scan range, each of the 28 plates is individually controlled for a tilting less than 4 deg, and for a sliding less than 0.5 cm. The sliding is done at discrete steps. At 230 GHz, a step size of 2 mil is sufficient. The plate positions must be reset for each frequency and for each scan direction. Once the position is set, the frequency bandwidth of the antenna is very narrow.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA-TM-102045 , E-4793 , NAS 1.15:102045
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  • 9
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    A comparison of reflector antenna designs for wide-angle scanning (1989)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Conventional reflector antennas are typically designed for up to + or - 20 beamwidths scan. An attempt was made to stretch this scan range to some + or - 300 beamwidths. Six single and dual reflector antennas were compared. It is found that a symmetrical parabolic reflector with f/D = 2 and a single circular waveguide feed has the minimum scan loss (only 0.6 dB at Theta sub 0 = 8 deg, or a 114 beamwidths scan). The scan is achieved by tilting the parabolic reflector by an angle equal to the half-scan angle. The f/D may be shortened if a cluster 7 to 19 elements instead of one element is used for the feed. The cluster excitation is adjusted for each new beam scan direction to compensate for the imperfect field distribution over the reflector aperture. The antenna can be folded into a Cassegrain configuration except that, due to spillover and blockage considerations, the amount of folding achievable is small.
    Keywords: COMMUNICATIONS AND RADAR
    Type: NASA-TM-101459 , E-4569 , NAS 1.15:101459 , NASA Technology Workshop for Earth Science Geostationary Platforms; Sep 21, 1988 - Sep 22, 1988; Hampton, VA; United States
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  • 10
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    Reflector spillover loss of an open-ended rectangular and circular waveguide feed (1990)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: An important and frequently used parameter in the design of a feed for a reflector antenna is the spillover loss. A set of spillover curves is presented for several common feeds, namely circular waveguides with TE11 or TE21 excitations, rectangular guides for the monopulse application, etc. The difference between two common methods of normalizing the feed input power is also discussed.
    Keywords: COMMUNICATIONS AND RADAR
    Type: IEEE Transactions on Antennas and Propagation (ISSN 0018-926X); 38; 940-942
    Format: text
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