共查询到20条相似文献,搜索用时 15 毫秒
1.
A propagation experiment is described in which a stratospheric balloon served as a transmitter platform at 870 and 1502 MHz in simulation of a land mobile satellite. A vehicle followed the drifting balloon along roads of western Texas and New Mexico, collecting at L -band amplitude and phase, and at UHF amplitude information only for elevation angles between 25° and 45°. The data obtained has been analyzed and is presented along with results from modeling of multipath scattering and roadside tree attenuation. The signal, with variations caused by multipath propagation and tree shadowing, was reduced by 3 dB at L -band and 2 dB at UHF for one percent of all locations. A median ratio of 3.9 was found between peak-to-peak phase (degrees) and power (dB) fluctuations. The ratio between L -band and UHF dB attenuation averages varied from 1.3 to 1.0 at fade levels from 6 to 23 dB. Optical sky brightness was measured and used to predict fade distribution with great accuracy. A single-scatterer multipath model is introduced. It is used to duplicate some of the measured data and to show the dependence of power variations on satellite elevation angle. Using Fresnel diffraction theory, the attenuation caused by a model tree was calculated to be near 10 dB and the maximum fade was found to increase by the logarithm of the number of branches 相似文献
2.
Field tests related to planned mobile satellite systems (MSS) were performed, and results that add to the existing database of propagation measurements at L -band (1.5 GHz) are described. They are considered particularly useful in that propagation effects were studied systematically with repeated and controlled runs pertaining to different path elevation angles, road types, and path geometries defining shadowing and line-of-sight modes. In addition, simultaneous L -band and UHF measurements were performed for the purpose of establishing scaling factors applicable to previous UHF (870 MHz) results. The control of the experimental parameters was made possible by using a helicopter as the source platform and a mobile van which housed the receiver 相似文献
3.
Wang J.R. Shiue J.C. Schmugge T.J. Engman E.T. 《Geoscience and Remote Sensing, IEEE Transactions on》1990,28(5):906-914
The NASA Langley Research Center's L -band pushbroom microwave radiometer (PBMR) aboard the NASA C-130 aircraft was used to map surface soil moisture at and around the Konza Prairie Natural Research Area in Kansas during the four intensive field campaigns of FIFE in May-October 1987. A total of 11 measurements were made when soils were known to be saturated. This measurement was used for the calibration of the vegetation effect on the microwave absorption. Based on this calibration, the data from other measurements on other days were inverted to generate soil moisture maps. Good agreement was found when the estimated soil moisture values were compared with those independently measured on the ground at a number of widely separated locations. There was a slight bias between the estimated and measured values, the estimated soil moisture on the average being lower by about 1.8% 相似文献
4.
The authors present the results of the construction and testing of three cryogenic low-noise GaAs FET amplifiers, based on a National Radio Astronomy Observatory design, to be used in a detector for the axion, a hypothetical particle. The amplifiers are centered on 1.1 GHz, and 2.4 GHz, have a gain of approximately 30 dB in bandwidths of 300 MHz, 225 MHz, and 310 MHz, and have minimum noise temperatures of 7.8 K, 8 K, and 15 K, respectively 相似文献
5.
A mobile satellite system (MSS) propagation experiment at 1.5 GHz was performed near the east coast of the United States in central Maryland during December 1987 using the MARECS-B2 satellite as a transmitter platform. A receiving system in a vehicle measured signal fades caused by shadowing and multipath from roadside trees and utility poles. The propagation degradations were characterized for a system of three roads previously examined using a helicopter as the transmitter platform. The objectives of the MARECS-B2 MSS tests were to: (1) establish cumulative fade distributions for the particular satellite geometry for both rural and suburban roads; (2) validate the consistency of previous roadside tree measurements which employed a helicopter as the transmitter platform for the same system of roads; (3) obtain an additional set of fade levels at a lower angle hitherto not measured in central Maryland; and (4) combine the satellite-acquired data set with previous helicopter results and establish an analytic, empirically derived function describing the cumulative fade distributions for a family of path angles. An analysis of the satellite data has demonstrated the successful achievement of these objectives 相似文献
6.
Madrangeas V. Aubourg M. Guillon P. Vigneron S. Theron B. 《Microwave Theory and Techniques》1992,40(1):120-127
High power L -band dielectric resonator microwave filters, used in satellite systems because of their good temperature and vibration characteristics, low weight, and size, are discussed. The electromagnetic and electrical parameters of different microwave dielectric resonator structures are computed by means of the two-dimensional and three-dimensional finite element method, which are applied both for free and forced oscillation systems. The design and response evaluation of an elliptic filter using rectangular dielectric resonators excited in their TM110 mode are presented 相似文献
7.
