升级或替换?--谈亚洲数字电视的天馈线系统

要使射频广播系统的架构能够支持数字电视系统,需要设些什么?对整个网络进行进个站点的深入研究是必需的,RFS公司无线技术小组的Mike Dallimore这样认为。本文介绍了RFS公司通过对各种因素的分析研究得出的将现有系统介级为数字电视系统的最经济方案。     

Chemical oxygen-iodine laser power generation with an off-axis hybrid resonator

A rectangular negative branch off-axis hybrid resonator was coupled to a 10 kW class chemical oxygen-iodine laser. Resonator setup and alignment turned out to be straightforward. The extracted power was 6.6 kW and reached approximately 70% of the power for an optimized stable resonator. The divergence of the emitted laser beam in the unstable direction was lower than two times the diffraction limit. Experimentally measured margins for mirror misalignment were found in close agreement with numerical calculations.     

Modeling ozone and aerosol formation and transport in the pacific northwest with the community Multi-Scale Air Quality (CMAQ) modeling system

The Community Multi-Scale Air Quality (CMAQ) modeling system was used to investigate ozone and aerosol concentrations in the Pacific Northwest (PNW) during hot summertime conditions during July 1-15, 1996. Two emission inventories (El) were developed: emissions for the first El were based upon the National Emission Trend 1996 (NET96) database and the BEIS2 biogenic emission model, and emissions for the second El were developed through a "bottom up" approach that included biogenic emissions obtained from the GLOBEIS model. The two simulations showed that elevated PM2.5 concentrations occurred near and downwind of the Interstate-5 corridor along the foothills of the Cascade Mountains and in forested areas of central Idaho. The relative contributions of organic and inorganic aerosols varied by region, but generally organic aerosols constituted the largest fraction of PM2.5. In wilderness areas near the 1-5 corridor, organic carbon from anthropogenic sources contributed approximately 50% of the total organic carbon with the remainder from biogenic precursors, while in wilderness areas in Idaho, biogenic organic carbon accounted for 80% of the total organic aerosol. Regional analysis of the secondary organic aerosol formation in the Columbia River Gorge, Central Idaho, and the Olympics/Puget Sound showed that the production rate of secondary organic carbon depends on local terpene concentrations and the local oxidizing capacity of the atmosphere, which was strongly influenced by anthropogenic emissions. Comparison with observations from 12 IMPROVE sites and 21 ozone monitoring sites showed that results from the two El simulations generally bracketed the average observed PM parameters and that errors calculated for the model results were within acceptable bounds. Analysis across all statistical parameters indicated that the NW-AIRQUEST El solution performed better at predicting PM2.5, PM1, and beta(ext) even though organic carbon PM was over-predicted, and the NET96 El solution performed better with regard to the inorganic aerosols. For the NW-AIRQUEST El solution, the normalized bias was 30% and the normalized absolute error was 49% for PM2.5 mass. The NW-AIRQUEST solution slightly overestimated peak hourly ozone downwind of urban areas, while the NET96 solution slightly underestimated peak values, and both solutions over-predicted average 03 concentrations across the domain by approximately 6 ppb.     

Surface-plasmon microscopy with a two-piece solid immersion lens: bright and dark fields

We report bright-field and dark-field surface-plasmon imaging using a modified solid immersion lens and a commercial objective of moderate NA in the epi configuration. The contrast and resolution are extremely good, giving well-resolved images of protein monolayers both in air and in water. We also describe a two-part solid immersion lens that allows the sample to be moved without degrading the image quality in any observable way. The merits of the two-part lens are discussed and compared to commercially available microscope objectives. Finally, we introduce a simple Green's function model that illustrates the key features of both bright-field and dark-field surface-plasmon imaging.     

Early use of PbS nanotechnology for an ancient hair dyeing formula

Lead-based chemistry was initiated in ancient Egypt for cosmetic preparation more than 4000 years ago. Here, we study a hair-dyeing recipe using lead salts described in text since Greco-Roman times. We report direct evidence about the shape and distribution of PbS nanocrystals that form within the hair during blackening. It is remarkable that the composition and supramolecular organization of keratins can control PbS nanocrystal growth inside a hair.     

