AlN-C(石墨)复相材料微波衰减性能的研究

采用热压烧结工艺,通过合理的烧结温度及保温时间的控制,制备了性能优异的AlN-C复相微波衰减材料。通过网络分析仪、SEM等测试手段,研究了衰减剂C含量对AlN-C复相材料微波衰减性能的影响,结果表明,当衰减剂C含量极小时(w≤1.0 %),AlN-C复相材料不具备衰减电磁波的特性;当衰减剂C含量为3.0 %～5.0 %时,AlN-C复相材料呈现良好的宽频衰减特性;当衰减剂C含量大于7.0 %时,AlN-C复相材料呈现明显的选频衰减特性,且随着C含量的增加,材料的衰减量增大,有效衰减带宽减小,中心谐振频率f0有向高频漂移的趋势。对AlN-C复相材料的频谱特性进行了初步的分析。探讨了AlN-C复相材料的微波衰减机理,电导损耗、界面极化损耗是其主要的微波衰减机理。     

TiO2对AIN-C复相材料性能的影响

以TiO2为前驱体,采用热压烧结工艺,高温原位合成AIN-C-TiC复相材料.通过矢量网络分析仪、SEM和XRD等测试手段,研究了添加剂TiO2对A1N-C复相材料密度,热导率、电阻率,微波衰减特性和显微结构的影响.结果表明,添加TiO2后有新相TiC生成,热导率降低,改善了材料的衰减性能.通过调整参数,使得该复相材料在w(TiO2)为3%时,微波衰减达到-0.8 dB.     

雨衰减预测中的降雨率调整因子概念

利用ITU-R数据库地面视距链路和地空链路雨衰减数据,分析了传统雨衰减预报中所使用的基于柱状雨胞模型的雨衰减物理模型导出的路径调整因子与实测数据之间的矛盾.在此基础上,基于指数雨胞建立的雨衰减物理模型,通过理论分析导出了降雨率调整因子的概念.利用降雨率调整因子,可以解释实测数据的主要特征,表明:基于指数雨胞的雨衰减物理模型是合理的,利用降雨率调整因子的概念,可为发展新的地面视距链路和地空链路雨衰减预报模式和方法提供理论依据.     

TiO2添加剂对AlN-SiC材料微波衰减性能的影响

在AlN-SiC复相材料中加入TiO2,采用热压工艺,制备了性能优异的AlN-SiC-TiO2复相微波衰减材料。通过矢量网络分析仪、SEM等测试手段,研究了AlN-SiC复相材料微波衰减性能与SiC含量的之间的关系,以及添加剂TiO2对AlN-SiC复相材料微波衰减特性和显微结构的影响。结果表明,SiC是良好的宽频微波衰减剂,对衰减频谱曲线特征起决定作用;TiO2的添加大大促进了AlN-SiC复相材料的烧结性能和微波衰减性能,添加了TiO2后6 GHz下衰减量由0.6 dB增加到0.85 dB。初步探讨了AlN-SiC-TiO2复相材料的微波衰减机理,电导损耗、介质损耗是其主要的微波衰减机理。     

TiO_2对AlN-C复相材料性能的影响

以TiO2[0]为前驱体,采用热压烧结工艺,高温原位合成AlN-C-TiC复相材料。通过矢量网络分析仪、SEM和XRD等测试手段,研究了添加剂TiO2对AlN-C复相材料密度、热导率、电阻率、微波衰减特性和显微结构的影响。结果表明,添加TiO2后有新相TiC生成,热导率降低,改善了材料的衰减性能。通过调整参数,使得该复相材料在w(TiO2)为3%时,微波衰减达到-0.8dB。     

长余辉材料SrAl2O4∶Eu2+ ,Dy3+的制备及发光性能研究

采用高温固相法,在碳粉还原条件下制备了发光亮度高、余辉时间长的SrAl2O4∶Eu2+,Dy3+长余辉发光材料,对其发光性能进行了研究.发光粉体经碳粉还原后的发射光谱表明,其发射主峰位于509 nm左右;余辉衰减曲线证明其余辉衰减过程存在快速衰减和慢衰减两个过程,该发光粉体的余辉时间能持续20 h以上.此外还考察了烧结温度、稀土掺杂量和助熔剂含量对此发光粉体发光性能的影响.     

