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  • COMMUNICATIONS AND RADAR  (5)
  • AIRCRAFT INSTRUMENTATION  (1)
  • Occupational Health and Environmental Toxicology
  • 1995-1999
  • 1990-1994  (7)
  • 1960-1964
  • 1993  (4)
  • 1990  (3)
  • 1
    Electronic Resource
    Electronic Resource
    New York, NY [u.a.] : Wiley-Blackwell
    Bioelectromagnetics 11 (1990), S. 37-45 
    ISSN: 0197-8462
    Keywords: cyclic magnetic fields ; embryogenesis ; developmental delay ; Life and Medical Sciences ; Occupational Health and Environmental Toxicology
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Biology , Physics
    Notes: Continuous exposure of sea urchin (Strongylocentrotus purpuratus) embryos at 18°C to a cyclic 60-Hz magnetic field at 0.1 mT rms beginning 4 min after insemination caused a significant developmental delay during the subsequent 23 hours. No delay in development was recorded for periods up to 18 hours after fertilization. At 18 h, most embryos were in the mesenchyme blastula stage. At 23 h, most control embryos were in mid-gastrula whereas most magnetic-field-exposed embryos were in the early gastrula stage. Thus an estimated 1-h delay occurred between these developmental stages. The results are discussed in terms of possible magnetic-field modification of transcription as well as interference with cell migration during gastrulation. The present study extends and supports the growing body of information about potential effects of exposures to extremely-low-frequency (ELF) magnetic fields on developing organisms.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
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  • 2
    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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  • 3
    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.
    Format: text
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  • 4
    Publication Date: 2013-08-31
    Description: NASA is currently investigating the readiness of Advanced Sensors and Instrumentation to meet the requirements of new initiatives in space. The following technical objectives and technologies are briefly discussed: smart and nonintrusive sensors; onboard signal and data processing; high capacity and rate adaptive data acquisition systems; onboard computing; high capacity and rate onboard storage; efficient onboard data distribution; high capacity telemetry; ground and flight test support instrumentation; power distribution; and workstations, video/lighting. The requirements for high fidelity data (accuracy, frequency, quantity, spatial resolution) in hostile environments will continue to push the technology developers and users to extend the performance of their products and to develop new generations.
    Keywords: AIRCRAFT INSTRUMENTATION
    Type: NASA, Washington, Space Transportation Avionics Technology Symposium. Volume 2: Conference Proceedings; p 521-555
    Format: application/pdf
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  • 5
    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
    Format: application/pdf
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  • 6
    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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  • 7
    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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