含卷浪Pierson-Moscowitz谱海面电磁散射研究

海尖峰的存在会导致雷达虚警概率的上升和多目标环境中检测性能下降,因此研究海尖峰现象意义重大.海尖峰现象的一个重要特点是海面的水平极化散射强度接近甚至大于垂直极化散射强度,卷浪被认为是产生海尖峰的一个原因.首先建立了卷浪和Pierson-Moscowitz谱海面的共同模型,利用矩量法研究了卷浪模型的水平和垂直后向电磁散射特征,包括入射频率、入射角、风速和风向对电磁散射特征的影响.发现在小擦地角情况和较大风速下超级现象(水平散射强度大于垂直极化散射强度)比较明显,从而推论出在小擦地角入射下产生海尖峰现象的概率较大.同时对时变卷浪在小擦地角入射时的海杂波幅值分布特性和多普勒谱进行了分析.     

应用改进的物理光学法和图形计算电磁学近似算法快速计算导体目标电磁散射特性

结合改进的物理光学法和图形计算电磁学法实现了考虑边缘绕射情况下复杂目标的高频电磁波散射的高效且精确求解.传统的考虑边缘绕射的物理光学算法不能直接计算出目标的雷达截面,它需要先计算绕射贡献,然后加上物理光学的散射贡献,最终才能得到目标的雷达截面.通过运用改进的物理光学法对图形计算电磁学法进行修正,直接修正表面法向量,从而修正了表面电流,这样就考虑了边缘处的绕射,提高了算法的效率.这不但充分利用了计算机硬件优势,借助于计算机显示技术,由图形加速卡完成最困难、最费时的消隐工作,而且利用图形计算电磁学的积分公式,将三维空间的积分转化为屏幕像素的二维空间积分,使得计算大幅简化.数值结果表明了所提出方法的精确性和高效性.     

二维导体微粗糙面与其上方金属平板的复合电磁散射研究

研究了二维导体微粗糙面与其上方金属平板复合电磁散射特征.应用互易性原理使求解二次散射场简化为求解包含平板上的极化电流和微粗糙面散射场的积分方程，从而降低了求解难度.应用物理光学近似和微扰法分别求解了平板上的极化电流和粗糙面的电磁散射场，得到了复合散射截面计算公式并进行了数值计算.尤其对该复合模型后向耦合电磁散射结果进行了详细分析和讨论. 关键词： 互易性原理 复合电磁散射 微粗糙面 平板     

圆波导劈形端口辐射器的数值分析

 高功率微波运用的圆波导劈形端口辐射器(Vlasov-Nakajima辐射器)的辐射场可视为由平开口圆波导的辐射场和在该辐射场中的劈形圆波导段的散射场叠加而成。运用等效电磁流原理数值求解Kirchhoff-kottler积分获得圆波导TM₀₁模的辐射场,再应用物理光学方法计算劈形段的散射场,并将计算的远场方向图与实验结果比较,获得较满意的一致。     

具有早期后除极化现象的可激发系统中螺旋波破碎方式研究

在Greenberg-Hasting元胞自动机模型中引入了正常元胞和老化元胞,并规定只有老化元胞存在早期后除极化现象且早期后除极化可以激发其他元胞.在正常元胞和老化元胞均匀分布的情况下,研究了早期后除极化对螺旋波演化行为的影响,重点探讨了早期后除极化导致的螺旋波破碎方式.数值模拟结果表明:早期后除极化在比率约为26.4%的少数情况下不对螺旋波产生影响,在其他情况下则会对螺旋波产生各种影响,包括使螺旋波漫游、漂移、波臂发生形变以及导致螺旋波破碎和消失等.观察到早期后除极化通过传导障碍消失和通过转变为反靶波消失,早期后除极化导致螺旋波破碎有8种方式,包括非对称破缺导致的破碎、对称破缺导致的破碎、同时激发双波导致的破碎、非对称激发导致的破碎、整体传导障碍导致的破碎、整体快速破碎等.分析发现这些螺旋波破碎现象都与早期后除极化产生回火波有关,得到螺旋波破碎的总比率通常约为13.8%,但是在适当选取老化元胞密度和早期后除极化的激发下,螺旋波破碎比率可达到32.4%,这些结果与心律失常致死的统计结果基本一致,本文对产生这些现象的物理机理做了简要分析.     

