降雨对微波传输特性的影响分析

在归一化雨滴谱分布、雨滴的介电模型和Mie散射理论的基础上计算群雨滴在微波波段的散射和衰减特性,讨论分析雨滴谱分布、降雨强度、入射波频率和温度等因素对微波传输特性的影响.数值模拟结果表明,不同谱分布的群雨滴散射能力从大到小依次为JD,MP,Gamma和JT分布,降雨强度对微波传输特性的影响最大,入射波频率次之,温度的影响最小.所得出的结论有助于准确评估降雨对微波传输的影响以及提高利用测雨雷达和毫米波雷达等定量探测降水的精度. 关键词： 微波传输 雨滴谱分布 Mie散射 散射衰减特性     

用染料激光增益降低二元混合物中少量化合物的受激拉曼散射可探测浓度

在二元混合物构成的圆形谐振腔中,用染料的激光增益增强了少量化合物(乙醇溶液中的甲醇、水溶液中的乙醇)的受激拉曼散射（SRS）光谱,将少量化合物在悬垂液滴中的瞬态SRS探测极限提高了近一个数量级. 用受激拉曼散射理论,解释了增强少量化合物SRS的机理. 关键词： 受激拉曼散射 染料激光增益 二元混合物 最小可探测浓度     

甲醇和乙醇的三维荧光光谱特性研究

为阐明甲醇和乙醇的光谱学特性,建立两者快速的辨析方法。采用三维荧光光谱,对甲醇和乙醇的荧光特性进行了研究。分析结果显示,甲醇的三维荧光光谱中出现了两个特征荧光峰,其荧光强度在甲醇体积浓度小于15%时与其浓度成正相关,乙醇的三维荧光光谱中出现了一个完整的特征荧光峰,其荧光强度在乙醇体积分数小于50%时与乙醇浓度成相关,甲醇比乙醇具有更高的荧光效率,以甲醇为有机溶剂研究化合物的荧光特性时,需要考虑甲醇的本底荧光。甲醇和乙醇的特征荧光峰位置差别较大,前者特征荧光峰出现在225/35 0nm和250/375nm处,而后者特征荧光峰位于240/310nm,据此可以有效辨析两种醇。     

蒙特卡罗方法评定峰值检波器标定不确定度北大核心CSCD

针对高功率微波(HPM)脉冲测试用峰值检波器标定不确定度评估中测量函数非显式,直接测量量与被测量间概率分布传递困难等问题,研究建立了蒙特卡罗分析方法(MCM),利用有限带宽S参数网络时域响应计算原理建立了检波器标定系统的时域仿真模型,模型包含标定系统各微波器件S参数、检波电压、监测峰值功率等直接测量量,以及微波脉冲反射、叠加及传输延迟等物理过程。根据MCM原理对模型中各直接测量量引入特定分布的随机误差,通过重复计算,获得了检波器标定曲线的不确定度及其概率分布。     

横向电感膜片微波矩形波导的传输特性

以微波传输线等效二端口网络模型等效电路理论近似分析了含有横向电感膜片矩形波导的结构。计算发现,矩形波导内置一对膜片时膜片的宽度会影响散射参数的相位和振幅,而膜片放置的位置对散射参数的幅值及带宽没有影响。根据微波无反射传输的条件,研究了微波矩形波导内置两组横向电感膜片结构的转移矩阵。采用Matlab编程计算,得到在插入衰减最小时两组膜片在波导中的最佳间距。等效电路理论计算结果与用CST软件仿真的结果相符。研究发现,在膜片间距取最佳值时,两种方法得到的相对带宽差值仅为0.59%。     

陈皮中黄酮类化合物的微波辅助提取

黄酮类化合物对人体具有多种生理活性,对黄酮的提取研究具有重要的实用意义。实验通过改装微波炉,采用微波法提取陈皮中的黄酮类化合物,研究了乙醇浓度、微波温度、微波时间、微波反应器的火力点对黄酮提取率的影响。通过正交试验,确定了最佳提取工艺条件。     

载波调制高斯脉冲激光水下传输特性的仿真分析

光在水中传输时的吸收和散射效应使得激光的水下传输特性变得复杂,尤其是后向散射会造成目标对比度降低,导致利用激光进行水下探测时面临挑战,而载波调制技术可以用来抑制水下激光雷达的后向散射。利用基于蒙特卡罗方法的计算机仿真技术建立余弦调制高斯脉冲激光水下传输模型,并讨论光载微波脉冲宽度、调制频率和调制深度等参数对探测结果的影响。结果表明:相比于传统脉冲激光雷达的仿真结果,载波调制相关检测技术可以有效提高水下目标对比度;脉冲宽度存在最佳值,调制频率对目标回波信号的信噪比和测距精度有影响,调制深度越大,目标回波信号的信噪比越高。     

