源在曲面上的NURBS-UTD方法的爬行波射线寻迹算法研究

辐射源位于凸曲面上的一致性几何绕射理论(UTD)在分析设计电大尺寸平台上天线的应用广泛,本文给出了一种源点位于任意光滑凸曲面上的UTD爬行波射线寻迹算法,使用数值的微分几何方法对暗区爬行波射线进行寻迹.对目标模型引入非均匀有理B样条(NURBS)曲面进行建模,这使得UTD方法可以应用于以往板、柱、锥模型以外的任意光滑曲面模型.本文算法可应用于任意曲面上,数值结果表明了本文算法的有效性及工程适用性.     

A UTD based asymptotic solution for the surface magnetic field on a source excited circular cylinder with an impedance boundary condition

An asymptotic solution based on the uniform geometrical theory of diffraction (UTD) is proposed for the canonical problem of surface field excitation on a circular cylinder with an impedance boundary condition (IBC). The radius of the cylinder and the length of the geodesic path between source and field points, both of which are located on the surface of the cylinder, are assumed to be large compared to a wavelength. Unlike the UTD based solution pertaining to a perfect electrically conducting (PEC) circular cylinder, some higher order terms and derivatives of Fock type integrals are found to be significantly important and included in the proposed solution. The solution is of practical interest in the prediction of electromagnetic compatibility (EMC) and electromagnetic interference (EMI) between conformal slot antennas on a PEC cylindrical structure with a thin material coating on which boundary conditions can be approximated by an IBC. The cylindrical structure could locally model a portion of the fuselage of an aircraft or a spacecraft, or a missile. Validity and accuracy of the numerical results obtained by this solution are demonstrated in comparison with those of an exact eigenfunction solution.     

Electromagnetic interaction between two parallel circular cylinderson a planar interface

Multiple interaction between two parallel and infinitely long circular cylinders on a planar interface separating two different media is analyzed theoretically. The scattering equations are derived from the so-called extinction theorem applied to this particular geometry. For simplicity, the surface is considered to be perfectly conducting although the method can be extended for any material. The equations, solved numerically by means of an appropriate discretization of the surface, provide the electric surface current density from which the scattered intensity can easily be calculated. Scattering of the transversal-magnetic and transversal-electric incident wave is studied as a function of the cylinder separation for cylinder diameters from 0.2λ to 4λ (λ being the incident wavelength). The effects of the interaction between cylinders are shown in the scattering cross section and in the surface current density of the planar substrate and of the cylinders     

Electrically small self-resonant wire antennas optimized using agenetic algorithm

One of the major limitations of electrically small antennas is that as the size of the antenna is decreased its radiation resistance approaches zero and its reactance approaches plus or minus infinity. Most small antennas are inefficient, nonresonant and, thus, require matching networks. In this investigation, we use a genetic algorithm (GA) in conjunction with the numerical electromagnetics code to search for resonant wire shapes that best utilize the volume within which the antenna is confined. Antenna configurations, over a ground plane, having from two to ten wire segments, were optimized near 400 MHz and then built and tested. As the cube size deceased from a side length of 0.096λ to 0.026λ, the computed Qs increased from 15.8 to 590. The measured Qs increased from 16.0 to 134 for cubes of 0.093 to 0.037λ on edge. This process for designing small antennas using a GA produced new self-resonant antenna configurations     

基于UTD和GA混合算法的无人机载天线布局优化

为克服无人机天线之间的相互干扰,采用遗传算法对天线进行优化布局,以减少天线间的耦合。通过采用一种新的目标函数,结合一致性几何绕射理论对圆柱体上的单极子天线和无人机表面上单极子天线的布局进行优化,得到了最佳的优化位置。通过Feko仿真对优化位置结果进行验证,得到了最佳位置处S参数值,可知天线优化位置后相互之间的耦合度变小,这也间接证明了该方法计算的正确性。该方法能够及时准确地对机载天线间的耦合度进行预测和评估,具有实际应用价值。     

用UTD方法分析电大导体表面上缝隙天线间的互耦

采用一致性几何绕射理论(UTD)结合互易定理准确分析了复杂电大导体表面上缝隙天线间的互耦。UTD是高频方法，对于解决电大系统问题十分有效。本文用UTD方法计算了电大导体圆柱面上缝隙天线间的互耦，与已有文献结果吻合得很好，对实际工程有一定的指导意义。     

并行UTD算法及在机载天线分析中的应用

要实现对复杂环境的EMC分析,最大的障碍是计算量过大,耗时太长.本文提出MPI结合UTD的并行算法,实现了大型复杂飞行器机载天线的方向图特性分析.数值结果表明,并行UTD算法可以大大节省计算时间.这一方法为在小型机系统下实现大型EMC分析软件提供了坚实的基础.     