Tree attenuation results at 870 MHz are described for experiments conducted in October 1985 and March 1986 in Central Maryland. These experiments employed a helicopter as a source platform and a van with receiver and data acquisition instrumentation. Tree attenuation results were obtained for the cases in which the van was stationary and in motion. The experiments were performed for the purpose of providing the designers of planned land mobile satellite systems with important elements in the determination of link parameter requirements; namely, the expected fading statistics due to roadside trees for both mobile and stationary vehicles. Single tree attenuation results gave worst case median fades as high as 15 dB although roadside tree values were noted to produce fades in excess of 20 dB for small percentages of time. The cumulative fade distributions and their relative contributions as a function of path elevation angle, right side versus left side driving, and different road types are derived from the field measurements. Upon comparing the attenuations from bare deciduous trees (March 1986) with those due to trees in full foliage (October 1985), the increase in dB attenuations were, in general, less than 25 percent for the dynamic cases, and less than 40 percent for the worst case static configuration. This result demonstrates thai the dominant fading is caused by the wooded tree branches as opposed to the leaves on these branches. The tail end of the observed fade distributions was observed to follow lognormal distributions with respect to dB attenuation. 相似文献
8.
Duh K.H.G. Pane-Chane Chao Smith P.M. Lester L.F. Lee B.R. Ballingall J.M. Ming-Yih Kao 《Microwave Theory and Techniques》1988,36(12):1598-1603
Quarter-micron-gate-length high-electron-mobility transistors (HEMTs) have exhibited state-of-the-art low-noise performance at millimeter-wave frequencies, with minimum noise figures of 1.2 dB and 32 GHz and 1.8 dB at 60 GHz. At Ka -band, two-stage and three-stage HEMT low-noise amplifiers have demonstrated noise figures of 1.7 and 1.9 dB, respectively, with associated gains of 17.0 and 24.0 dB at 32 GHz. At V -band, two stage and three-stage HEMT amplifiers yielded noise figures of 3.2 and 3.6 dB, respectively, with associated gains of 12.7 and 20.0 dB and 60 GHz. The 1-dB-gain compression point of all the amplifiers is greater than +6 dBm. The results clearly show the potential of short-gate-length HEMTs for high-performance millimeter-wave receiver application 相似文献
9.
10.
Durden S.L. Klein J.D. Zebker H.A. 《Antennas and Propagation, IEEE Transactions on》1991,39(10):1537-1539
Fluctuations of an L -band, horizontally polarized signal that was transmitted from the ground through a coniferous forest canopy to an airborne radar are examined. The azimuth synthetic aperture radar (SAR) impulse response in the presence of the measured magnitude fluctuations shows increased sidelobes over the case with no trees. Statistics of the observed fluctuations are similar to other observations 相似文献
11.
12.
Quantitative analysis of the coherent and incoherent power of sea-reflected waves at 1.5 GHz was performed using multipath-fading data obtained by field experiments. The experiments were carried out for shore-to-shore, satellite-to-shore, and satellite-to-ship paths, and antennas with gains from 13 dBi to 21 dBi were used for signal reception. Results indicate that in rough sea conditions where the incoherent component is dominant, the power of the incoherent component can reach the maximum power of the coherent component (generated in calm sea) in most cases, except for a few cases such as when measurements using narrow-beam antennas or elevation angles below 7° are made. On the results obtained, a simple prediction method for multipath fading caused by sea reflection is presented 相似文献
13.
A fully digital implementation of digital modems is the preferred option of system designers because high performance can be achieved at reasonable cost. The author explains the beneficial features inherent in fully digital demodulator implementations. Other features which are required for land mobile satellite communication systems are also discussed. Recently reported techniques for the synchronisation and detection of M -ary PSK and M -ary QAM modulation schemes are reviewed with emphasis placed on those which are well suited to digital implementation 相似文献
14.
Behavior of networks of very small aperture satellite terminals (VSATs) operating at Ku band during the solar transit period, is compared to more traditional C or Ku -band satellite networks. Based on analyses and experiments, it is explained why solar transit outages are rarer in Ku -band VSAT systems than conventional satellite communications systems. In many cases, Ku -band VSAT systems will operate through periods of Sun transits without any significant increase in transmission error rates or incidences of link outages 相似文献
15.