TBT causes regime shift in shallow lakes

Tributyltin (TBT) is an organotin compound used since the early 1960s as a biocide in boat antifouling paints. Its use has been linked to a host of negative effects in marine ecosystems including malformations and imposex in Mollusca and acute toxicity in many other aquatic animals. Yet, the consequences of TBT use in freshwaters are largely unknown. Here, for the first time we reveal that TBT may have caused hitherto unsuspected damage to freshwater ecosystems. Through an analysis of dated sediment cores collected from a system of recreationally boated, shallow lakes, we show that first evidence of TBT is associated with a dramatic loss of submerged vegetation and associated diverse animal communities. Cause and effect are difficult to unravel in our study. However, we hypothesize that TBT, through reducing populations of grazing organisms in lakes already affected by eutrophication, promoted the replacement of macrophytes by phytoplankton, ultimately leading to a regime shift in the ecosystem. Our findings may have parallels in freshwater ecosystems all over the world.     

Combination of NMR and MRI quantitation of moisture and structure changes for convection cooking of fresh chicken meat

This study demonstrates that a combination of bulk NMR and magnetic resonance imaging measurements of the T(2)-values of water protons can be used to determine the heat-induced changes in the structure and moisture content of fresh chicken meat which had been cooked in a convection oven at 200°C for a range of times. The gravimetric moisture content was also determined for both the raw and cooked meat. Multi-exponential fitting of the bulk NMR T(2) relaxation time data demonstrated three distinct water populations T(21) (39-43ms), T(22) (82-99ms) and T(23) (2-3ms) for raw meat which changed to 18-31ms (T(21)), 61-208ms (T(22)) and 3-7ms (T(23)) after the meat had been cooked. The T(1) and T(2) values obtained by MRI for cooked meat decreased progressively with increased heating time. There are highly significant correlations between the T(2) values from MRI and the T(21) values from bulk NMR measurements of cooked meat (r=0.986; p<0.01), and also between the normalised M(0) values from MRI and the gravimetric moisture content (r=0.953; p<0.01).     

A novel performance descriptor for ultrasonic strain imaging: a preliminary study

Ultrasonic strain imaging that uses signals from conventional diagnostic ultrasound systems is capable of showing the contrast of tissue elasticity, which provides new diagnostically valuable information. To assess and improve the diagnostic performance of ultrasonic strain imaging, it is essential to have a quantitative measure of image quality. Moreover, it is useful if the image quality measure is simple to interpret and can be used for visual feedback while scanning and as a training tool for operator performance evaluation. This report describes the development of a novel quantitative method for systematic performance assessment that is based on the combination of measures of the accuracy of motion tracking and consistency among consecutive strain fields. The accuracy of motion tracking assesses the reliability of strain images. The consistency among consecutive strain images assesses the signal quality in strain images. The clinical implications of the proposed method to differentiate good or poor strain images are discussed. Results of experiments with tissue-mimicking phantoms and in vivo breast-tissue data demonstrate that the performance measure is a useful method for automatically rating elasticity image quality.     

Meeting modern grid codes with large direct‐drive permanent magnet generator‐based wind turbines—low‐voltage ride‐through

Ambitious offshore wind energy targets continue to drive technological innovation, with the latest direct‐drive permanent magnet generator‐based wind turbines promising higher efficiency and availability. However, these machines have fixed rotor flux, provided by the magnets, which means that their voltage rises with speed. Further, high machine stator reactance leads to significant magnetic energy storage in the stator windings. Both these aspects provide new challenges for the power converter when designing to meet modern low‐voltage ride‐through requirements. This paper therefore proposes a novel control strategy, using a minimally rated chopper and dynamic brake resistor (DBR) integrated with the wind turbine's power converter, to help these systems to meet the demands of modern grid codes. This control method may allow the chopper and DBR to be rated at only 40% of a fully rated version. Despite only partially rating the DBR system, the control method minimizes the torsional oscillations in the drive train, thereby protecting the mechanical system. Copyright © 2011 John Wiley & Sons, Ltd.     

Atmospheric turbulence effects measured along horizontal-path optical retro-reflector links

The scintillation measured over close-to-ground retro-reflector links can be substantially enhanced due to the correlations experienced by both the direct and reflected echo beams. Experiments were carried out at China Lake, California, over a variety of ranges. The emphasis in this paper is on presenting the data from the 1.1?km retro-reflecting link that was operated for four consecutive days. The dependence of the measured irradiance flux variance on the solar fluence and on the temperature gradient above the ground is presented. The data are consistent with scintillation minima near sunrise and sunset, rising rapidly during the day and saturating at irradiance flux variances of ～10. Measured irradiance probability distributions of the retro-reflected beam are compared with standard probability density functions. The ratio of the irradiance flux variances on the retro-reflected to the direct, single-pass case is investigated with two data sets, one from a monostatic system and the other using an off-axis receiver system.