Comparison of measured and predicted bandwidth of graded-index multimode fibers

Measurements of pulse spreading in three graded-index fibers have been performed over an extensive range of wavelengths, including regions in which the index profiles become optimal and the bandwidths correspondingly high. The refractive index distributions and profile dispersion parameter have been measured and used in a ray-tracing algorithm in order to predict bandwidths. Comparison reveals that averaging processes on the index data are usually necessary to account for noncircularity of the fiber and small variations in the deduced profile caused by the wavelength dependence of the near-field intensity distribution. Results obtained by this means usually tend to slightly underestimate the true fiber bandwidth, while α-profile predictions always result in overestimates by about one order of magnitude. Remaining discrepancies between measured and predicted bandwidths are attributed to small variations of the index profiles along the fiber length.     

低频多跳天波时延估计算法比较

低频天波传播时延的准确预测对其在远程导航授时中的应用潜力挖掘具有重要意义. 为了获得地-电离层波导中低频多跳天波模式的传播时延特性,同时验证典型多径时延估计算法在不同信道环境下的作用性能,文中首先采用时域有限差分(finite-difference time-domain, FDTD)电磁计算方法对不同电离层反射情况下的距发射台400 km处地面接收的低频天地波耦合总场进行正演预测,然后分别基于快速傅里叶变换(fast Fourier transform, FFT)/快速傅里叶逆变换(inverse fast Fourier transform, IFFT)频谱相除、多重信号分类(multiple signal classification, MUSIC)和旋转不变技术信号参数估计(estimating signal parameters via rotational invariance techniques, ESPRIT)三种算法对电磁场数值预测结果进行后处理,解耦得到不同模式（地波、一跳天波、二跳天波、三跳天波及四跳天波）的时延,并在此基础上分析比较了无噪声和信噪比(signal-noise ratio, SNR)为0 dB、?5 dB以及?10 dB情况下三种算法对多跳天波的时延估计结果. 结果表明,波跳次数越高,算法的检测能力越差. 对于文中所模拟的信道条件,在弱噪声（SNR=0 dB）、电离层强反射时,FFT/IFFT算法结果精度最高,时延误差不超过400 ns;而在强噪声（SNR= ?10 dB）、电离层弱反射时,ESPRIT算法稳定性最好,误差范围在5 μs以内.     

Rain-attenuation measurements of millimetre waves over short paths

Results of rain-attenuation measurements at millimetre waves are presented that have been obtained with a shuttle-pulse technique. This technique requires a path length through rain of only a few metres, so that the rainfall rate and drop-size distribution can be considered to be uniform along this path.     

MAPP在线频响预测与实测响应的比较

１引言MeyerSound的MAPP（多用途声学预测程序）在线是服务于专业声系统设计者的一个高精度的可视化工具。用一台可上因特网的个人电脑，音响设计者可选定MeyerSound音箱的型号、放置的位置、指向，并且可随意地设定场地的大小，同样可设定环境因素如大气压力、温度和相对湿度。这些信息通过因特网传送到一台超级的远程服务器电脑上，高精确地描述每只音箱指向特性的数据进行复杂的声学预测运算。服务器将会预测出音箱的声域图，同样可以预测出在指定传声器位置的平均和峰值声压级以及频率和脉冲响应（复杂的音箱极性数据：大小和相位，…     

区域海面蒸发波导预报与监测试验对比分析

基于中尺度数值天气预报模式（The Fifth-Generation NCAR/Penn State Mesoscale Model,MM5）建立了大气波导预报平台,使用GFS数据对包含我国黄海、东海以及南海海域的区域蒸发波导分布进行预报.将2015年12月份海上航行过程中监测的不同时刻、不同海域处的蒸发波导高度同预报结果进行对比分析,结果表明二者具有较好的一致变化趋势.考虑到实时测量时间分辨率高以及数值预报预报区域广的特点,在实际舰船雷达效应评估中可充分利用两种数据,实现海上微波传播效应的预测和预报.     

