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991.
Lampariello P. Frezza F. Shigesawa H. Tsuji M. Oliner A.A. 《Antennas and Propagation, IEEE Transactions on》1998,46(7):1032-1041
A new leaky-wave antenna is presented that possesses many desirable features and is suitable for application to both the millimeter-wave and microwave ranges. These desirable features, some of which are unusual, include a simple configuration, a wide flexibility in the range of available beamwidths, the ability to control the beamwidth and the direction of the beam essentially independently, and negligible cross polarization at all scan angles. The antenna structure consists of a parallel-plate stub guide of small height, less than a half wavelength, located off center on the top of rectangular waveguide. The beamwidth is easily controlled from very wide to very narrow by adjusting the stub width or location. The article presents the principle of operation and the theory, employing a new transverse equivalent network that is accurate, but also simple, so that it permits rapid and inexpensive numerical calculations 相似文献
992.
Pilot-channel-assisted binary phase-shift keying (PCA-BPSK) in direct-sequence code-division multiple-access (DS-CDMA) systems is investigated. The loss due to imperfect channel estimation is calculated, and the pilot-to-data channel power ratio is optimized analytically. The results show the effects of the various parameters and provide a suitable tool for system design 相似文献
993.
B.K. Srinivas R.P. Chhabra 《Chemical Engineering and Processing: Process Intensification》1991,29(3):121-131
In this work, new experimental measurements of the minimum fluidization velocity and velocity-voidage characteristics are reported for a variety of liquid-particle systems in glass columns of two different diameters. Three types of liquids, namely, Newtonian, visco-inelastic, and visco-elastic fluids, were used to fluidize the beds of glass particles of four different sizes (1.27–15.8 mm). The results obtained with Newtonian liquids conform to the expected behaviour. The applicability of a variety of equations has been examined with a view to predicting the values of the minimum fluidization velocity and fluidization index for non-Newtonian systems. The experimental results reported herein embrace the following ranges of conditions: 1.27 < Dp < 15.8 mm; DT = 50.8 and 101.6 mm, and 0.382 n 1.00. 相似文献
994.
995.
996.
In principle, any desired amount of gain can be developed from an antenna of arbitrary size. The phenomena of high gain from very small antennas is called “supergain”. To see why this statement might be so, the author recalls the construction used in optics, known as Huygens' principle. This states that every point on a wavefront can be regarded as a source of radiation. At the end of a short period of time, the envelope of all of these individual wavelets forms the new wavefront. For example, this construct explains why a shadow is not perfectly sharp, and why interference fringes form. End fire antennas, dipole antennas, Yagi antennas and quad antennas are examined 相似文献
997.
K. P. Seleznev Yu. B. Galerkin V. P. Mitrofanov M. R. Poles É. I. Sergacheva 《Chemical and Petroleum Engineering》1992,28(2):76-80
Translated from Khimicheskoe i Neftyanoe Mashinostroenie, No. 2, pp. 4–7, February, 1992. 相似文献
998.
Several boron-containing organosilicon polymers were synthesized from a sodium-coupling reaction of silicon and boron halides with and without alkyl halide in hydrocarbon solvents. The B–Si preceramic polymers were characterized using techniques such as IR, UV, and NMR spectrometry, gel permeation chromatography, elemental analysis, molecular weight measurement, and thermal analyses (TGA, DSC, DTA, and TMA). The chemical structures of the preceramic polymers were postulated based on the analytical results. Black ceramic materials were obtained from the precursor polymers upon thermal degradation at temperatures above 1000°C in an inert atmosphere. The precursor polymers had a ceramic yield of up to 70%. Thermogravimetric analysis of the ceramic material in air at a flow rate of 100 mL/min showed it was stable up to 1000°C with little weight gain or loss. Several methods were used to characterize the ceramic materials: XRD, solid NMR, high-temperature DTA, elemental analysis, and acid digestion. The analyses indicated that the ceramic materials comprised a mixture of silicon carbide (SiC), silicon borides (SiB4, SiB6), and amorphous Si–B–C ceramics, with small amounts of silica and free silicon. 相似文献
999.
Micromechanical milling has been shown to be a rapid and direct method for the fabrication of structures with the geometry
and size suitable for use as x-ray mask absorbers. While the micromilling process can not duplicate the size and resolution
of absorber patterns created by high energy electron beam or optical lithography methods, micromilling can repeatedly create
absorber line widths down to 10 micrometers, or less, with a one-sigma tolerance of 0.5 micrometers. A method for easily characterizing
milling tool run out has been adapted so tool change out can be more routine. The milling process leaves some absorber burrs
and the absorber is apparently tapered at the machined wall which introduce process biases, both of which add to exposure
degradation. Nevertheless, based on work to date, it appears both of these effects can be reduced to acceptable limits.
Received: 25 August 1997/Accepted: 3 September 1997 相似文献
1000.
Buso S. Mattavelli P. Rossetto L. Spiazzi G. 《Power Electronics, IEEE Transactions on》1998,13(5):814-823
This paper presents the practical implementation of a fully digital control for boost power factor preregulators (PFPs). The control algorithm, which is simple and fast, provides a significant improvement in the system's dynamic performance compared to the usual analog control techniques. The paper discusses the design criteria and the actions taken for the implementation of the digital control, which is performed by means of a standard microcontroller (Siemens 80C166). The effectiveness of the approach is assessed by experimental tests 相似文献