AI-SI法分析X波段机载雷达天线罩

采用口径积分-表面积分(AI-SI)算法,对带罩机载阵列天线进行了分析。雷达罩的介质反射是引起天线方向图副瓣抬升的主要原因,在传统的AI-SI法分析天线罩过程中没有考虑天线金属阵面对天线罩介质反射的影响,因此,带来较大的计算误差。为了避免上述问题,文中在分析带罩天线过程中,采用了理想源阵列加反射板模型。天线罩总的透射场为阵列天线的一次透射场与反射板产生的二次透射场的叠加。为验证算法的正确性,将AI-SI算法计算结果与CST软件仿真结果进行了比较,两者吻合较好。     

旋转体天线罩远场方向图的快速计算

介绍了一种用矩量法分析旋转体天线罩问题中远场方向图的快速计算方法.利用旋转体天线罩结构上的特性,可以对天线罩上的场及等效电磁流进行模式分解,将二维问题转化为若干一维问题,这样求解得到的等效电流和磁流分布是以模式叠加的形式表示的.在求解远场方向图时,可以利用每个模式沿罩面环向的分布特性及天线罩的结构特性,通过解析的方法来处理方位角积分,从而可以快速计算远场方向图.     

基于高斯积分的平面波谱法分析天线罩电性能

将平面波谱表面积分法与高斯积分相结合分析天线罩的电特性。在计算了口径天线的差波谱以求解带罩天线差方向图的基础上,将高斯积分应用到由罩外表面切向场求远场方向图中,提高了平面波谱法的计算精度。最后通过具体算例表明了本文方法的有效性。     

基于近场场图的带罩天线近副瓣畸变分析

采用FEKO软件仿真带罩天线远场方向图和近场场图, 采用近场场图分析远场方向图近副瓣畸变的原因.仿真过程中采用等效源技术, 兼顾仿真精度和仿真效率.带罩天线方向图近副瓣畸变主要是由天线罩尖部区造成的, 近场场图表明三种效应导致近副瓣抬升:其一, 电磁波经过天线罩尖部发生明显的衍射, 波阵面分叉; 其二, 金属雨蚀头被激励出表面电流, 进而产生二次辐射; 其三, 部分电磁波能量经反射后透过天线罩尖部区域, 反射-透射电磁波能量叠加在副瓣方向上.针对上述效应, 对天线罩改进设计, 改进后可明显抑制带罩天线近副瓣的抬升.近场场图分析方法揭示了天线罩局部细节和远场方向图之间的联系, 对天线罩电性能优化设计有很好的指导作用.     

曲面双层带通频率选择表面天线罩设计

针对复杂曲面外形严重影响频率选择表面（ FSS)天线罩传输特性的问题,提出了一种基于表面寻迹技术的曲面FSS天线罩设计方法。首先通过平面网格剖分对曲面进行拟合及表征,然后采用表面寻迹算法对FSS阵列的排布位置进行计算,最后将平面FSS结构投影于曲面外形,从而提高了曲面FSS阵列的排布及建模精度。采用该方法完成了某K频段A夹层曲面FSS天线罩的设计及测试,结果表明该曲面FSS罩的传输特性与平面FSS基本一致,且对天线辐射方向图影响较小,有效消除了复杂曲面外形对天线罩传输特性的影响。     

大型机载雷达罩对线阵天线影响的分析

提出了一种口径积分-表面积分和射线跟踪混合方法,分析了某大型雷达罩对SSR线天线及单元方向图的影响,给出了带罩天线方向图中直射瓣和反射瓣的矢量合成的解法,数值计算结果与实验数据符合较好.     

基于天线-罩体综合设计的反射面赋形设计方法

介质夹层式天线罩在保护内部天线的同时,使得透过罩体后天线口径场的幅度和相位分布发生变化,其中相位分布的变化是引起天线远场方向图畸变的主要因素.文章从天线罩体综合设计的角度出发,提出一种反射面赋形设计方法,通过赋形反射面来改变天线口径场的相位分布,使其与天线罩引起的相位分布的趋势相反,以保证透过天线罩后的口径场仍为同相场.对某地面卫星通信站的9.14m口径天线罩进行了数值实验,结果表明:所提方法可有效改善带罩天线系统的电性能,对于方向图畸变导致的副瓣和零深变化其效果尤为显著.     

