A uniform GTD solution for the radiation from sources on a convex surface

A compact approximate asymptotic solution is developed for the field radiated by an antenna on a perfectly conducting smooth convex surface. This high-frequency solution employs the ray coordinates of the geometrical theory of diffraction (GTD). In the shadow region the field radiated by the source propagates along Keller's surface diffracted ray path, whereas in the lit region the incident field propagates along the geometrical optics ray path directly from the source to the field point. These ray fields are expressed in terms of Fock functions which reduce to the geometrical optics field in the deep lit region and remain uniformly valid across the shadow boundary transition region into the deep shadow region. Surface ray torsion, which affects the radiated field in both the shadow and transition regions, appears explicitly in the solution as a torsion factor. The radiation patterns of slots and monopoles on cylinders, cones, and spheroids calculated from this solution agree very well with measured patterns and with patterns calculated from exact solutions.     

基于自由曲面的离轴三反光学系统研制

针对宽波段、大视场机载光学系统的设计需求,采用二次成像光路形式和XY多项式自由曲面,研制了一套基于640×512@24μm长波红外制冷型探测器的离轴三反光学系统。相比传统离轴三反光学系统,该系统解决了制冷型探测器冷光阑匹配问题和子午视场较小的设计难点,具有宽波段、大视场、透过率高、体积紧凑、无中心遮拦、无热化等技术优点。光学系统焦距160 mm,工作波段8～12μm,F数2,视场5.5°×4.4°,主镜和次镜均为二次曲面,三镜为XY多项式自由曲面。光学系统波前测试结果表明,系统波像差全视场平均值0.067λ(λ=9.11μm),具有较好的成像质量。     

基于Abaqus圆柱圆周表面激光淬火温度场仿真

针对圆柱圆周表面激光淬火的温度场形成特点,利用Abaqus软件对其进行温度场仿真分析。通过对圆柱进行三维实体建模,细化作用面,细分作用时间步长等操作,实现了对圆柱圆周表面激光淬火的温度场瞬态形成仿真。通过对仿真所得结果进行分析,得出对于圆柱圆周表面的激光淬火温度场来说淬火路径开始区域的淬火温度比淬火路径末端区域的淬火温度小。从而说明在圆柱圆周表面激光淬火时,由于激光光斑扫描的方向是趋于已淬火区域,所以已淬火区域的残留温度会对淬火区域的基温有影响,从而使得激光淬火路径末端区域的淬火温度高。研究为利用Abaqus软件对圆柱圆周表面激光淬火温度场形成仿真分析提供了指导,对圆柱圆周表面激光淬火工艺应用于实际生产有很大的帮助。     

HF Ground Wave Propagation over Mixed Land, Sea, And Sea-Ice Paths

Ground wave propagation is analyzed for a two-section path on a spherical earth. Each section can be a two-layer medium which is characterized by a surface impedance. Specific calculations for a land-to-sea path indicate that the well-known recovery effect in amplitude and phase is more extreme at higher frequencies but is reduced for elevated observer heights. Calculations for a sea-to-sea ice path indicate a brief recovery because of the excitation of the trapped surface wave over sea ice. At greater distances from the boundary, the field may be seriously degraded due to the sea ice.     

基于伪随机罗斯轨迹的光学表面中频误差抑制研究

光学表面的中频误差的控制长期以来一直是光学加工中的热点、难点问题。对于接触式的计算机控制光学表面成形技术,传统的加工路径如光栅式路径和螺旋线式路径通常会引入固有的中频误差,而新型的伪随机轨迹对机床的动态性能要求过高,使其难以普遍应用。基于对伪随机轨迹的研究,本文提出了利用伪随机罗斯轨迹对光学表面中频误差进行抑制的方法。在保证加工路径不重合和遍历工件表面的基础上,建立了伪随机罗斯轨迹的参数模型。针对传统的加工路径和伪随机罗斯轨迹进行了工艺对比实验,并对干涉结果进行功率谱密度分析。实验结果表明,三种路径对于低频误差均可以实现有效地抑制,而传统路径对于1/0.07 mm以上的频段误差会则会产生不良的影响,而伪随机罗斯轨迹则对于中频误差可以实现有效地抑制。     

大口径主镜热边界层热控对成像质量影响分析

随着望远镜口径的增大,主镜热惯性增大,主镜温度相对于环境温度的滞后性引起反射面热边界层形成热湍流波动,严重影响望远镜成像质量。介绍了一种基于CFD仿真和光程差累积的计算方法评估主镜反射面热边界层的湍流波动对成像质量的影响。以3.0 m口径的主镜为例,仿真计算了自然对流下和强迫对流下不同温升的反射面热边界层分布,将热边界层的流场参数转化成折射率场分布,采用光程差累积法得到反射面热边界层的成像质量。结果可以定量地描述反射面热边界层对成像质量的影响;验证了现有天文观测要求主镜反射面与环境温差小于2 K的合理性;同时证明了主镜反射面热控措施使得强迫对流下的光程差比自然对流下的下降了一个数量级,显著提高了主镜视宁度,进一步表明反射面热控措施对改善成像质量具有重要意义。     