An asymptotic high-frequency analysis of the radiation by a sourceon a perfectly conducting convex cylinder with an impedance surfacepatch [antennas]

A high-frequency approximation is presented for the fields radiated by a magnetic line or line dipole source which is located on an impedance surface patch that partly covers an electrically large, perfectly conducting convex cylinder. Relatively simple asymptotic approximations are developed for the currents induced on the impedance surface by the line sources, and the radiation patterns are calculated by incorporating these surface currents into the radiation integral. The latter integral exists only over the patch region as it uses a perfectly conducting cylinder Green's function which is expressed in terms of a uniform geometrical diffraction (UTD) solution. Numerical results are presented and shown to compare very well with other independent calculations and measurements     

Approximations to the radiation resistance and directivity ofcircular-loop antennas

The purpose of this article is to derive approximate formulas for the radiation resistance (R), and the directivity (D) of circular loop antennas. It is shown that simple approximations to the Bessel functions can be employed, to accurately model the oscillatory behavior of the Bessel function integral for both small and intermediate-sized loop antennas. Furthermore, when these approximations are combined with the usual asymptotic contributions to the integral in the case of a large input parameter ka (a=loop radius, a=2π/λ) accurate and relatively simple results for R and D can be secured for all loop sizes. Numerical results can, if necessary, be obtained using a simple pocket calculator     

A capacitively loaded PIFA for compact mobile telephone handsets

A capacitively loaded planar inverted-F antenna (PIFA) is proposed and studied. It is found that the capacitive load reduces the resonance length of the PIFA from λ/4 to less than λ/S. A design with a bandwidth of 178 MHz centered at 1.8 GHz is provided to demonstrate that compact antennas for mobile telephone handsets can be constructed using this approach. The finite-difference time-domain (FDTD) method is used in the study and experimental verification is also provided     

High-frequency scattering by objects buried in lossy media

The uniform geometrical theory of diffraction (UTD) is extended so that it can be used to calculate the scattering from an object buried in a lossy medium. First, the accuracy of this high frequency method is examined by comparing numerical results for the scattering by a polygonal cylinder in a lossy medium of infinite extent with calculations based on a method of moments (MoM) solution. Next, the more difficult scattering problem of a polygonal cylinder in a lossy half space is treated. The UTD solution for the unbounded region is employed together with the fields of rays introduced by the interface between air and the lossy medium to obtain expressions for the scattered field in air and in the lossy medium     

UTD solution for electromagnetic scattering by a circular cylinderwith thin lossy coatings

A uniform geometrical theory of diffraction (UTD) solution is obtained for the field exterior to a two-dimensional circular cylinder with a thin lossy dielectric coating. The solution is convenient for engineering applications due to its simple ray format. In the lit region, the geometrical optics (GO) solution consists of the direct incident ray and the reflected ray. In the shadow region, the geometrical theory of diffraction (GTD) uses the creeping-wave format to calculate the diffracted field. In the transition regions adjacent to the shadow boundaries, where the pure ray optical solution fails, a `universal' transition integral is used for the UTD solution to calculate the field. Numerical values for the essential transition integral are deduced, by a heuristic approach, from alternative representations of the Green's function for a circular cylinder with coating. Numerical results obtained from the UTD solution show excellent agreement with the eigenfunction results for cylinders with thin dielectric coatings     

有限长金属圆锥柱侧微带天线辐射特性计算

本文应用一致性几何绕射理论(UTD),导出了有限长金属圆锥柱侧微带天线三维辐射场和增益的计算公式.这些公式同样适用于有限长金属圆柱和圆锥.应用上述公式计算了锥侧微带天线及其排阵的任意平面方向图和增益,并与实测结果进行了比较.两者吻合良好.     

A probe antenna for in situ measurement of the complex dielectricconstant of materials

A new probe for the in situ measurement of the complex dielectric constant of materials in the microwave frequency region is described. The probe uses two stub antennas mounted on a cylinder. The cylinder is inserted in the material and the transmission coefficient between the two antennas is measured. By comparing this signal with that predicted from a numerical model of the probe obtained by the finite-difference time-domain (FDTD) method we can determine the dielectric constant of the material. The measurement setup and numerical model of the probe are described and several measurement examples in natural and artificial media are presented     

光滑曲面上测地线射线寻迹算法及爬行波计算

提出了一种光滑凸曲面上的一致性几何绕射理论(UTD)爬行波射线寻迹算法,该算法结合泰勒级数,并根据测地线的相关性质,使用计算几何方法对暗区爬行波射线进行寻迹;在参数曲面片上根据测地线性质求解爬行波轨迹点,并对爬行波在任意一点处的测地曲率进行计算,结果准确度高。最后使用该算法结合UTD方法计算二维圆柱的散射场,得到的结果与级数解吻合,验证了本文算法的有效性,扩展了UTD方法在实际工程中的应用。     

