Higher order diffractions from a circular disk

The backscattering from a circular disk is analyzed using the geometrical theory of diffraction (GTD). First-, second-, and third-order diffractions are included in the hard polarization analysis, while first-, second-, and second-order slope diffractions are included for soft polarization. Improvements in the prediction of the monostatic radar cross section (RCS) over previous works are noted. For hard polarization, an excellent agreement is exhibited between experimental and theoretical results, while a very good agreement is noted for soft polarization. To further improve the soft polarization results for wide angles, a model for the creeping wave or circulating current on the edge of the disk is obtained and used to find an additional component of the backscattered field. The addition of this component significantly improves the results for wide angles, leading to excellent agreement for soft polarization also. An axial-caustic correction method using equivalent currents is also included in the analysis.     

差压式流量传感器的数值模拟与优化设计

利用RNGk-ε模型对100mm口径锥体差压式流量传感器进行了CFD数值模拟实验,其等效直径比β值分别为0.50,0.65,0.85,前锥角分别为40°,45°,50°,后锥角分别为120°,130°,140°,共27种锥体组合。实验结果表明:β值越大,流出系数越小,且流出系数更易受雷诺数的影响;β值相同时,前锥角对流出系数具有较大影响,且较大的前锥角可减弱雷诺数对流出系数的影响;后锥角对流出系数的线性度具有一定程度的影响。     

Compound Fresnel Zone Lens Conformal to Truncated Cone: Antenna Application

A millimeter wave antenna consisting of two Fresnel zone plate lenses, plane and conical, is examined numerically by use of the vector diffraction theory. The lenses are of Wood-Wiltse (double-dielectric) or Soret (half-open) type, and are designed for the frequency of 117 GHz. The lenses are made conformal to a truncated circular cone with a base diameter of 500 mm and a plateau diameter of 250 mm. Designs for two opening semi-angles, 45° and 75° each of them with a particular lens thickness are presented. For the angle of 90° the cone lens becomes a plane ring lens, which in combination with the plateau zone lens forms a plane lens of size equal to the cone base diameter. Illuminated by directive feeds set at a focal distance of 525 mm from the cone apex, the double-dielectric and half-open compound and plane lenses, form three pairs of Fresnel zone lens antennas, the co-polar and cross-polar radiation characteristics of which have been compared numerically. The double-dielectric lens antennas examined are about 5 dB superior in gain to the half-open lens antennas, which has a gain of approximately 45 dBi. Because all lenses are of equal transverse aperture, the corresponding lens antennas exhibit the same ?3 dB beamwidth of about 0.33 degrees. The plane zone lens antenna is very thin and simple. Instead, the antenna comprising a 3-D compound Fresnel zone lens is thicker but can be made conformal to a specific surface shape and possesses more levels of design and optimization freedom.     

Tapered windows in SiO2by ion implantation

The enhanced etch rate of ion damaged SiO2has been used to controllably taper steps in thermally grown SiO2. A 50-keV Ar⁺implantation with a dose of 3 × 10¹³/cm²produces a uniform taper of 35-45° with no vertical step at the top edge of the window. These results are observed by viewing the sample on edge with a scanning electron microscope (SEM). The taper angle of the oxide varies from near vertical (90°) at a dose of 4 × 10¹²/cm²to very small angles at large dosages.     

Radar determination of winds at sea

This paper reviews the history and the current status of the relationship between the backscattering coefficient σ° and the wind-speed u at microwave frequencies. When σ° is assumed to be proportional to u^γ, measurements have indicated that γ lies in the vicinity of 1.4 to 2.0. Under similar conditions with incidence angle between 30° and 80°, γ for HH polarization is usually larger than that of VV polarization and γ for upwind or downwind is larger than that of the crosswind. Better surface truth and controlled experiments are still needed to obtain a more specific value for γ under a given condition. Theoretical modeling of radar sea scatter indicates that near vertical incidence the physical optics method is applicable while for incidence angles approximately in the range 30° to 80° the return can be explained by Bragg scattering due to capillary waves when the tilting effect of the large-scale waves is also included. The methods used in extracting wind vectors from SEASAT scatterometer measurements are outlined. Areas where further experimental and theoretical studies are needed are indicated.     