甲醇汽油、乙醇汽油定性判别及其醇含量测定模型研究

甲醇汽油和乙醇汽油都为清洁能源,但甲醇汽油和乙醇汽油的优缺点各有不同,其中甲醇、乙醇的含量决定了汽油性能的优劣,对甲醇汽油和乙醇汽油进行判别区分以及醇类汽油中醇含量进行定量测定非常重要。通过中红外光谱技术对醇类汽油的类型进行判别并对其含量进行定量分析。首先通过对比分析甲醇汽油和乙醇汽油的中红外光谱图,采用随机森林(random forest, RF)对甲醇汽油和乙醇汽油样品进行判别;在建立甲醇汽油和乙醇汽油样品定性判别模型之后,分别建立甲醇汽油和乙醇汽油的定量测定模型,从而精确测定汽油中对应的醇的含量。为减小在实验过程中实验仪器振动、噪声等原因导致的光谱漂移、光散射等现象,对中红外光谱进行预处理。首先采用不同预处理,如(savitzky-golay, S-G)卷积平滑、多元散射校正(multiplicative scatter correction,MSC)、标准正态变量变换(standard normal variable transformation,SNV)、导数(derivatives)等方法进行校正,分别建立适合甲醇汽油和乙醇汽油的检测模型。预处理后的数据分别建立甲醇汽油、乙醇汽油的最小二乘支持向量机(least square support vector machine,LS-SVM)模型。采用随机森林(random forest, RF)对甲醇汽油和乙醇汽油样品进行判别,发现当决策树个数为61时,判别正确率达到98.28%。对于LS-SVM模型,比较建模结果可知:无论是甲醇汽油还是乙醇汽油,标准正态变量变换(SNV)预处理效果最好,经SNV校正处理后建立的甲醇汽油甲醇含量测定LS-SVM模型的预测相关系数R_p为0.9519,均方根误差(root mean square error of prediction, RMSEP)为1.766 3;经过标准正态变量变换后建立的乙醇汽油乙醇含量测定LSSVM模型的预测相关系数R_p为0.951 5,均方根误差RMSEP为1.770 3。该研究可为甲醇汽油、乙醇汽油的定性判别和其含量测定提供技术参考和理论依据,为甲醇汽油产业提供测量醇类汽油检测的新方法,具有较为重要的现实意义,也为其他类型的化工产品的检测奠定了基础。     

雪层覆盖的粗糙地面与上方矩形截面柱复合电磁散射的时域有限差分法研究

采用土壤介电常数的四成分模型和雪的介电常数模型分别表示实际的地面和雪层的介电特性, 应用指数型分布粗糙面模型和Monte Carlo方法模拟实际的粗糙地面, 运用时域有限差分方法研究了雪层覆盖的粗糙地面与上方矩形截面柱的复合电磁散射问题. 得出了复合电磁散射系数的角分布曲线, 计算了双站复合散射系数随土壤与雪层粗糙度参数、介电参数、 矩形截面柱几何参数、介电参数等的变化情况, 并做了详细分析与讨论. 得到了雪层覆盖的粗糙地面与上方矩形截面柱复合电磁散射特性.     

一层非球形粒子散射的标量辐射传输迭代解的求逆

给出了一层有下垫反射面的非球形粒子散射的标量辐射传输方程一、二阶迭代解,推导了相函数各阶Legendre展开系数与随机小椭球粒子相函数的对应关系,提出了一层随机小椭球粒子介电常数和单位面积粒子数的迭代反演方法.通过两次各方位角上双站散射测量,反演随机小椭球粒子的介电常数和单位面积粒子数.本方法比现有文献的一些反演方法易于实现,可应用于颗粒性复合材料介电与结构特性以及地表背景参数的反演研究 关键词： 辐射传输方程 迭代解 反演 相函数 介电常数     

Stability of biomimetic ferrofluids established by a systematic study using microwave irradiation at defined wattages

An established biomimetic process for the synthesis of aqueous ferrofluids using polymers has been subjected to systematic microwave irradiation at different wattages primarily to see if the magnetization could be increased by microwave irradiation and if so how would it affect the stability of the fluid. Care has been taken to maintain ambient conditions of synthesis even after three cycles of microwave irradiation before oxidation and ten cycles after it, so as not to violate the basic principles of the process. Detailed characterization using, x-ray diffractometry, transmission electron microscopy, fourier transform infra-red spectroscopy, dynamic light scattering, thermo-gravimetric analysis, differential thermal analysis and vibrating sample magnetometry showed that these fluids containing iron oxide nanoparticles-poly(vinyl) alcohol nanocomposites are highly stable in the proportions established by us. Measurements at five different wattages double the saturation magnetization but the stability remains unaffected compared to the one without microwave irradiation, forcing us to believe that the incubation of the iron salt solution and the polymer in the right proportion before oxidation is what contributes to the stability and that increasing the number of cycles of microwave irradiation at this stage, perhaps, would have led to a more pronounced effect.     