Al2O3-SiC复相微波衰减材料的性能研究

采用热压烧结工艺制备了Al2O3-SiC复相微波衰减材料。通过网络分析仪,研究了SiC含量对材料的微波衰减性能的影响。结果表明,当w(SiC)<3%,复相材料具有选频衰减的频谱曲线,谐振峰位置基本不变;当w(SiC)为3%~16%,材料的谐振损耗峰向低频方向移动;当w(SiC)>16%时,材料呈现出宽频衰减特性。对复相材料的微波衰减机理作了初步探讨,弛豫损耗和电导损耗是其主要的衰减机理。     

AlN-TiC复相微波衰减材料性能的研究

采用热压烧结工艺制备了AlN-TiC复相微波衰减材料。通过XRD、SEM和网络分析仪,研究了TiC含量对材料的微波衰减性能的影响。结果表明,当w(TiC)低于10%时,材料呈选频衰减且衰减非常小;当w(TiC)为25%~50%时,材料呈现良好的多点选频衰减,且其衰减量随w(TiC)的增加而增加,最大达–18 dB。中心谐振频率有向高频漂移的趋势。初步探讨了AlN-TiC复相材料微波衰减曲线的频谱特性与衰减机理。     

超声产前诊断胎儿畸形漏诊7例分析

采用热压工艺在1 500℃下保温1 h制备了MgO-Mg2 SiO4-SiC复相微波衰减损耗钮扣材料,通过矢量网络分析仪测试了不同MgO、Mg2SiO4配比的复相材料在X波段(8～12 GHz)的微波衰减性能,并研究了SiC 添加量对复相材料微波衰减性能的影响.结果表明,热压烧结MgO和Mg2SiO4的衰减频率分别为10.68 GHz和11.80 GHz,MgO-MgzSiO4复相材料的衰减频率介于两者之间,随着复相材料中Mg2SiO4相的增加,衰减频率向高频方向移动;SiC的加入使复相材料的致密度下降,衰减频率向低频方向移动.从测试原理和波导性质方面分析了直径ф6.1 mm、高度4.2 mm的损耗钮扣材料需满足9.9<εr<18.66,可抑制行波管的边带震荡,此时衰减频率f0为37.7×εr-0.5(GHz).     

基于FPGA的QDPSK调制系统的设计与实现

在分析四相相对移相QDPSK信号调制解调原理的基础上,设计一种由FPGA实现的改进的数字化QDPSK信号调制系统,在信号的调制过程中将码元的调制波形分为稳定区和过渡区,并通过调用不同包络衰减函数的正弦函数样值序列来产生调制波形,该系统调制信号高频分量少,解码误码率较低.文中给出了数字化QDPSK调制系统的电路模型图及仿真结果.     

Comment on "Limitations of the Thevenin and Norton equivalent circuits for a receiving antenna"

The author comments that Collin (see IEEE Antennas and Propagation Magazine, vol.45, p.119-124, 2003) points out some of the attributes of the constant-power equivalent circuit for a receiving antenna. However, he disagrees with the writer's statement that the internal power dissipated in the circuit can be equated to the reradiated, or scattered, power from the antenna. A reflector antenna is not at all like the slightly lossy dielectric bodies and the perfectly conducting targets treated in the classical literature. It is a highly "lossy" structure that absorbs nearly all the power incident upon it.     

An Improved Equivalent Circuit for the Thin-Film Lumped-Element Circulator

A program written for the Hewlett-Packard automatic network analyzer permits the S-parameter eigenvalue phases and magnitudes to be displayed. The thus measured eigenvalues of a lumped-element circulator lead to an improved equivalent circuit which explains the observed "double hump" characteristic. The influence of different circuit parameters on the eigenvalues is measured and found in good agreement with the author's previously published theory. It is concluded from this theory and the measurements that, for the lossy circulator in general, maximum isolation, return loss, and minimum forward loss do not occur at the same frequency.     

Near-lossless and lossy compression of imaging spectrometer data: comparison of information extraction performance

We investigate the ability to derive meaningful information from decompressed imaging spectrometer data. Hyperspectral images are compressed with near-lossless and lossy coding methods. Linear prediction between the bands is used in both cases. Each band is predicted by a previously transmitted band. The residual is formed by subtracting the prediction from the original data and then is compressed either with a near-lossless bit-plane coder or with the lossy JPEG2000 algorithm. We study the effects of these two types of compression on hyperspectral image processing such as mineral and vegetation content classification using whole- and mixed pixel analysis techniques. The results presented in this paper indicate that an efficient lossy coder outperforms near-lossless method in terms of its impact on final hyperspectral data applications.     