反射罩光学设计与均匀性仿真

运用非成像光学的光学扩展量理论,根据反射罩曲面上一定区域内入射光通量分布比例与反射光通量分布比例对等的原则,确定每一条入射光线所对应的反射光线方向,求出所需反射罩的形状.根据光源的配光曲线、辐照面积、辐照强度和辐照不均匀性等要求,进行大面积太阳模拟器的反射罩设计,并运用TracePro软件对所设计的系统进行光线追迹仿真.结果表明,运用这种方法设计反射罩曲面,大大缩短设计时间,提高了效率,并且所设计的曲面能较好地符合设计要求.     

用于毫米波焦面阵成像的小F数透镜焦区矢量衍射场分析

采用射线追迹法和Stratton-Chu衍射积分公式研究透镜像空间的矢量衍射场，说明了决定衍射场分布样式的口径场相位分布的确定方法，对双曲面透镜给出了不同入射倾角时的焦区场功率分布，计算结果与实验吻合，并考察了焦区场的矢量特性，计算与实验在毫米波频段完成。     

金属丝加载天线罩等效平板透波特性的GA优化

应用遗传算法(GA),对加载金属丝的天线罩等效平板结构的透波特性进行多参量、宽频带优化.采用传输矩阵法导出了等效平板结构的透射系数公式,并据此构造遗传算法的适应度函数,以金属丝加载天线罩等效平板中介质罩壁的厚度、金属丝的半径和布置间距为优化参量,编写了遗传算法程序并在指定带宽内对透波性能进行优化.通过与有限元法计算结果的对比,验证了适应度函数的正确性,分别对金属丝加载于天线罩壁中间和内表面两种等效平板结构的罩壁厚度、金属丝布置参数等进行了优化,优化结果可为天线罩工程研制提供设计参数.该算法优化速度快、计算效率高,可以为这种结构的电性能设计提供优化工具.     

Rigorous asymptotic analysis of transmission through a curved dielectric slab

A circular cylindrical dielectric layer is an idealized but rigorously analyzable model for radome covers. With a line source located on the concave side, an exact integral formulation is derived for the field transmitted to the convex side. Alternative representations are developed therefrom in terms of discrete guided modes and continuous spectra, and of ray integrals which, asymptotically at high frequencies, yield geometric optical fields that experience multiple internal reflections between the layer boundaries and also multiple reflections on the concave side. It is then shown that the higher order multiple reflected contributions can be expressed collectively as a ray field with a weighted transmission coefficient that is equivalent to the plane wave transmission coefficient for a plane parallel layer but includes a simple curvature correction. When source and observer are close to the inner and outer layer boundaries, respectively, and are also separated by a large angular interval, guided mode effects may have to be included as well. The result is a general and novel representation of the transmitted field in terms of a certain number of ordinary multiple reflected geometric optical ray fields, a single "collective" ray field, which includes in a composite manner all of the remaining internal reflections, and, possibly, the guided modes along the layer.     

Complex ray analysis of beam transmission through two-dimensional radomes

Assessment of the radiation characteristics of certain types of radome covered antennas poses the problem of transmission of amplitude tapered large-aperture fields through a curved dielectric layer. Modeling the amplitude taper by a Gaussian, the incident illumination can be generated by a source at a complex coordinate location. The radome problem is then addressed by tracing complex ray fields from the complex source point through the complex extension of the radome configuration to the real location of the observer. No integrations over equivalent apertures are required here. The complex ray tracing has been performed for conventional complex geometric trajectories as well as more accurate trajectories with lateral shifts at the radome interfaces, and takes into account multiple reflections inside the radome. As previously for real rays, the multiple internally reflected complex rays can be combined into a "collective ray" that is weighted with a curvature and (or) taper corrected slab transmission coefficient. Numerical calculations for various two-dimensional circular cylindrical and wedge-tapered prototypes and various beam illuminations demonstrate the feasibility of the complex ray method, the efficiency of the collective ray concept, and the adequacy of the ray model without lateral shifts for the radome application.     

Specular points and critical gimbal angles of ogival radomes

Results on critical gimbal angles of ogival radomes have been presented as a function of fineness ratios and source point locations. It is shown that, for a given source point and reflected ray direction, no more than two specular points generally exist on the radome inner surface. The critical gimbal angle, beyond which reflected rays contribute to geometrical optics fields, is obtained in terms of a turning-point effect. Critical gimbal angles computed are significantly different from previously published results which overlooked the turning-point effect. Special techniques to determine the contribution of specular points near the turning point are briefly discussed. The techniques proposed can be applied to rotationally symmetric geometries other than ogives     

Moment method analysis of the magnetic shielding factor of aconducting TM shield at ELF

