线形小区中分布式天线协同的定时捕获改进方法

不精确的定时捕获造成分布式天线系统性能的严重下降,现有的改善分布式天线系统定时捕获性能的方法存在诸多不足。为进一步提高分布式天线系统定时捕获时的正确捕获概率,提出了一种基于分布式天线协同的定时捕获方法。该方法针对线形小区的平坦瑞利衰落信道场景,利用两根分布式接收天线接收来自单天线移动台的发射信号;随后,推导了两分布式接收天线的时延差先验信息,建立了协同定时捕获的协同条件;最后,根据门限检测方法在协同条件下进行定时捕获。分析与仿真结果表明,在定时捕获时,无论移动台处于协同区域的哪个位置,提出方法均能有效改善各分布式接收天线的正确捕获概率。     

CHARACTERISTICS OF CAVITY-BACKED NARROW SLOT ANTENNA IN A LOSSY MEDIUM

Cavity-backed slot antenna;Lossy medium;Moment method;Dyadic Green'sfunction     

Asymptotic theory for dipole radiation in the presence of a lossy slab lying on a conducting half-space

The basic theory for dipole radiation in the presence of a two-layer half-space is outlined with special reference to its use as a model for studying radio propagation through and over heavily vegetated terrain. The source dipole may be located above or below the top surface of the slab. The dipole orientation is either vertical or horizontal. The asymptotic derivations for the field expressions are carried out without making the usual assumption that the refractive index of the uppermost layer is large compared with unity. The final results exhibit the expected inverse square dependence of the field on horizontal range.     

常阻抗地平面上方垂直电偶极源电磁场的精确和有效解

地平面上方偶极源的电磁场可以分解成直达波场、镜像波场和Somm erfeld型积分三部分之和。应用圆柱波函数的球面波展开式和超几何函数理论,常阻抗地平面上方垂直电偶极源电磁场中的Somm erfeld型积分被表达成圆柱波函数(表面波模)与快速、绝对收敛的球面波函数集展开式之和;其级数展开式的系数是以表面阻抗为宗量的勒让德函数。该展开式数学物理意义明显,并且便于计算,它是此类Somm erfeld半空间问题的精确、有效解答。     

Plane-wave expansion for arrays of arbitrarily oriented piecewise linear elements and its application in determining the impedance of a single linear antenna in a lossy half-space

An infinite array of arbitrarily oriented identical elements with arbitrary identical currents is considered. The field from this array is expanded into plane inhomogeneous waves, and the mutual impedance between the array elements and an exterior arbitrarily oriented element is derived. The formulation is particularly useful when the array is located adjacent to a dielectric interface. Numerical examples are given and the relationship to earlier formulations pointed out. It is further shown that the impedance of a single element can be obtained as the average of the scan impedance taken over the entire hemisphere (called the array scanning method (ASM)). This technique has a clear physical interpretation which greatly facilitates its uses, which include the moment method solutions of wire antennas as applied to the Sommerfeld integral. Numerical evaluation is straightforward when the dipole is in the lossy half-space, and the utility of the method is demonstrated by the presentation of results for the input impedance of dipoles in a variety of half-space environments. Solution is by Galerkin's method with a piecewise sinusoidal expansion for the current. Computer time is proportional tod^{-1}, wheredis the distance of the dipole to the interface. For conducting media and low frequencies an approximation is made to reduce computation time. The moment method solution of a dipole buried at a depth as small as 1/150000 wavelength in the earth is presented.     

半空间环境中任意位置三维导体目标的电磁建模

采用基于混合位积分方程的数值方法对半空间环境中任意位置(包括界面上方、下方以及跨界面情形)的三维导体目标进行电磁建模,并通过以下两个途径来提高求解效率:一是优化阻抗矩阵的生成过程来避免格林函数的重复计算;二是在不同场源距离区域使用不同的索末菲积分计算方法(包括折合积分路径方法、最陡下降路径方法以及离散复镜像方法)来提高格林函数的单次计算效率.数值结果表明,该方法能方便地对半空间环境中任意位置目标电磁辐射与散射进行精确数值计算,同时具有较高的计算效率.     

