全文获取类型
收费全文 | 10562篇 |
免费 | 2480篇 |
国内免费 | 600篇 |
专业分类
化学 | 1117篇 |
晶体学 | 25篇 |
力学 | 118篇 |
综合类 | 46篇 |
数学 | 294篇 |
物理学 | 2413篇 |
无线电 | 9629篇 |
出版年
2024年 | 60篇 |
2023年 | 207篇 |
2022年 | 273篇 |
2021年 | 272篇 |
2020年 | 334篇 |
2019年 | 291篇 |
2018年 | 381篇 |
2017年 | 419篇 |
2016年 | 435篇 |
2015年 | 579篇 |
2014年 | 882篇 |
2013年 | 740篇 |
2012年 | 954篇 |
2011年 | 909篇 |
2010年 | 705篇 |
2009年 | 623篇 |
2008年 | 665篇 |
2007年 | 783篇 |
2006年 | 670篇 |
2005年 | 569篇 |
2004年 | 467篇 |
2003年 | 442篇 |
2002年 | 333篇 |
2001年 | 299篇 |
2000年 | 239篇 |
1999年 | 176篇 |
1998年 | 169篇 |
1997年 | 137篇 |
1996年 | 131篇 |
1995年 | 131篇 |
1994年 | 95篇 |
1993年 | 59篇 |
1992年 | 60篇 |
1991年 | 40篇 |
1990年 | 27篇 |
1989年 | 23篇 |
1988年 | 15篇 |
1987年 | 11篇 |
1986年 | 9篇 |
1985年 | 7篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 6篇 |
1977年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Madhurika Mahajan T. Chakarvarty Sunil Kumar Khah 《AEUE-International Journal of Electronics and Communications》2011,65(11):954-957
A numerical model for resonant frequency using asymmetric loading is presented using cavity model. In asymmetric loaded antenna, the posts are not equispaced on annular ring antenna and resonant frequency is independent of the angular location of posts. The posts are located away from the centre of patch. The diameter of post is thin as compared to the diameter of annular ring antenna. The first few dominant modes calculated for asymmetric loading are compared to symmetric loading where the posts are to be located at specific angular locations. The theoretical model is compared to the simulated results of IE3D based on Method of Moments. The numerical model developed can also be verified further by the full wave analysis. 相似文献
992.
In this paper, an Ultra-Wideband (UWB) planar antenna is proposed for the reinforced concrete detection, which consists of a pair of planar waterdrop arms, a microstrip to coplanar par- allel-strips transition and a shallow rectangular cavity. In order to overcome the disadvantages of the shallow cavity, some absorbing material is loaded to weaken the narrow-band effect of the cavity and the crosstalk interference. The simulated and measured results show that the proposed antenna has a large bandwidth from 0.48 GHz to 3.6 GHz with Voltage Standing Wave Ratio (VSWR) below 2 and a fractional bandwidth about 200% under the center frequency of 1.6 GHz, directional radiation char- acteristics and small late-time ringing in the time domain, which can be suitable for nondestructive detection of the reinforced concrete. 相似文献
993.
The characteristics of TCP and UDP lead to different network transmission behaviours. TCP is responsive to network congestion whereas UDP is not. This paper proposes two mechanisms that operate at the source node to regulate TCP and UDP flows and provide a differential service for them. One is the congestion‐control mechanism, which uses congestion signal detected by TCP flows to regulate the flows at the source node. Another is the time‐slot mechanism, which assigns different number of time slots to flows to control their flow transmission. Based on the priority of each flow, different bandwidth proportions are allocated for each flow and differential services are provided. Simulation results show some insights of these two mechanisms. Moreover, we summarize the factors that may impact the performance of these two mechanisms. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
994.
Li Junsong Farahvash Shayan Kavehrad Mohsen Valenzuela Reinaldo 《International Journal of Wireless Information Networks》2002,9(4):227-241
There are many benefits in using time division duplex (TDD) instead of frequency division duplex (FDD) schemes in fixed wireless cellular systems. For example, channel reciprocity for a single carrier frequency used on both uplinks and downlinks will allow easy access to channel state information, reduced complexity of RF design, much higher flexibility in handling dynamic traffic, simpler frequency plan, etc. However, there exists a serious limiting factor in using dynamic TDD (D-TDD) in cellular systems. This is due to a steady interference on an uplink in any cell caused by downlink transmissions in other cells. Simulation results show in D-TDD cellular systems, performance is unacceptable, when an omnidirectional antenna is used at base stations. Simulation results also suggest great potential for smart antennas in achieving substantial performance improvement in fixed D-TDD bandwidth-on-demand wireless systems. 相似文献
995.
Teruhiro Shirakura 《Annals of the Institute of Statistical Mathematics》1986,38(1):145-159
Summary For a given fractional 2
m
factorial (2
m
-FF) designT, the constitution of a block plan to divideT intok (2
r−1<k≦2
r
) blocks withr block factors each at two levels is proposed and investigated. The well-known three norms of the confounding matrix are used
as measures for determining a “good” block plan. Some theorems concerning the constitution of a block plan are derived for
a 2
m
-FF design of odd or even resolution. Two norms which may be preferred over the other norm are slightly modified. For each
value ofN assemblies with 11≦N≦26, optimum block plans fork=2 blocks with block sizes [N/2] andN−[N/2] minimizing the two norms are presented forA-optimal balanced 24-FF designs of resolutionV given by Srivastava and Chopra (Technometrics,13, 257–269). 相似文献
996.
《Journal of separation science》2003,26(18):1635-1642
Capillary electrochromatography using a monolithic matrix was employed to develop a rapid and highly efficient separation methodology for the analyses of mixtures of agrochemical importance. Using this method, ppm‐ppb detection limits for urea, carbamate, and phenoxy acid herbicides were achieved without a preconcentration technique. The detection limits were further decreased to low‐ppb levels for the same class of compounds using an on‐column preconcentration technique. 相似文献
997.
移动通信中智能天线自适应算法简介 总被引:6,自引:0,他引:6
智能天线是第三代移动通信系统中的关键技术之一,其自适应算法按照是否需要利用系统的导频信息分成非盲算法、盲算法和半盲算法三类,文中对每类中的各种算法进行了相应的阐述。 相似文献
998.
大口径改型卡氏天线栅状反射面成型工艺研究 总被引:1,自引:1,他引:0
主要论述了大口径曲面栅状反射面的结构特点、反射面上高精度线条的制作及其关键制造技术,通过合理的模具设计,运用湿法成型和真空袋压相结合的工艺,成功研制了形面精度高、电气指标一致性好的天线反射体。 相似文献
999.
BGA封装技术与SMT/SMD 总被引:5,自引:0,他引:5
文中就BGA与SMT/SMD之间有关的主要问题进行了较为详细的介绍和分析。 相似文献
1000.
PHS系统中的智能基站 总被引:5,自引:1,他引:4
文章介绍了智能天线技术基本原理,分析了PHS智能基站的实现方法,探讨了PHS智能基站的技术特点及应用方式. 相似文献