The correct ETLC model for anisotropic lossy materials-a comment on "Transient propagation in anisotropic laminated composites"

S. He comments on a paper by Lin et al. (IEEE Trans. Electromagn. Comp., vol.25, no.3, p.357-65, Aug. 1993). Lin et al. wanted to calculate the transient response for a plane wave obliquely incident on a multilayered structure consisting of anisotropic lossy materials. They first derived an equivalent transmission-line-circuit (ETLC) model for the anisotropic lossy materials, and then used a standard circuit analysis program (PSPICE) to calculate the transient response. The method is efficient and it enables one to use the existing computer code. However, He believes that the Lin et al. ETLC model for anisotropic lossy materials is not correct. He derives and presents the correct one.<>     

Source coding, large deviations, and approximate pattern matching

We present a development of parts of rate-distortion theory and pattern-matching algorithms for lossy data compression, centered around a lossy version of the asymptotic equipartition property (AEP). This treatment closely parallels the corresponding development in lossless compression, a point of view that was advanced in an important paper of Wyner and Ziv in 1989. In the lossless case, we review how the AEP underlies the analysis of the Lempel-Ziv algorithm by viewing it as a random code and reducing it to the idealized Shannon code. This also provides information about the redundancy of the Lempel-Ziv algorithm and about the asymptotic behavior of several relevant quantities. In the lossy case, we give various versions of the statement of the generalized AEP and we outline the general methodology of its proof via large deviations. Its relationship with Barron (1985) and Orey's (1985, 1986) generalized AEP is also discussed. The lossy AEP is applied to (i) prove strengthened versions, of Shannon's(1948, 1974) direct source-coding theorem and universal coding theorems; (ii) characterize the performance of "mismatched" codebooks in lossy data compression; ( iii) analyze the performance of pattern-matching algorithms for lossy compression (including Lempel-Ziv schemes); and (iv) determine the first-order asymptotic of waiting times between stationary processes. A refinement to the lossy AEP is then presented, and it is used to (i) prove second-order (direct and converse) lossy source-coding theorems, including universal coding theorems; (ii) characterize which sources are quantitatively easier to compress; (iii) determine the second-order asymptotic of waiting times between stationary processes; and (iv) determine the precise asymptotic behavior of longest match-lengths between stationary processes. Finally, we discuss extensions of the above framework and results to random fields     

低碳含金阳极帽封接气泡的产生与控制

本文研究低碳封接合金阳极帽产生封接气泡的原因及控制措施。结果表明，低碳含金阳极帽产生封接气泡，主要是由于氧化膜破损和封接温度偏高造成，其实质是由于氧化膜表面因破损产生凹坑、膜上附着物等缺陷，封接时这些地方吸附的气体未能逃逸形成了气泡。通过降低封接温度和控制送料斗振动强度的措施，基本解决了封接气泡大量产生的问题。     

通过Visual Basic编程实现多种按钮效果

命令按钮是用户与计算机应用程序进行交互的最简便的方法。面向对象编程语言Visual Basic中的按钮控件就是命令按钮,命令按钮是应用程序中最为常用的控件,但是Visual Basic中的按钮控件过于简单,不够美观,支持的事件少。为实现多彩多姿命令按钮,可以利用图像框装载漂亮的图片模拟命令按钮,通过事件过程实现"命令按钮"的多种特殊效果,包括按钮的默认效果、鼠标划过时按钮的效果、鼠标单击时按钮的效果,按击回车键或逃逸键时按钮的效果。在实际编程中,设计好具有多种效果的按钮,是能否赢得用户的一件法宝。     

The use of surface impedance concepts in the finite-differencetime-domain method

Surface impedance concepts are introduced into the finite-difference time-domain (FDTD) method. Lossy conductors are replaced by surface impedance boundary conditions (SIBC), reducing the solution space and producing significant computational savings. Specifically, a SIBC is developed to replace a lossy dielectric half-space. An efficient implementation of this FDTD-SIBC based on the recursive properties of convolution with exponentials is presented. Finally, three problems are studied to illustrate the accuracy of the FDTD-SIBC formulation: a plane wave incident on a lossy dielectric half-space, a line current over a lossy dielectric half-space, and wave propagation in a parallel-plate waveguide with lossy walls     

A moment method solution for the shielding properties of three-dimensional objects above a lossy half space

In this paper, a moment method (MM) solution for analyzing the electromagnetic (EM) shielding properties of three-dimensional (3-D) lossy dielectric and magnetic objects over a lossy half space is presented. An MM, based on a volume formulation and a special Green's function in the spectral domain, is developed. Plane waves with TMz and TEz polarizations incident upon 3-D lossy material structures are demonstrated for the shielding effects of those bodies in the presence of a lossy ground. Some of the results are compared with those evaluated by applying the finite difference time domain method, and good agreements are obtained. The MM solution can be used to study the shielding problems for 3-D objects located above a lossy ground.     

Processing characteristics of harmonic signals against noises in case of their interaction in <Emphasis Type="Italic">K</Emphasis>-space

In this paper it is shown that processing of harmonic signals with unknown nonrandom amplitude and phase, against noises in K-space allows achieving better processing characteristics than ones in linear space. It is proposed signals processing quality criteria, based on metrical properties of an estimation space.