《Microwave Theory and Techniques》1992,40(12):2332-2336
The design, fabrication, and evaluation of a W -band image-rejection downconverter based on pseudomorphic InGaAs-GaAs HEMT technology are presented. The image-rejection downconverter consists of a monolithic three-stage low-noise amplifier, a monolithic image-rejection mixer, and a hybrid IF 90° coupler with an IF amplifier. The three-stage amplifier has a measured noise figure of 3.5 dB, with an associated small signal gain of 21 dB at 94 GHz while the image-rejection mixer has a measured conversion loss of 11 dB with +10 dBm LO drive at 94.15 GHz. Measured results of the complete image-rejection downconverter including the hybrid IF 90° coupler and a 10 dB gain amplifier show a conversion gain of more than 18 dB and a noise figure of 4.6 dB at 94.45 GHz 相似文献
16.
Chang K.W. Wang H. Bui S.B. Chen T.-h. Tan K.L. Ton T.-N. Berenz J. Dow G.S. Lin T.S. Garske D.C. Liu L.C.T. 《Microwave Theory and Techniques》1991,39(12):1972-1979
The design, fabrication, and evaluation of a fully integrated W -band monolithic downconverter based on InGaAs pseudomorphic HEMT technology are presented. The monolithic downconverter consists of a two-stage low-noise amplifier and a single-balanced mixer. The single-balanced mixer has been designed using the HEMT gate Schottky diodes inherent to the process. Measured results of the complete downconverter show conversion gain of 5.5 dB and a double-sideband noise figure of 6.7 dB at 94 GHz. Also presented is the downconverter performance characterized over the -35°C to +65°C temperature range. The downconverter design was a first pass success and has a high circuit yield 相似文献
17.
Tan K.L. Streit D.C. Dia R.M. Wang S.K. Han A.C. Chow P.-M.D. Trinh T.Q. Liu P.H. Velebir J.R. Yeii H.C. 《Electron Device Letters, IEEE》1991,12(5):213-214
The author's present the DC and RF power performance of planar-doped channel InGaAs high-electron-mobility transistors (HEMTs). The planar-doped channel (PDC) pseudomorphic GaAs HEMT with 400 μm of gate width exhibited an output power of 184 mW, corresponding to 460 mW/mm, with 4.6-dB saturation gain and 25% power-added efficiency at 55 GHz. Although higher power density is possible, the authors have designed the device to operate at less than 500 mW/mm for thermal and reliability reasons. Devices with unit gate finger widths ranging from 30 to 50 μm were fabricated and characterized, with no performance degradation observed from using the longer gate fingers 相似文献
18.
Tan K.L. Dia R.M. Streit D.C. Han A.C. Trinh T.Q. Velebir J.R. Liu P.H. Lin T. Yen H.C. Sholley M. Shaw L. 《Electron Device Letters, IEEE》1990,11(7):303-305
Low-noise planar doped pseudomorphic (PM) InGaAs high-electron-mobility transistors (HEMTs) with a gate length of 0.1 μm for W -band operation are discussed. These devices feature a multiple-finger layout with air bridges interconnecting the sources to reduce gate resistance. The device exhibits a minimum noise figure of 2.5 dB with an associated gain of 4.7 dB at 92.5 GHz. This result demonstrates the feasibility of using PM InGaAs HEMTs for W -band low-noise receivers without the need for using lattice-matched InP HEMTs 相似文献
19.
Herman M.I. Lan G.-L. Chen J.C. Pao C.-K. 《Proceedings of the IEEE. Institute of Electrical and Electronics Engineers》1991,79(3):342-354
The development of a W -band (75-110 GHz) monolithic receiver, culminating in a three-chip multifunctional monolithic microwave integrated circuit (MMIC) receiver front-end, is described. The heart of the receiver is a four-channel multiplexer, with each channel possessing its own single balanced mixer and low-noise IF amplifier, all integrated onto a single GaAs chip. Two dual-channel monolithic Gunn oscillators with the drive level and spectral parity to meet system requirements have been developed. The key to the development of the monolithic front-end has been to ensure process compatibility between individual components and the careful partitioning of the chip architecture 相似文献
20.
AlGaAs/InGaAs P-n-p heterojunction bipolar transistors (HBTs) were fabricated using carbon-doped material grown by nonarsine metal-organic vapor-phase epitaxy (MOVPE). F max of 39 GHz and f t of 18 GHz were obtained. Operated in common-base mode, a P-n-p HBT achieved 0.5-W output power with 8-dB gain at 10 GHz; saturated output power was 0.69 W. Results are presented for devices with emitter lengths from 120 to 600 μm 相似文献