一种用于动中通天线的新型波导极化器

低剖面动中通系统中由于天线形式和空间的限制, 通常需要单独的极化器来进行极化跟踪。提出一 种用于动中通天线的新型波导极化器组件, 用于输出相位相同、功率比可调的两路信号。通过一个同轴式正交模耦 合器(OMT)和一个可旋转的180°介质移相器来实现任意角度的线极化, 采用垂直-水平通道相位一致的OMT 进行极化分离, 保证了要求天线处于45°极化角时垂直-水平通道的幅度误差和相位误差降低到可接受的范围, 从而保证了系统的交叉极化隔离度。     

The monostatic/bistatic approximation

Many radar cross section (RCS) prediction codes are limited to one monostatic return per run. However, such codes can calculate multiple bistatic returns per incident angle for a relatively small amount of additional computer resources. This article describes a method of using bistatic returns to generate multiple monostatic predictions for each incident angle computed. Typical results are presented, and show that the accuracy is initially high, and then degrades as the separation angle between the incident and viewing angles becomes large     

某型隐身飞机雷达隐身截面积特性分析

现代高科技战争中,提高战机隐身性能是战机设计的重要指标之一,直接影响着战机的生存力与战斗力。飞机的雷达散射截面积是飞机隐身性能评估的重要指标之一,文中依据某型隐身飞机的公开报道信息,建立了该型战机的数字模型,采用多层快速多极子算法分析了战机在频率为10 GHz,水平极化方式与垂直极化方式,俯仰角分别为0°、5°、10°、15°、20°、25°、30°条件下,以及加了尾翼情况下的雷达散射截面积,依据分析结果,系统分析了战机减少飞机雷达隐身截面积的一些措施。     

A UTD type analysis of the plane wave scattering by a fullyilluminated perfectly conducting cone

A uniform high-frequency asymptotic solution, based on the physical optics (PO) approximation, is obtained in the format of the uniform geometrical theory of diffraction (UTD) to describe the fields diffracted by the tip of a semi-infinite, perfectly conducting cone when it is fully illuminated by an electromagnetic plane wave. The solution is expressed in terms of an integral, over finite limits which can be integrated numerically without difficulty. The results computed from the uniform asymptotic PO solution compare well with previously published results given for narrow-angle semi-infinite cones. In addition, they compare well with measurement and with an independent moment method (MM) solution for the scattering by a finite flat-backed cone in which several higher order wave interactions are found to be significant; one such interaction is between the tip and the base of the cone. Expressions are provided which are useful for calculating this tip-base interaction and confirm its relative importance. These expressions also provide tip diffraction effects which are important within the forward paraxial zone for the radiation by antennas on cones     

Monostatic Reflectivity and Transmittance of Radar Absorbing Materials at 650 GHz

Transmittance and monostatic reflectivity of different radar absorbing materials at 650 GHz are presented. The reflectivity was measured in plane-wave conditions in a radar cross-section (RCS) range with vertical polarization. The lowest reflectivity level (-70 dB) was achieved with commercial absorbers TK THz RAM and Firam-500 with oblique incidence angles. Floor carpets were also studied, and the reflectivity level of those was found to be sufficiently low (from -50 to -60 dB) for use in antenna test ranges. Results agree with earlier studies and indicate the applicability of the RCS method in reflectivity measurements also at 650 GHz.     