瞬变等离子体微波诊断初步研究

采用微波诊断技术对气压300 Pa,S波段HPM条件下形成的低温、弱电离、非均匀和强碰撞大气等离子体特性参数开展了初步诊断研究。获取了9~15 GHz连续波穿过HPM等离子体区域的透射波形,并对波形进行了归一化处理;通过不同频率微波的透射特性,假设等离子体电子数密度分布呈Epstein分布,分析了可能的电子数密度最大值。该研究成果的获取为HPM大气等离子体在隐身、阻断信息链等方面的应用提供了重要的技术支撑。     

Microwave conductivity studies on some semiconductors

The cavity perturbation technique is employed for the characterisation of semiconductors at microwave frequency for its conductivity. Temperature variation of microwave conductivity studies provide the information regarding the band gap, scattering parameter and impurity ionization energy. Change in the real part of the dielectric permittivity with conductivity indicates the change in the momentum relaxation time.     

Design optimization for microwave coupling structures of a digital-to-analog converter based on series arrays of high-temperature superconducting Josephson junctions

A new coupling scheme of the external microwave signal for a digital-to-analog converter based on series arrays of high-temperature superconducting Josephson junctions has been proposed. It is suggested to couple the meander-like array inductively to a parallel microwave transmission line. Within the resistively shunted junction model different types of microwave coupling structures were analyzed with the goal to reduce the mutual phase locking and to achieve high output power and phase-locking stability. A harmonic balance technique was used to derive an analytic expression connecting the amplitudes of microwave voltages and currents in the array and the frequency of the external signal. Numerical simulations were performed to determine the boundaries of stable locking as a function of circuit parameters.     

Model of a microwave beam coupling to CO2 laser plasma

We have designed a transmission line model of the microwave coupling mechanism for a microwave pumped CO₂ laser. The model is a total loss ridge waveguide transmission line having nonuniform impedance. The laser plasma is modeled as a frequency-dependent lossy dielectric and acts as a distributed resistance in the length of the microwave cavity. The coupling structure of the microwaves is designed not to be resonant at the microwave source frequency of 2.45 GHz at 1 kW and propagating the total microwave field energy to be absorbed without internal reflection. An exact solution to this general transmission line propagation constant for a shunt resistance along length of the guide is found. The measurements and predictions of the parameters of the plasma conductivity as a function of the attenuation constant agree closely.     

Analysis of waveguide inserted by a metallic sheet of arbitrary shape with the method of lines

In this paper, one type of waveguide three-dimension discontinuity, rectangular waveguide inserted by a metallic sheet of arbitray shape with zero thickness, is firstly analysed by using the Method of Lines. This waveguide discontinuity can be regarded as basic unit of microwave and MMW passive components and active circuits. Numerical results are in good agreement with physical meanings and other published data. Numerical results of several characteristics are presented at the end of the paper, including convergent characteristic of which the scattering parameters are depended on the number of discretization lines, dependence of scattering parameters on the size of metallic sheet and frequency response of scattering parameters.     

Aqueous sample in an EPR cavity: sensitivity considerations

The radial mode matching (RMM) method has been used to calculate accurately the microwave field distribution of the TE(011) mode in a spherical EPR cavity containing a linear aqueous sample, in order to understand in detail the factors affecting sensitivity in EPR measurements at X band. Specific details of the experiment were included in the calculations, such as the cavity geometry, the presence of a quartz dewar, the size of the aqueous sample, and the sample's dielectric properties. From the field distribution, several key physical parameters were calculated, including cavity Q, filling factor, mean microwave magnetic field at the sample, and cavity efficiency parameter Lambda. The dependence of EPR signal intensity on sample diameter for a cylindrical aqueous sample was calculated and measured experimentally for non-saturated and half-saturated samples. The optimal aqueous sample diameter was determined for both cases. The impact of sample temperature, conductivity, and cavity Q on sensitivity of EPR is discussed.     

The effect of bound states in microwave waveguides on electromagnetic wave propagation

The transmission of a TE microwave field with a frequency ω through Λ, T, and X waveguide junctions filled with a ferromagnetic is considered. These junctions are known to have bound states with below-cutoff frequencies. A probing microwave radiation with a frequency Ω applied to the scattering region generates magnetic oscillations with frequencies ω+nΩ (where n=0, ±1, ±2, ...), which resonantly combine with the bound waveguide states. This effect provides for a new method of studying bound waveguide states and efficiently controlling the transmission of microwave radiation.     

57-km single-ended spontaneous Brillouin-based distributed fiber temperature sensor using microwave coherent detection

We present a novel technique for performing single-ended distributed fiber temperature measurements by use of microwave heterodyne detection of spontaneous Brillouin scattering. Brillouin frequency-shift measurements were obtained for a sensing length of 57 km, with a spatial resolution of 20 m. The rms error in frequency measurements at the far end of the sensing fiber was less than 3 MHz, and the overall frequency dependence on temperature was 1.07+/-0.06 MHz/K.