26,351 research outputs found
Shaped cassegrain reflector antenna
Design equations are developed to compute the reflector surfaces required to produce uniform illumination on the main reflector of a cassegrain system when the feed pattern is specified. The final equations are somewhat simple and straightforward to solve (using a computer) compared to the ones which exist already in the literature. Step by step procedure for solving the design equations is discussed in detail
RFI emitter location techniques
The possibility is discussed of using Doppler techniques for determining the location of ground based emitters causing radio frequency interference with low orbiting satellites. An error analysis indicates that it is possible to find the emitter location within an error range of 2 n.mi. The parameters which determine the required satellite receiver characteristic are discussed briefly along with the non-real time signal processing which may by used in obtaining the Doppler curve. Finally, the required characteristics of the satellite antenna are analyzed
Bifocal dual reflector antenna
A bifocal dual reflector antenna is similar to and has better scan capability than classical cassegrain reflector antenna. The method used in determining the reflector surfaces is a modification of a design method for the dielectric bifocal lens. The three dimensional dual reflector is obtained by first designing an exact (in geometrical optics sense) two-point corrected two dimensional reflector and then rotating it around its axis of symmetry. A point by point technique is used in computing the reflector surfaces. Computed radiation characteristics of the dual reflector are compared with those of a cassegrain reflector. The results confirm that the bifocal antenna has superior performance
Understanding of the phase transformation from fullerite to amorphous carbon at the microscopic level
We have studied the shock-induced phase transition from fullerite to a dense
amorphous carbon phase by tight-binding molecular dynamics. For increasing
hydrostatic pressures P, the C60-cages are found to polymerise at P<10 GPa, to
break at P~40 GPa and to slowly collapse further at P>60 GPa. By contrast, in
the presence of additional shear stresses, the cages are destroyed at much
lower pressures (P<30 GPa). We explain this fact in terms of a continuum model,
the snap-through instability of a spherical shell. Surprisingly, the relaxed
high-density structures display no intermediate-range order.Comment: 5 pages, 3 figure
Pressure effects on the magnetic structure in La0.5Ca0.5-xSrxMnO3 (0.1 -< x -< 0.4) manganites
The effect of high pressure (0 - 8 GPa) on the magnetic structure of
polycrystalline samples of La0.5Ca0.5-xSrxMnO3 (0.1 -< x -< 0.4) manganites at
5 K is investigated using neutron diffraction technique. Application of
pressure is found to modify the previously reported magnetic structure,
observed under ambient conditions, in these compounds [I. Dhiman et al., Phys.
Rev. B 77, 094440 (2008)]. In x = 0.1 composition, at 4.6(2) GPa and beyond,
A-type antiferromagnetic structure is found to coexist with CE-type
antiferromagnetic phase, observed at ambient pressure, with TN ~ 150 K. For x =
0.3 sample, as a function of pressure the CE-type phase is fully suppressed at
2.3(1) GPa and A-type antiferromagnetic phase is favored. Further Sr doping at
x = 0.4, the A-type antiferromagnetic phase is observed at ambient pressure and
for T < TN (~ 250K). This phase is retained in the studied pressure range.
However, the magnetic moment progressively reduces with increasing pressure,
indicating the suppression of A-type antiferromagnetic phase. The present study
brings out the fragile nature of the CE-type antiferromagnetic state in half
doped manganites as a function of pressure and disorder \sigma 2. We observe
that pressure required for destabilizing the CE-type antiferromagnetic state is
reduced with increasing disorder \sigma 2. External pressure and changing
A-site ionic radii have analogous effect on the magnetic structure.Comment: 9 pages, 6 figures, 1 table, To appear in Physical Review
Vibration Characteristics of Aircraft Engine-Bladed-Disk Assembly
This paper is concerned with the vibration characteristics of a gas-turbine blade-disk assembly and a third stage of compressor blade-disk assembly of an orpheus aircraft engine. The assembly is analyzed by considering each component individually and then combining them together with a receptance coupling technique by matching forces and displacements at each junction point. The blade is modelled by number of free-free aerofoil section beams staggered at different angles to the plane of the disk, and the non-uniform disk is modelled as numbers of concentric annuli. The natural frequencies and mode shapes for each case have been obtained. Results obtained are verified by testing both the above assemblies on a microprocessor based vibration exciter and real time analyzer. The mode shape corresponding to each natural frequency was obtained by probing with hand held accelerometer
The interparticle interaction and crossover in critical lines on field-temperature plane in PrSrMnO nanoparticles
The magnetic properties and the effects of interparticle interaction on it
have been studied in nanoparticles of half doped PrSrMnO.
Three samples consisting of nanoparticles of different average particle sizes
are synthesized to render the variation in interparticle interaction. Though
all the samples crystallize in the same structure to that of their bulk
compound, the low temperature ferromagnetic-antiferromagnetic transition, which
is present in bulk compound, is not evident in the nanoparticles. Linear as
well as nonlinear ac susceptibility coupled with dc magnetic measurements have
shown the superparamagnetic behavior of these nanoparticles where the blocking
temperature increases with the increasing particle size. Presence of
interparticle interaction is confirmed from the temperature variation of
coercive field and the analysis of frequency dependent ac susceptibility. We
have identified the nature of this interaction to be of dipolar type, and show
that its strength decreases with the increasing particle size. The effect of
this dipolar interaction on magnetic properties is intriguing as the compounds
exhibit crossover from de Almeida-Thouless to Gabay-Toulouse like critical
lines on field-temperature plane above their respective interaction field. In
agreement with theoretical prediction, we infer that this crossover is induced
by the unidirectional anisotropy arising from interparticle interaction, and
this is confirmed from the presence of exchange bias phenomenon.Comment: To appear in Phys. Rev.
Transport in quantum wires
With a brief introduction to one-dimensional channels and conductance
quantisation in mesoscopic systems, we discuss some recent experimental puzzles
in these systems, which include reduction of quantised conductances and an
interesting {\it odd-even} effect in the presence of an in-plane magnetic
field. We then discuss a recent non-homogeneous Luttinger liquid model proposed
by us, which addresses and gives an explanation for the reduced conductances
and the {\it odd-even} effect. We end with a brief summary and discussion of
future projects.Comment: Talk presented at the International Discussion Meeting on Mesoscopic
and Disordered systems, December, 2000, 16 pages, 2 figure
Autonomous detection and anticipation of jam fronts from messages propagated by inter-vehicle communication
In this paper, a minimalist, completely distributed freeway traffic
information system is introduced. It involves an autonomous, vehicle-based jam
front detection, the information transmission via inter-vehicle communication,
and the forecast of the spatial position of jam fronts by reconstructing the
spatiotemporal traffic situation based on the transmitted information. The
whole system is simulated with an integrated traffic simulator, that is based
on a realistic microscopic traffic model for longitudinal movements and lane
changes. The function of its communication module has been explicitly validated
by comparing the simulation results with analytical calculations. By means of
simulations, we show that the algorithms for a congestion-front recognition,
message transmission, and processing predict reliably the existence and
position of jam fronts for vehicle equipment rates as low as 3%. A reliable
mode of operation already for small market penetrations is crucial for the
successful introduction of inter-vehicle communication. The short-term
prediction of jam fronts is not only useful for the driver, but is essential
for enhancing road safety and road capacity by intelligent adaptive cruise
control systems.Comment: Published in the Proceedings of the Annual Meeting of the
Transportation Research Board 200
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