11,875 research outputs found
Land vehicle antennas for satellite mobile communications
The RF performance, size, pointing system, and cost were investigated concepts are: for a mechanically steered 1 x 4 tilted microstrip array, a mechanically steered fixed-beam conformal array, and an electronically steered conformal phased array. Emphasis is on the RF performance of the tilted 1 x 4 antenna array and methods for pointing the various antennas studied to a geosynchronous satellite. An updated version of satellite isolations in a two-satellite system is presented. Cost estimates for the antennas in quantities of 10,000 and 100,000 unites are summarized
Estimation and control of non-linear and hybrid systems with applications to air-to-air guidance
Issued as Progress report, and Final report, Project no. E-21-67
Concepts and cost trade-offs for land vehicle antennas in satellite mobile communications
Several antenna design concepts, operating at UHF (821 to 825 MHz transmit and 866 to 870 MHz receive bands), with gain ranging between 6 and 12 dBic, that are suitable for land mobile vehicles are presented. The antennas may be used within CONUS and ALASKA to communicate to and from a geosynchronous satellite. Depending on the type of steering mechanism, the antennas are broken down into three categories; (1) electronically scanned arrays with phase shifters, (2) electronically switched arrays with switchable power dividers/combiners, and (3) mechanically steered arrays. The operating characteristics of two of these design concepts, one a conformal antenna with electronic beam steering and the other a nonconformal design with mechanical steering, were evaluated with regard to two and three satellite system. Cost estimates of various antenna concepts were made and plotted against their overall gain performance
Breakdown performance of vacuum circuit breakers using alternative CF3I-CO2 insulation gas mixture
In this paper, sulphur hexafluoride (SF6) insulated vacuum interrupter circuit breakers have been used to explore replacing SF6 gas with a trifluoroiodomethane and carbon dioxide (CF3I-CO2) gas mixture. The search for an alternative insulation gas is driven by the well known extreme global warming potential of SF6. For this purpose, the circuit breaker gas compartment of a piece of gas insulated switchgear (GIS) was filled with a CF3I gas mixture and then tested using lightning impulses up to the rated withstand strength. The tested ring main unit was initially designed to be insulated with SF6 gas. The unit is a three-phase switchgear containing two switches per phase; a selector interlock and a vacuum bottle circuit breaker per phase. The test programme performed in this investigation demonstrated the breakdown performance and insulation strength of the new gas mixture as well as the vacuum circuit breakers behavior when insulated with a new insulation medium. Data on the dielectric properties of the proposed gas mixture is presented, and the performance of the tested vacuum circuit breaker is discussed. Promising results are obtained which indicate the suitability of this more-environmentally friendly gas for high voltage insulation purposes
Detection of vibronic bands of C in a translucent cloud towards HD 169454
We report the detection of eight vibronic bands of C, seven of which have
been hitherto unobserved in astrophysical objects, in the translucent cloud
towards HD~169454. Four of these bands are also found towards two additional
objects: HD~73882 and HD~154368. Very high signal-to-noise ratio (1000
and higher) and high resolving power () UVES-VLT spectra (Paranal,
Chile) allow for detecting novel spectral features of C, even revealing
weak perturbed features in the strongest bands. The work presented here
provides the most complete spectroscopic survey of the so far largest carbon
chain detected in translucent interstellar clouds. High-quality laboratory
spectra of C are measured using cavity ring-down absorption spectroscopy in
a supersonically expanding hydrocarbon plasma, to support the analysis of the
identified bands towards HD~169454. A column density of N(C) = cm is inferred and the excitation of the molecule
exhibits two temperature components; K for the low-
states and K for the high- tail. The rotational
excitation of C is reasonably well explained by models involving a
mechanism including inelastic collisions, formation and destruction of the
molecule, and radiative pumping in the far-infrared. These models yield gas
kinetic temperatures comparable to those found for . The assignment of
spectral features in the UV-blue range 3793-4054 \AA\ may be of relevance for
future studies aiming at unravelling spectra to identify interstellar molecules
associated with the diffuse interstellar bands (DIBs).Comment: 15 pages, 13 figures, submitted to MNRA
PERFORMANCE OF PARALLEL SURGE ARRESTERS
The operation of parallel surge arresters can improve energy absorption capability if the arresters are
similar and are installed close to each other. However, it has been reported that any small difference in
the individual V-I characteristics can lead to unbalance in current sharing. When the arresters are
installed some distance away from each other, travelling wave effects can modify the effectiveness of
parallel arresters for surge overvoltage protection and, in this case, detailed simulations are required to
ascertain the level of protection. Such a situation occurs in practice with large substations or short
underground cable connections. Various studies have shown that a requirement for two-arrester
protection is closely dependent upon the type and length of cable used.
In the case of overhead lines, the distances are much bigger and the main objective of line arresters is
to reduce the flashover rates due to surge overvoltages. This is especially relevant to lines located in
regions of high lightning activity, lines with compact/uprated design where the phase-to-phase and
phase-to-earth air clearances are reduced.
In this paper, we present a study of parallel arresters considering the separation distance and their
application to overhead lines. Various scenarios of overhead line configurations were considered and
the overvoltage levels were calculated for each case. Assessment of flashover performance is also
conducted for a number of conditions. A number of calculation techniques were used and compared
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