Comparison of measured and predicted time dispersion and directionof arrival for multipath in a small cell environment

A number of three-dimensional (3-D) direction-of-arrival measurements in a small campus environment at 1845 MHz are described. The impulse responses and the power have been measured using an 80-MHz wide-band receiver applying correlation techniques. These measurements are compared with theoretical predictions made with a site-specific ray-based propagation predictor tool to determine the coherently reflected part of the signal. Furthermore, radiosity is applied as a model to determine the power scattered by the surfaces     

Experimental demonstration of soliton data transmission over unlimited distances with soliton control in time and frequency domains

A 2/sup 9/-1 pseudorandom-binary-sequence soliton signal has been transmitted experimentally over one million km for the first time with no degradation in the bit error rates. The synchronous modulator was driven by a timing clock signal extracted from the transmitting data signal. These results mean that it is possible to send soliton data signals over unlimited distances through the use of soliton control in the time and frequency domains.<>     

用Klippel系统对扬声器频响近场测试的分析

近年出现了一款室内条件下测试扬声器特性的Klippel系统,在普通房间里近场测试频响特性.借助一只普通的圆形扬声器,从理论基础和实际测试两个方面对扬声器进行分析对比,检验了该测试设备获得频响测试结果的可信性,并以此为基础,为选择扬声器的适用频段提供技术支持.     

谁持彩练当空舞--第四届"中国信息安全发展趋势与战略"高层研讨会企业风采录

国内众多知名厂商积极参加了第四届“中国信息安全发展趋势与战略”高层研讨会。记者辑录的其中几家业内企业,不见得完全能代表中国信息安全产业风貌,但他们从各自的历史里走来,共同编织中国信息安全产业的摇篮。在市场中拼搏生存下来的企业一定有其自己的生存法则,对这些企业不一定以成功案例分析,但我们可以从中“管窥一斑”,晓知其奋斗历程,解析其所以然。     

Comparison of measured and calculated mutual coupling in the near field between microwave antennas

Measurements of near-field mutual coupling between two moderate sized microwave antennas were performed and compared to coupling calculated using recently developed computer programs. Required input data for the programs are the complex far-field radiation patterns of the antennas and various geometrical factors describing the relative positions and orientations of the two antennas. Measured and calculated coupling as a function of both transverse and radial displacement showed good agreement.     

One-directional gain and frequency selection of electromagnetic waves in a medium with a running grating of inverse population

The effects of one-directional amplification, frequency selection and transformation of time structure of electromagnetic waves (the generation of ultra-short pulses) are considered theoretically for a medium, where the creation of a running wave of atomic states inverse population is possible under excitation by coherent light beams with different frequencies and directions. These effects are due to the possibility of synchronism of a group velocity of an amplified electromagnetic wave with the phase velocity of a wave of inverse population. The general theory is applied for describing two specific cases which are observed in CdS crystals where the gain effects are caused by: 1) radiative transitions on lines generating the P-band of recombination radiation (the visible range) and 2) radiative transitions between different exciton subbands (the far IR range).     

The electrically short sheathed dipole: Experimental relationship between its measured admittance and the permittivity of the external medium

Motivated by a study of the electrical properties of deep polar ice, a technique was developed to interpret the measured admittance of an electrically short sheathed dipole probe in terms of the complex relative permittivity of the surrounding material. With the related analytical problem unsolved, the experimental study described herein provides new information for evaluating theoretical models as well as guidelines for the full-scale design of systems to measure the properties of ice or other materials in similar circumstances, and bounds on the measured sample volume. Reduction of the admittance data is accomplished with the aid of a three-element equivalent circuit resulting from a laboratory model study that is also described. Potential applications include measure-merit of the in situ properties of rock and naturally occurring ice masses surrounding deep drill holes. Preliminary results from the Antarctic ice sheet at Byrd Station are included by way of illustration.     