八木天线有源折合振子馈电的新结构

文中给出了八木天线有源振子为折合振子不平衡－平衡变换器的一种新的馈电结构，实践证明，此种结构形式具有制作简单、结构合理、长期使用不用维护等优点，因此值得推广使用。     

腔体内单极天线的电流分布及输入阻抗的研究

本文利用矩量法与并矢格林函数相结合,研究了腔体内单极天线(或探针)的电流分布及输入阻抗。通过选择分段正弦基函数,不但使矩阵阶数大大减小,而且改善了矩阵元素级数的收敛性。本文的数值结果还给出了腔体内有耗介质对输入阻抗的影响。     

导电媒质平面上方垂直磁偶极源赫兹位函数的球面波展开式

精确有效地计算索末菲尔德型积分是分析导电媒质半空间电磁波辐射和散射问题的关键.本文给出求解有限导电媒质平面上方垂直磁偶极源赫兹位函数的解析方法.利用圆柱波函数的球面波函数展开式和超几何函数理论,位函数中的索末菲尔德积分被表示成快速、绝对收敛的球面波展开式;展开系数是以物性参数为复宗量的勒让德多项式.该展开式易于电磁场的计算和分析,并且不受场点和源点的位置、媒质的物性参数和频率等条件的限制.     

非对称电偶极在地层中的热效应解析解

用近似法分析了有耗媒质中非对称电偶极的激励场,得到一个场和焦耳热的简单的解析表达式。与传统的对称型半波振子公式相比较,其下端导体的电流分布表达式相同,当下端导体为四分之一波长时激励近场表达式相近。结合典型储油层给出一些计算结果,对电磁波采油技术有参考价值。     

Wire antennas in the presence of a dielectric/ferrite inhomogeneity

A moment method solution for treating thin-wire antennas in the presence of an arbitrary dielectric and/or ferrite inhomogeneity is presented. The wire is modeled by an equivalent surface current density, and the dielectric/ferrite inhomogeneity is modeled by equivalent volume polarization currents. The conduction currents on the wire and the polarization currents in the dielectric/ferrite inhomogeneity are treated as independent unknowns and determined in the moment method solution. The method is applied to the problem of a loop antenna loaded with dielectric or ferrite. Numerical results are presented, and are in good agreement with measurements and previous calculations.     

An equivalent model for the analysis of electromagnetic scatteringfrom a long, lossy tether structure

An equivalent lossy wire model is introduced for the analysis of electromagnetic scattering from a long lossy tether structure, which is used to suspend a satellite from a shuttle in free space. The equivalent model incorporates the electrical characteristics of the uniform TM₀₁ mode of a coated wire configuration and the geometrical properties of a lossy wire configuration. By considering residue contributions and the resulting surface wave quantities alone, and by adopting two additional simplifications in the calculation of induced currents and scattered field, the equivalent lossy wire model is used to determine the effect of a long, lossy tether on communications between its two ends. It is shown that for tether lengths of up to 30 km with the primary source being located close to one end of a lossy, straight tether, the presence of the tether does not have a significant effect on the near-zone fields in the neighborhood of its `receiving end'     

Radiation from aperture antennas radiating in the presence of a dielectric sphere

A more systematic and rigorous procedure for analyzing the radiation patterns of an open-ended wavegnide radiating in the presence of a dielectric sphere is presented. The analysis is based on the formula developed earlier [5] for radial translation of the spherical vector wave functions. An experimental program is described which confiras the validity of the analysis presented.     

Linkage between two microwave antennas in the presence of a cylindrical reflector

It is shown how it is possible to calculate the damping of a microwave scattered by a cylinder and a plane which contains the transmitting and receiving antennas. Experiments agree very well with theory.     