Characteristics of vector propagation channels in dynamic mobilescenarios

In wireless communications, the performance of a smart antenna system depends heavily upon vector channels describing channel propagation between an antenna array and a mobile subscriber. The smart antennas perform quite well in stationary mobile environments in which channel propagation characteristics are stable. However, in dynamic wireless environments where the mobile user is in motion, knowledge of how vector channels are affected is necessary for the proper operation of smart antennas. Here, we experimentally investigate the variation of vector channel parameters such as spatial signatures, directions-of-arrival (DOAs), and complex path attenuations with small movement (2λ) of the mobile under typical line-of-sight (LOS), line-of-sight with local scatterer (LOSLS), and nonline-of-sight (NLOS) propagation scenarios. The experiments are conducted using a 1.8-GHz smart antenna testbed developed at The University of Texas at Austin and a mobile transmitter. The results show that with small displacements, DOAs remain approximately unchanged and spatial signatures change due primarily to complex attenuations. Spatial signatures are very susceptible to the movement in the NLOS scenario, reaching up to 90% relative angle change within 2λ displacement. However, in the LOS scenario, they exhibit small and periodic fluctuations with a period of 0.6λ     

Efficient Analysis of Aperture Antennas on Generally Shaped Convex Multilayered Surfaces Using a Hybrid SD-UTD Method

A novel hybrid method is described for analyzing convex multilayered conformal array antennas. The hybrid method is based on the spectral domain approach in combination with the ray-based uniform theory of diffraction (UTD) method. The analysis is divided in two parts. First, the spectral domain approach is accelerated by using an asymptotic extraction technique where the extracted term of the Green's function is calculated using UTD. It is shown that this new approach results in significant acceleration of the existing spectral domain algorithm without losing accuracy. The modified spectral domain method is then used in the second part where generally shaped convex multilayered surfaces are analyzed by using sets of canonically shaped surfaces (spheres and/or circular cylinders). Their radii are obtained using the UTD formulation, which contains important information such as distance and curvature of the generally shaped surface along each geodesic. The results obtained using the new algorithm are compared to the available results (calculated and measured) for different conformal antennas, showing very good agreement.      

An approximation of the radiation integral for distorted reflectorantennas using surface-error decomposition

The physical optics/aperture integration (PO/AI) formulation is often used to analyze the radiation patterns of reflector antennas. In this study, the PO/AI radiation integrals for distorted reflector antennas are addressed. The surface error of the antennas is approximated by a series of surface expansion functions. The radiation integral is decomposed into a series of radiation-type integrals, each of which corresponds to one of the surface expansion functions. Each of these radiation-type integrals is then weighted by amplitude coefficients. The advantage of performing the decomposition is that each of the radiation-type integrals can be computed and the pattern data stored. The computation of the pattern for a distorted reflector antenna with a changing error profile is performed by recalling the pattern data for each perturbation term and weighting it with the amplitude coefficient. This facilitates rapid evaluation of the radiation integral in cases where the error profile is changing (for example, time-varying errors). The superposition of integrals presented in this paper was shown to be valid for surface-error profiles up to 0.1 λ rms amplitude     

An implementation of the UTD on facetized CAD platform models

The Aircraft inter-Antenna Propagation with Graphics (AAPG) 2000 computer code relies on realistic computer-aided design (CAD) platform geometrical modeling to support uniform geometrical theory of diffraction (UTD) predictions of antenna-to-antenna coupling for aircraft-mounted antennas. The code employs novel ray-tracing techniques that permit the computation of UTD propagation paths over facetized objects of general shape. Moreover, geometrical data, required by UTD formulations for wedge and smooth-surface diffraction and reflection, are computed entirely from path and facetized-object geometries. Since the entire object representation in AAPG 2000 is in terms of facets, division of path-object interactions into wedge and smooth-surface cases must be performed heuristically     

理想导电凸曲面上振子电磁辐射UTD解的并矢格林函数方法

P.H.Pathak,Wang Nan等人在研究典型问题几何绕射理论之后,于1981年发表了任意导电凸曲面振子天线高频电磁辐射一致性几何绕射理论近似解。本文应用并矢格林函数方法,通过典型曲面高频电磁辐射一致性近似解的研究和推广,导出了理想导电凸曲面上电、磁振子电磁辐射场在高频近似下一致性几何绕射理论近似解。与P.H.Pathak,Wang Nan等人的结果相比,主项并矢转移函数除个别系数外完全相同,高阶并矢转移函数在几何光学区略有差异。