Total-and scattered-field decomposition technique for the finite-element time-domain method

A new finite-element time-domain (FETD) volumetric plane-wave excitation method for use with a total- and scattered-field decomposition (TSFD) is rigorously described. This method provides an alternative to the traditional Huygens' surface approaches commonly used to impress the incident field into the total-field region. Although both the volumetric and Huygens' surface formulations theoretically provide for zero leakage of the impressed wave into the scattered-field region, the volumetric method provides a simple path to numerically realize this. In practice, the level of leakage for the volumetric scheme is determined by available computer precision, as well as the residual of the matrix solution. In addition, the volumetric method exhibits nearly zero dispersion error with regard to the discrete incident field.     

Automatic aneurysm neck detection using surface Voronoi diagrams

A new automatic approach for saccular intracranial aneurysm isolation is proposed in this work. Due to the inter- and intra-observer variability in manual delineation of the aneurysm neck, a definition based on a minimum cost path around the aneurysm sac is proposed that copes with this variability and is able to make consistent measurements along different data sets, as well as to automate and speedup the analysis of cerebral aneurysms. The method is based on the computation of a minimal path along a scalar field obtained on the vessel surface, to find the aneurysm neck in a robust and fast manner. The computation of the scalar field on the surface is obtained using a fast marching approach with a speed function based on the exponential of the distance from the centerline bifurcation between the aneurysm dome and the parent vessels. In order to assure a correct topology of the aneurysm sac, the neck computation is constrained to a region defined by a surface Voronoi diagram obtained from the branches of the vessel centerline. We validate this method comparing our results in 26 real cases with manual aneurysm isolation obtained using a cut-plane, and also with results obtained using manual delineations from three different observers by comparing typical morphological measures.     

Calculating the High-Frequency Electrical Conductivity of a Thin Metallic Layer for an Ellipsoidal Fermi Surface

The conductivity of a thin metallic layer in an ac electric field is calculated with respect to different specular reflection coefficients of the layer surfaces for an ellipsoidal Fermi surface. The ratio between the free path of electron conduction and layer thickness is not limited. The dependences of the absolute value and the argument of dimensionless conductivity on the dimensionless layer’s thickness, dimensionless external electric field frequency, and specular reflection coefficient of one of the layer surfaces at different ellipticity parameters of the Fermi surface are analyzed.     

Overwater propagation of millimeter waves

A field test program conducted to determine the short-and long-term fading characteristics and atmospheric absorption effects on an overwater millimeter-wave transmission path is described. A 6.1-nautical-mile overwater link was established using a 53-GHz transmitting frequency. Low grazing angles were used with antenna heights of 15 to 30 feet above sea level. The results of the program indicate 1) that short-term fading is Rayleigh distributed in the severest cases and is caused by surface reflections, and 2) that long-term fading variations occur more frequently and with greater intensity during warm weather periods.     

Surface electric fields and impedance matrix elements of stratifiedmedia

For the various geometrical configurations of waves in stratified media, we consider the important case when both source and field points are located on the same interface separating two different dielectric media. We denote this configuration as surface electric field case. In this paper, the electric fields are calculated numerically without using potentials. For the surface electric field case the integrand of the electric field grows with k_ρ^3/2 for large κ_ρ making the Sommerfeld integral singular. To calculate the surface electric fields in the spatial domain, we previously applied a technique of higher order asymptotic extraction. In the higher order asymptotic extraction, the higher order asymptotic parts were calculated analytically. The remainder, which has an integrand decays as κ_ρ^-3/2 was calculated numerically along the Sommerfeld contour path of integration. In this paper, we use a different extraction technique, the half-space extraction. After the half-space extraction, the integrand of the Sommerfeld integral of stratified media decays exponentially and the integral is calculated along the Sommerfeld integration path. The half-space extraction part is calculated by numerical integration along the vertical branch cuts. The surface electric fields for stratified media using half-space extraction and higher order asymptotic extraction are in good agreement. To validate the accuracy of the solution, we also compute the impedance matrix elements using surface electric fields, testing, and basis functions all in the spatial domain. The results are then compared with the results of the spectral domain method. The comparisons of the complex impedance matrix elements are tabulated and show that the difference is less than 2%     

A new measurement technique for obtaining the complex relativepermittivity of terrain surfaces

A new method for measuring the effective complex relative permittivity of a reflecting surface is presented. The approach is based on the two-ray model. We derive an equation of a circle representing the complex reflection coefficient which relates the incidence angle, frequency, and received power from the path gain using the two-ray model. The intersection point of three such circles at different heights, while maintaining the same incidence angle, yields the correct complex reflection coefficient value. By measuring the received power for both the vertical and horizontal polarizations, the relative permittivity of the surface can be determined. The technique is validated using computer simulation, as well as field measurements of typical terrain surfaces, such as asphalt, grass, and bare soil. A major advantage of this method is that it obviates the need to use antennas with a narrow beam pattern     

On Observation of Surface Electromagnetic Waves in Enclosed Spaces

An experiment on the study of waveguide propagation of surface electromagnetic waves with a frequency of 1 GHz in an enclosed space on free curved metal–dielectric interface was conducted. The distribution of the field of the electromagnetic wave along the propagation path was obtained for two cases: when a metallized tape was placed between radiating and receiving antennas and without the tape. It has been found that the field strength in the presence of the metallized tape is higher than the field strength without the tape.     

