7,844 research outputs found
RF noise suppression using the photodielectric effect in semiconductors
Technique using photodielectric effect of semiconductor in high-Q superconductive cavity gives initial improvement of 2-4 db in signal-to-noise enhancement of conventional RF communication systems. Wide band signal plus noise can be transmitted through a narrow-band cavity due to parametric perturbation of the cavity frequency or phase
System performance conclusions
The advantages and disadvantages of reducing power levels and using antennas with diameters smaller than 1 Km were evaluated if rectenna costs and land usage requirements become major factors, operating at 5800 megahertz should be considered. Three sequences (random, incoherent phasing, and concentric rings - center to edge) provided satisfactory performance in that the resultant sidelobe levels during startup/ shutdown were lower than the steady-state levels present during normal operations. Grating lobe peaks and scattered power levels were used to determine the array/subarray mechanical alignment requirements. The antenna alignment requirement is 1 min or 3 min depending on phase control configuration. System error parameters were defined to minimize scattered microwave power
Smaller SPS system sizing tradeoffs
The solar power satellite and associated microwave system was reoptimized with larger antennas (at 2.45 GHz), reduced output powers, and smaller rectennas. Four constraints were considered: (1) the 23 mw/sq cm ionospheric limit; (2) a higher (54 mW/sq cm) ionospheric limit; (3) the 23 KW/sq cm thermal limit in the antenna; and (4) an improved thermal design allowing 33% additional waste heat. The differential costs in electricity for seven antenna/rectenna configurations operating at 2.45 GHz and five satellite systems operating at 5.8 GHz were calculated. It is concluded that a larger antenna/smaller rectenna configurations are economically feasible under certain conditions; a transmit antenna diameters should be limited to 1 to 1.5 Km for 2.45 GHz operation and .75 to 1.0 Km for 5.8 GHz; the present ionospheric limit of 23 mw/sq cm is probably too low and should be raised after ionospheric heating tests and studies are completed; the 5.8 GHz configurations are constrainted by antenna thermal limitations, rather than ionospheric limits; and multiple (two to four) antennas on a single solar satellite are recommended regardless of the particular antenna/rectenna configuration chosen
Microwave system performance summary
The design of the microwave system for the solar power satellite is described. Design modifications recommended include changes in phase control to the power module level, a reduction in allowable amplitude jitter, the use of metal matrix waveguides, and sequences for startup/shutdown procedures. Investigations into reshaping the beam pattern to improve overall rectenna collection efficiency and improve sidelobe control are surveyed
Polycomb group protein complexes exchange rapidly in living Drosophila
Fluorescence recovery after photobleaching (FRAP) microscopy was used to determine the kinetic properties of Polycomb group (PcG) proteins in whole living Drosophila organisms (embryos) and tissues (wing imaginal discs and salivary glands). PcG genes are essential genes in higher eukaryotes responsible for the maintenance of the spatially distinct repression of developmentally important regulators such as the homeotic genes. Their absence, as well as overexpression, causes transformations in the axial organization of the body. Although protein complexes have been isolated in vitro, little is known about their stability or exact mechanism of repression in vivo. We determined the translational diffusion constants of PcG proteins, dissociation constants and residence times for complexes in vivo at different developmental stages. In polytene nuclei, the rate constants suggest heterogeneity of the complexes. Computer simulations with new models for spatially distributed protein complexes were performed in systems showing both diffusion and binding equilibria, and the results compared with our experimental data. We were able to determine forward and reverse rate constants for complex formation. Complexes exchanged within a period of 1-10 minutes, more than an order of magnitude faster than the cell cycle time, ruling out models of repression in which access of transcription activators to the chromatin is limited and demonstrating that long-term repression primarily reflects mass-action chemical equilibria
Spontaneous Breaking of Translational Invariance in One-Dimensional Stationary States on a Ring
We consider a model in which positive and negative particles diffuse in an
asymmetric, CP-invariant way on a ring. The positive particles hop clockwise,
the negative counterclockwise and oppositely-charged adjacent particles may
swap positions. Monte-Carlo simulations and analytic calculations suggest that
the model has three phases; a "pure" phase in which one has three pinned blocks
of only positive, negative particles and vacancies, and in which translational
invariance is spontaneously broken, a "mixed" phase with a non-vanishing
current in which the three blocks are positive, negative and neutral, and a
disordered phase without blocks.Comment: 7 pages, LaTeX, needs epsf.st
Analysis of the low-energy differential cross sections of the CHAOS Collaboration
This paper presents the results of an analysis of the low-energy
differential cross sections, acquired by the CHAOS Collaboration at TRIUMF
\cite{chaos,denz}. We first analyse separately the and the
elastic-scattering measurements on the basis of standard low-energy
parameterisations of the - and p-wave -matrix elements. After the removal
of the outliers, we subject the truncated elastic-scattering
databases into a common optimisation scheme using the ETH model \cite{glmbg};
the optimisation failed to produce reasonable values for the model parameters.
We conclude that the problems we have encountered in the analysis of these data
are due to the shape of the angular distributions of their
differential cross sections
An updated analysis of NN elastic scattering data to 1.6 GeV
An energy-dependent and set of single-energy partial-wave analyses of
elastic scattering data have been completed. The fit to 1.6~GeV has been
supplemented with a low-energy analysis to 400 MeV. Using the low-energy fit,
we study the sensitivity of our analysis to the choice of coupling
constant. We also comment on the possibility of fitting data alone. These
results are compared with those found in the recent Nijmegen analyses. (Figures
may be obtained from the authors upon request.)Comment: 17 pages of text, VPI-CAPS-7/
Extraction of the coupling constant from NN scattering data
We reexamine Chew's method for extracting the coupling constant from
np differential cross section measurements. Values for this coupling are
extracted below 350 MeV, in the potential model region, and up to 1 GeV. The
analyses to 1~GeV have utilized 55 data sets. We compare these results to those
obtained via mapping techniques. We find that these two methods give
consistent results which are in agreement with previous Nijmegen
determinations.Comment: 12 pages of text plus 2 figures. Revtex file and postscript figures
available via anonymous FTP at ftp://clsaid.phys.vt.edu/pub/n
Electroexcitation of the Roper resonance from CLAS data
The helicity amplitudes of the electroexcitation of the Roper resonance on
proton are extracted at 1.7 < Q2 < 4.2 GeV2 from recent high precision
JLab-CLAS cross sections data and longitudinally polarized beam asymmetry for
pi+ electroproduction on protons. The analysis is made using two approaches,
dispersion relations and unitary isobar model, which give consistent results.
It is found that the transverse helicity amplitude for the gamma* p -->
P11(1440) transition, which is large and negative at Q2=0, becomes large and
positive at Q2 ~ 2 GeV2, and then drops slowly with Q2. Longitudinal helicity
amplitude, that was previously found from CLAS data as large and positive at
Q2=0.4,0.65 GeV2, drops with Q2. These results rule out the presentation of
P11(1440) as a 3qG hybrid state, and provide strong evidence in favor of this
resonance as a first radial excitation of the 3q ground state.Comment: 3 pages, 2 figures, Talk on the Workshop on "The Physics of Excited
Nucleons", Bonn, Germany, October 200
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