Comparison of measured and calculated antenna sidelobe couplingloss in the near field using approximate far-field data

Computer programs presently exist to calculate the coupling loss between two antennas provided that the amplitude and phase of the far field are available. It is shown that when this far-field information is not available it is possible to specify approximate far fields from a knowledge of the sidelobe of each antenna along the axis of separation and the electrical size of each antenna. The ENVLP computer program developed by M.H. Francis and A.D. Yaghjian (ibid. vol.AP-34, p.952-5, July 1986) was modified for this purpose. Measurements of near-field coupling loss between two moderately sized microwave antennas were made to determine the effectiveness of using approximate sidelobe level data instead of the detailed far fields. Comparison of the measured and computed coupling indicates that the use of approximate far fields gives an estimate of the coupling loss with an uncertainty of about ±5 dB     

A comparison of predicted and measured environmental effects on amicrowave radar range

The instrumentation system used to monitor the environment to determine its effects on the vertical signal distribution of a microwave radar reflection range is described. Path-averaged refractivity changes derived from observed intensity fluctuations were significantly smaller than the refractivity changes obtained from the meteorological data. Signal intensity changes measured at ten different heights were compared with the predicted intensity changes obtained from calculations which used the measured refractivity profile. The model predicts the observed nocturnal changes but overestimates the magnitude of the change     

Propagation of short optical pulses in dispersive medium

A theoretical analysis of the propagation of transform-limited short pulses in the medium of oscillators (Lorentz model) is presented. In particular, a temporal-modes expansion of the propagating field is introduced for the nonresonance and the resonance cases. It is shown that the medium dispersion causes severe distortions in the pulse shape. The proper choice of medium parameters and/or spectral filtering of the different temporal modes may allow the formation of arbitrary pulse shapes     

On excitation of waves over a plane surface of a highly conductive medium,the Zenneck wave

Effective excitation of a surface wave, in particular, the Zenneck wave, over a plane boundary of a highly conductive medium is considered.     

不规则地面低频地波传播预测方法的性能比较

对不规则地面低频地波传播预测中积分方程方法、Millington经验公式以及FDTD算法的性能进行了分析比较.首先介绍了三种方法的模型和理论;然后分别采用这几种方法预测了传播路径中有高斯形山脉时低频地波的场强和二次时延分布,对几种方法的优势、局限性及误差进行了比较分析.结果显示:在地形起伏较小的区域,积分方程方法和FDTD方法吻合较好,Millington方法不能反映地形起伏对地波传播影响规律;当地形起伏较大时,积分方程方法也不再适用,FDTD则可以在山前及山脉区域预测到由地形反射引起的振荡信号.     

短波天波信号快衰落对干扰效果的影响

针对短波天波信号快衰落的瑞利分布规律,分析了信号快衰落对通信干扰效果的影响,结合有效干扰的准则,研究推导了在短波快衰落情况下,形成有效干扰所需要的干扰信号和通信信号场强中值之比,为短波通信对抗装备的设计提供技术参考.     

Spatial and frequency selection of waves in pseudoperiodic slow-wave structures

The principles of the joint selection of spatial harmonics and the extension of passbands of pseudoperiodic slow-wave structures (SWSs) are considered. Relationships for calculating the continuous spectrum of spatial harmonics are presented. Recurrence formulas for determining the input reflection coefficient from an SWS section are derived and used for the analysis of frequency characteristics. Selection of spatial harmonics and transformation of passbands in pseudoperiodic folded SWSs (a folded waveguide, an interdigital structure, etc.) are investigated. The possibility of extraction of either the first or the fundamental spatial harmonic in a frequency band from 30% to one octave and simultaneous suppression of spurious harmonics, including backward waves, is demonstrated. Owing to this feature, pseudoperiodic SWSs can be used in high-power wideband traveling-wave tubes.     

Plane waves and canonical sources in a gyroelectromagnetic uniaxial medium

A characteristic of a gyroelectromagnetic uniaxial medium is that its permittivity and permeability tensors have the same preferred axis. Dyadic analysis is used to obtain expressions for plane waves in such a medium, and their characteristics are explored. Conditions for incidental and pathological unirefringence are identified; and expressions for time-averaged Poynting vectors and time-averaged energy densities are obtained. Canonical sources for this medium are also identified.