用微波辐射计测量大气积分水汽的改进算法

提出选择具有最佳频率组合的双通道微波辐射计,通过测量路径上的湿延迟来解算积分水汽含量的改进算法。由于在湿延迟测量模式中,消除了云中液态水对湿延迟测量的\     

A hybrid technique for analyzing wire antennas in the presence of a plane interface

A hybrid technique which combines the method of moments (MM) with ray methods is employed to analyze the radiation of wires in the presence of a plane interface. In this technique, which is an extension of that proposed for combining the MM with the uniform geometrical theory of diffraction (GTD), a piecewise sinusoid (PWS) Galerkin method formulation is used. In this paper the basic assumption is made that a PWS dipole can be replaced by three sources of transverse, spherical waves, so that their fields can be treated separately by standard ray methods. Via this procedure the MM matrix can be easily augmented to account for the wire-interface interactions. Calculations of the field both radiated in the antenna half-space and transmitted through a plane interface are found in very good agreement with those performed by using the rigorous Sommerfeld integral representation. These results show that this technique provides an accuracy which is widely acceptable in most engineering applications, even when the wire is placed very close to the interface. This approach, which employs ray methods to calculate reflected and transmitted field contributions, appears promising to treat the case of curved interfaces.     

竖井大长隧道火灾通风的CFD模拟

随着城市建设发展,竖井型隧道越来越多地应用于城市交通隧道中。采用计算流体力学方法对火灾工况下竖井型隧道的气流场进行了数值模拟,分析了竖井型自然通风口对高温烟气扩散的影响。认为,竖井自然通风口引入隧道外冷空气,显著降低火源端部温度,可在一定程度上减少高温烟气对火源处隧道顶板的破坏。通过竖井引入新风,显著降低火源附近的有毒气体浓度,改善了火灾救援条件。火灾产生的有毒烟气由隧道洞口集中排放转变为竖井自然通风口分散排放,这对制定火灾救援、人员疏散方案有重要的指导作用。本隧道所设置的竖井自然通风口方案可满足火灾情况下人员逃生要求。     

半空间VMD 的磁场垂直分量精确镜像理论解

精确有效计算Sommerfeld型广义积分是分析导电媒质半空间电磁波辐射和散射问题的关键。基于精确镜像理论,详细推导了导电媒质平面上方垂直磁偶极子激励的磁场垂直分量。该表达式快速收敛,易于数值计算,适用于源点与场点为任意位置、导电媒质参数任意的情形。首先利用源点与场点均位于分界面处特殊情形下磁场垂直分量的精确解析表达式,验证了精确镜像理论退化至特殊条件下给出的磁场表达式。然后计算了源与场点位于某一高度的场强。文中结果在可以等效为磁偶极子的小电流圆环场强计算中具有重要的应用价值。     

Reliable information embedding for image/video in the presence of lossy compression

Lossy compression is inherently contradictory with information hiding and may result in unreliable retrieval of embedded message. For most of the existing data hiding algorithms, the process of dealing with the conflict and trade-off of invisibility, capacity and robustness is done in an experimental fashion. We have developed an image and video unified data-embedding technique. The algorithm is capable of accurately retrieving hidden data after the embedded signals are subjected to JPEG/MPEG compression as long as its compression degree is above the predefined one. Embedding strategy is to integrate data hiding into the JPEG/MPEG coding structures and to combine the adjustment factor of the strength and quantity of data hiding with the control parameter of lossy compression, according to the constraint of human visual system, the demand of hiding capacity and distortion impact of lossy compression. Experimental evaluation demonstrates that the proposed algorithm allows us to hide data invisibly in an accurate retrievable way with effective and flexible trade-off mechanism in the presence of lossy compression. In addition, the scheme is robust to some common attacks including low-pass filtering, noisy interference, frame dropping and transcoding.     

On the behaviour of a compact antenna system with non-resonant elements in the presence of the human head

A novel handset antenna technique to solve the increasing demand of mobile bands, the loading effects (mismatching and efficiency losses) and the power absorption introduced by the head is analysed in terms of bandwidth, efficiency and SAR (specific absorption rate). The technique proposed integrates non-resonant elements and its results are compared with those obtained by a planar inverted-F antenna. The main antenna parameters (bandwidth, efficiency in free-space, efficiency regarding the human head presence and SAR) are compared in terms of electromagnetic simulation and measurements. The study concludes that the novel antenna architecture achieves multiband operation from 824–960 MHz and 1710–2170 MHz and become robust to human loading while occupying a reduced volume of just 250 mm³ in a typical handset phone.