This paper presents an integral equation and method of moments (MoM) solution to the problem of TM transmission by a metallic conducting shield at extremely low frequencies (ELF). In order to accurately compute the total fields interior to the shield, equivalent problems are formulated which avoid the numerically difficult problem of computing the total fields as the sum of the incident plus scattered fields. In particular, the total electric field on the interior surface of the shield is obtained by a volume current equivalent problem, and then the total magnetic field interior to the shield is formulated in terms of equivalent magnetic surface currents flowing on the interior surface of the shield replaced by a perfect conductor     

Calculation of the electromagnetic field behind a building, based on the equivalence principle

The electromagnetic field behind buildings is calculated by making use of the equivalence principle in two-dimensional environments. We derive equivalent currents on the building walls and other boundaries. The electromagnetic field at a receiver behind the building is calculated from those currents. We also visualize the Poynting vector field strength behind the building, which makes clear how the field is reflected or diffracted around the top of the building. We compare our result with a knife-edge model and a ray-tracing model, and we confirm that the equivalence principle provides predictions of the fields accurately. The calculation method is applied to multiple buildings by using it consecutively to obtain the field behind the last building.     

Electromagnetic scattering from an arbitrarily shaped three-dimensional homogeneous chiral body

The method of moments technique for analyzing electromagnetic scattering from an arbitrarily shaped three-dimensional homogeneous chiral body is presented based on the combined field integral equations. The body is assumed to be illuminated by a plane wave. The surface equivalence principle is used to replace the body by equivalent electric and magnetic surface currents. These currents radiating in unbounded free space produce the correct scattered field outside. The negatives of these currents produce the correct total internal field, when radiating in an unbounded chiral medium. By enforcing the continuity of the tangential components of the total electric and magnetic fields on the surface of the body, a set of coupled integral equations is obtained for the equivalent surface currents. The surface of the body is modeled using triangular patches. The triangular rooftop vector expansion functions are used for both equivalent surface currents. The coefficients of these expansion functions are obtained using the method of moments. The mixed potential formulation for a chiral medium is developed and used to obtain explicit expressions for the electric and magnetic fields produced by surface currents. Numerical results for bistatic radar cross sections are presented for three chiral scatterers - a sphere, a finite circular cylinder, and a cube.     

Calculation and experimental validation of induced currents on coupled wires in an arbitrary shaped cavity

An efficient numerical technique is presented for the calculation of induced electric currents on coupled wires and multiconductor bundles placed in an arbitrary shaped cavity and excited by an external incident plane wave. The method is based upon the finite-difference time-domain (FD-TD) formulation. The concept of equivalent radius is used to replace wire bundles with single wires in the FD-TD model. Then, the radius of the equivalent wire is accounted by a modified FD-TD time-stepping expression (based on a Faraday's law contour-path formulation) for the looping magnetic fields adjacent to the wire. FD-TD computed fields at a virtual surface fully enclosing the equivalent wire are then obtained, permitting calculation of the currents on the wires of the original bundle using a standard electric field integral equation (EFIE). Substantial analytical and experimental validations are reported for both time-harmonic and broad-band excitations of wires in free space and in a high-Qmetal cavity.     

Modeling of magnetic-field coupling with cable bundle harnesses

A field-to-line coupling model is developed for cable bundle harnesses in terms of the scattering currents and total line voltages. The equivalent distributed sources representing the effects of electromagnetic coupling are expressed as a function of the incident magnetic-field components. Such a formulation is particularly suitable to be used for the analysis of multiconductor transmission lines excited by a transient field, when data for the incident electric field are either inaccurate or not available. This model allows the accurate calculation of the induced voltages and currents on complex cable bundles. The effects on the induced voltages and currents due to ground losses and to the presence of the dielectric sheath in shielded and unshielded cables is discussed, considering bundles excited by either slow or fast transient fields. Numerical applications demonstrate the validity of the proposed procedure.     

基于滤波器电路设计多层带通频率选择表面

针对传统多层频率选择表面（frequency selective surface，FSS）的设计存在计算量大、耗时长等问题，提出了一种基于滤波器理论和FSS等效电路设计多层FSS的方法.该方法先由滤波器理论综合得到多层FSS的等效电路，再利用FSS等效电路的反演方法将其电路模型转换成多层FSS的结构参数.物理过程简单直观，可以实现任意层数的高阶FSS的快速精确设计.为了验证该方法的准确性，完成了一种K波段带通型三阶FSS天线罩设计以及实物样件加工、测试.测试结果表明该天线罩的传输曲线与滤波器理论设计结果基本一致，且对天线辐射方向图的影响较小，为多层FSS的精确设计提供了一种精确理论设计方法.