基于平面波斜入射的人体信道远场路径损耗建模

人体信道路径损耗计算对植入式通信链路预估具有重要意义。文章利用有耗媒质的电磁场边界条件、反射和透射定理并引入切向等效波阻抗定义,推导出平面波向人体斜入射时各人体组织分界面上的入射角、透射角、反射系数、透射系数、切向等效波阻抗以及各人体组织中的电磁合成波,提出了一种基于平面波向多层有耗媒质斜入射的人体信道远场路径损耗解析模型。然后以植入在肌肉为例,计算了TM波和TE波在5个常用工业通信频率以不同角度斜入射的人体信道电磁场分布与路径损耗,结果显示,电磁波在入射面的反射是影响人体信道路径损耗的关键因素,当频率在1.4 GHz附近时总路径损耗最小,TM波性能优于TE波,且当入射角小于等于30°时,总路径损耗基本保持不变。最后采用COMSOL Multiphysics建立了有限元仿真模型验证解析模型,二者结果高度吻合,最大误差仅为0.039,有力证明了解析模型的正确性和有效性。     

嵌入式系统中电子地图的路径寻优

最优路径算法是地理信息科学与计算机科学等领域的研究热点。由于嵌入式系统的特点,其中电子地图的最优路径规划算法,要求快速而高效,为此对一些最优路径搜索算法进行了分析,提出了用基于人工智能的A*算法对电子地图进行最优路径寻优,实现了准确而高效的最优路径搜索。最后给出了一个基于Linux嵌入式系统中的电子地图的路径寻优实例。     

HF ground wave propagation over sea ice for a spherical Earth model

Calculations of the field strength at 10 MHz are reported for a ground wave signal where the path is sea water covered by a uniform layer of sea ice. It is shown that a surface wave type mode may dominate the conventional ground wave modes at the shorter ranges (i.e., < 10 km). At larger ranges the field strength due to the ice layer may be seriously degraded.     

A local description of the directional characteristics of thepropagation of electromagnetic fields

A method is presented for determining a local measure of the amount of electromagnetic field propagation as a function of direction. This determination is made for the region near a given point by using the known field in the vicinity of that point within a given plane. The method is developed by assuming the parabolic equation approximation. Then the resulting equations are cast in the path integral form. A reformation of the path integral formulation is then developed. This reformation contains a quantity which is interpreted as a reasonable measure of the amount of field (amplitude) propagating in each direction. The relationship of this model to the plane-wave expansion technique is noted. The use of the method in two dimensions is illustrated by several numerical calculations. These numerical calculations are shown to be consistent with the predictions of the geometric theory of diffraction     

An analytical model for minimum drift region length of SOI RESURFdiodes

An analytical model for calculating the minimum drift region length of SOI RESURF diodes is presented with an expression for the maximum breakdown voltage of the device. The minimum drift region length is determined from the condition that the maximum breakdown voltage due to the one-dimensional field along the vertical path equals that of the lateral electric field along the surface. Analytical results agree well with the simulations using PISCES II, and qualitatively with the experimental results     

Dual slew-rate limiting for reducing gradient field interference in cardiac gated magnetic resonance imaging

A method is presented for minimising gradient field interference on the electrocardiogram (ECG) of a patient undergoing cardiac magnetic resonance imaging. A two lead ECG amplifier uses a single slew-rate limiter in the main signal path, this path providing the ECG signal plus slew-rate limited gradient interference from the scanner. A secondary signal path, also containing a slew-rate limiter, is derived from the main path and provides only slew-rate limited gradient interference. This signal can then be subtracted from the main path to cancel the original interference. This simple arrangement operates in real-time, needs no control signals from the scanner and has been successfully employed in a clinical environment.     

Efficient computation of surface fields excited on adielectric-coated circular cylinder

An efficient method to evaluate the surface fields excited on an electrically large dielectric-coated circular cylinder is presented. The efficiency of the method results from the circumferentially propagating representation of the Green's function as well as its efficient numerical evaluation along a steepest descent path. The circumferentially propagating series representation of the appropriate Green's function is obtained from its radially propagating counterpart via Watson's transformation and then the path of integration is deformed to the steepest descent path on which the integrand decays most rapidly. Numerical results are presented that indicate that the representations obtained here are very efficient and valid even for arbitrary small separations of the source and field points. This work is especially useful in the moment-method analysis of conformal microstrip antennas where the mutual coupling effects are important