用板块法计算任意复杂形体目标的雷达截面

本文在D Klement等人(1988)工作的基础上提出了一种计算任意复杂形体目标雷达截面(RCS)的新方法板块法,该方法运算速度快、使用灵活。利用该方法,本文计算了锥、柱等典型散射体的RCS。计算结果与测试结果吻合较好。在此基础上,本文计算了一飞机模型在不同姿态角下的RCS。经与实测比较,结果令人满意。     

Anisotropic Wettability of Biomimetic Micro/Nano Dual‐Scale Inclined Cones Fabricated by Ferrofluid‐Molding Method

Learning from nature, a series of cone‐shaped structures resembling trichomes of plants are fabricated by ferrofluid molding to understand the influence of geometry on wettability. Experimentally, ferrofluid microdroplets are generated under an external magnetic field, and their shape can be changed from right cones into oblique cones by tilting the external magnetic field. Followed by hard molds made with UV‐curable tri(propylene glycol) diacrylate, polydimethylsiloxane microcones with different inclination angle (θ) are subsequently generated. Nickel thin film is deposited onto the microcones to form micro/nano dual‐scale structures. The largest contact angle (CA) is obtained in nickel‐deposited right cones (CA = 163.1° ± 2.5°). Anisotropic wettability is exhibited in oblique cones and the retention forces in the pin and release directions differ up to 12 μN (cones θ = 50°). As explained by a model as a function of the inclination angle of the cone structures, the contact and retention forces of droplet move in pin and release directions exhibit considerable differences. Results suggest the inclination of the trichomes assist the balance between repellency and retention of water in a direction‐selective manner.     

二维分形海面雷达后向散射截面

本文中通过坐标变换,在局部坐标系中,建立起了散射场与入射场之间的关系表达式,推导出二维分形海面散射矩阵,利用此结果数值模拟二维分形海面的全极化后向雷达截面不同入射角的分布,并数值模拟了不同风向时,后向散射雷达截面的变化.发现四种极化方式下的雷达后向散射截面随入射角和风向分布图形完全相同,仅在数值上相差常数.     

Biconical antennas with unequal cone angles

The problem of radiation and reception of electromagnetic waves associated with a spherically capped biconical antenna having unequal cone angles ψ₁ and ψ₂ is investigated. Both cones that comprise a bicone are excited symmetrically at the apices by a voltage source so that the only higher order modes are TM. A variational expression for the terminal admittance is derived. Under the wide-angle approximation, expressions for the radiated field, the effective height, and the terminal admittance are obtained. In addition, limiting values of these quantities are derived for electrically small and electrically large wide-angle bicones. The results for arbitrary cone angles are new and subsume results that appear in the existing literature as special cases such as where ψ₁=ψ₂ or ψ₂=π/2. Moreover, the approximations of this paper are more accurate than many in the literature. It is argued that the radiation pattern of an electrically small cone is proportional to sin &thetas;, which is similar to that of a short dipole; whereas the pattern behaves like 1/sin &thetas; for electrically large cones. The parameter &thetas; is the angle from the bicone's axis of symmetry to the observation direction. Consequently, the direction of maximum radiation changes with exciting frequency for a bicone of fixed length. Although most of the analyses are presented in the frequency-domain, time-domain responses of bicones are discussed for some special cases that are similar to situations considered by Harrison and Williams. In particular, the time-domain radiated field and the received voltage are shown to depend on the input's passband and on the match between the source and the bicone     

随机分布多粒子侧向散射光偏振特性分析

为了研究随机分布的复合多粒子侧向散射光偏振度和粒子直径大小之间的关系,采用直径0.22μm或0.494μm的粒子与过滤的蒸馏水混合构成不同浓度的悬浮液,作为研究粒子侧向散射光偏振特性散射介质的实验方法,进行了理论分析和实验验证,取得了直径0.22μm的粒子侧向散射光强在不同深度、不同角度的偏振度和直径0.22μm或0.494μm的粒子在3种浓度中侧向散射中水平偏振度、垂直偏振度的数据。结果表明,粒子侧向散射光的退偏振对粒子直径的变化非常敏感,直径小的粒子其散射光水平方向偏振度远大于直径大的粒子,而其散射光垂直方向的偏振度却远小于直径大的粒子。这一结果对多粒子存在状况下的粒子直径检测是有帮助的(尤其适用于粒子直径小于1μm的情况)。     

基于人工磁导体的低RCS微带天线设计

基于人工磁导体(AMC)的工作机理,设计了一款工作频率在X波段的低雷达散射截面(RCS)微带天线。设计了一种AMC单元,经X和Y极化波垂直入射在8.6~14.6 GHz的频带范围内,获得180°±37°的反射相位差;将其进行正交排列组成AMC棋盘结构的反射屏,反射屏中AMC阵列块由3×3的单元组成。仿真结果显示,该反射屏较相同尺寸的PEC板具有更小的后向RCS,将此AMC结构与工作频点为10 GHz的微带天线共面排布,在保持原有天线良好辐射性能和剖面高度的同时,在8.4~14.8 GHz的频率范围内对两种极化波垂直入射实现了不低于7.5 dB的RCS缩减量。     

VHF频段隐身目标缩比模型的雷达散射截面测量

介绍了甚高频(VHF)频段、工作在水平(H)或垂直(V)极化发射-接收组合(即HH、VV、HV、VH)情况下隐身目标缩比模型的雷达散射截面积(RCS)测量方法;给出了低频段RCS测量与计算的详细过程,采用背景杂波对消和时域加窗处理的方法减少了低频段RCS测量的误差;并给出了两种隐身目标缩比模型的RCS测量结果。测量结果表明:由于谐振效应,在VHF下端的低频段,隐身目标的RCS在平方米的量级,远大于在微波段的测量值;在部分频点,交叉极化的RCS甚至比同极化还强。这为利用隐身目标在频率域的谐振效应和极化域的极化特征,设计具有探测隐身目标能力的现代雷达提供了理论依据。     

基于分块压电悬臂梁的压电地板单元研制

为了提高压电地板的发电功率,将压电圆盘或正方形压电结构分块成悬臂梁。有限元静态分析表明,当承重和材料体积相近时,三角形分块的输出功率分别是方形分块和压电圆盘的20倍、7倍;对比分析了顶角为60°、90°、120°的三角形悬臂梁压电地板单元的发电性能,发现减薄压电片和基板厚度使踩踏位移相同时,3种压电地板单元的输出功率相同;若压电片和基板厚度相同,顶角为60°的三角形分块压电地板单元输出功率最大。用6个60°三角形压电悬臂梁制作压电地板单元,以手按压,可轮流点亮两个发光二极管。     

3.11THz标准体雷达散射截面测量

针对太赫兹近场散射特性测量特点,基于CO2激光抽运的太赫兹激光器和双层独立转动平台搭建了一套高频段太赫兹雷达散射截面（RCS）测量系统。利用不锈钢光滑金属球体作为标准定标体验证了系统的可靠性,测量结果与理论值误差小于3 dBsm,系统的信噪比优于24 dB。首次利用该系统开展了3.11 THz频点处不同材料及涂覆层圆形金属平板及不同底面直径圆锥体RCS的测量。通过比较分析发现了表面阳极氧化和喷漆处理的航空铝及P304不锈钢与纯航空铝平板的RCS区别,以及不同底面直径的圆锥体RCS差异,为太赫兹频段复杂目标体RCS的研究奠定基础。     

On the derivation of bistatic RCS from monostatic measurements

The factors which govern bistatic scattering are discussed and the relations between bistatic and monostatic scattering are presented in terms of the combined effect of individual scattering centers on the target. The concept of reradiation lobe patterns of the individual scattering centers is used to define the bistatic pattern in terms of the monostatic pattern and the bistatic angle β. Analytical arguments then demonstrate that for small bistatic angles, the bistatic RCS is very closely approximated by the monostatic RCS measured on the bisector of the bistatic angle and measured at a frequency lower than the true frequency by the factor cos β/2. The limitations to this approximation which are based on the angular width of the lobe patterns reradiated from individual scattering centers are noted. The approximation is of particular value in translating measured monostatic RCS values into bistatic RCS for small bistatic angles.