层状介质时域有限差分方法斜入射平面波引入新方式

应用二维时域有限差分方法分析层状介质中的目标散射时,在总场-散射场边界斜入射平面波源用常规方法难以引入,因为在总场-散射场边界处设置的入射波实际上包含了入射脉冲以及各分层界面的反射和多次反射.为解决这个问题,提出了斜入射平面波的混合引入方式,即对总场-散射场的四个边界面采取不同的处理方式.对于总场-散射场的纵向侧边界,用含有斜入射角度的修正一维时域有限差分方法,只要在自由空间位置加入入射脉冲就会自行产生由各分层界面形成的反射波,包括多次反射.同时,把纵向总场-散射场侧边界向下延伸,使得总场-散射场下边界位于完全匹配层内,这样透射波和散射波均为外向行波而被吸收.对于总场-散射场的上边界,由于完全位于自由空间中,边界上各点的入射波将是总场-散射场纵向边界角点处入射波的带有时间延迟的复制.数值模拟结果表明了本文所提出方法的正确性和有效性. 关键词： 时域有限差分 层状介质 斜入射平面波 修正一维麦克斯韦方程     

基于TF/SF方法的三维复合目标电磁散射FDFD研究

分析三维频域有限差分(FDFD)中通过总场-散射场(TF/SF)边界引入入射波的方法.基于等效原理,在TF/SF边界处设置等效面电磁流,将入射波引入总场区.根据边界附近节点的位置特点,将方程式中的相关节点加入或扣除入射波的相应分量修正FDFD方程式,使得方程式中的各节点满足同为总场或同为散射场的条件.算例结果验证了该方法的正确性.利用FDFD方法计算涂敷吸波材料的复合von Karman型弹头和某导弹模型的双站RCS(radar cross section),数值结果说明该方法是分析复合目标电磁散射问题的一种有效方法.     

考虑棱边散射的半空间复杂导体目标高频分析方法

研究了半空间内复杂导体目标棱边散射的高频求解方法.分析半空间电磁波的传播规律和复杂目标棱边的电磁散射特性,将半空间并矢格林函数引入等效电磁流方法中,给出了基于等效电磁流法和物理绕射理论的半空间棱边散射场计算式,同时结合图形电磁学,对半空间复杂目标进行消隐处理,判断目标的棱边像素及获得棱边参数,与半空间目标的面元散射场相叠加,快速有效地计算了半空间复杂目标的雷达散射截面.数值结果证明了方法的有效性和准确性. 关键词： 半空间等效电磁流方法 半空间并矢格林函数 图形电磁学 雷达散射截面     

沙丘粗糙面的二次极化电磁散射

研究了新月形沙丘粗糙面的二次极化电磁散射. 结合射线追踪理论, 由一次散射面元的反射场照射到二次散射面元, 采用基尔霍夫近似推导了二次散射面元的二次极化散射场. 计算结果表明二次极化散射结果在特定的角度和类型范围内有显著影响. 在电磁波射向背风坡时可以发现其同极化散射截面在入射角较大时大于其他入射方向的结果, 入射角在休止角附近时的交叉极化散射截面出现峰值, 以及前后狭长沙丘之间的二次极化散射特别突出. 本文结果可用于反演分析沙漠地区的风场信息. 关键词： 新月形沙丘 二次极化散射 射线追踪 休止角     

准全向平板超材料吸波体的设计

本文设计了一种具有准全向吸波特性的平板超材料吸波体,其准全向吸波特性是由超材料吸波单元的双面吸波、极化不敏感和宽入射角实现的.理论分析和仿真结果表明:该吸波体在6.18 GHz的确有一个双面吸波的吸收点,且吸收率对极化角和入射角均不敏感.提取的等效阻抗表明可以调节超材料的电磁响应使其在吸收频率处与自由空间阻抗匹配来抑制反射.仿真的能量损耗分布表明:该吸波体对电磁波的吸收主要源于基板的介质损耗;采用两种不同介质基板的设计可使前吸波体与后吸波体的耦合度明显降低、抑制耦合所导致的传输.该吸波体可能在许多领域具有 关键词： 准全向吸波 双面吸波 极化不敏感 宽入射角     

Investigation of EM scattering from electrically large sea surface with breaking wave at low grazing angles

According to the high-resolution radar data, backscattering scattering from sea surfaces may give rise to super events of strong scattering and large Doppler offsets for horizontal (HH) polarization at low grazing angles (LGA), which are usually considered to be caused by breaking waves. In this paper, the mesoscale breaking wave model is introduced to account for the contribution of breaking waves, which can be responsible for sea spikes and high Doppler shift. The backscattering radar cross-section (RCS) from a single breaking wave is computed and the scattering from sea surfaces is analyzed by a composite surface scattering model, in which the phase factor of every facet is modified by the capillary waves. By using the slope criterion, the spatial distribution of the breaking waves is obtained. At the same time, the RCS of sea surfaces with breaking waves is calculated and the results agree with measured data well at LGA for HH polarization. Considering the Bragg phase velocity, orbital motion of facets and wind drift, the Doppler spectrum of sea surface with breaking wave is simulated, and when compared with the result without breaking wave, there is a good improvement for HH polarization, and for the vertical polarization, the revision is little, which are agreeable with the results of recent researches.     

Study of scattering from turbulence structure generated by propeller with FLUENT

In this article, the turbulence structure generated by a propeller is simulated with the computational fluid dynamics (CFD) software FLUENT. With the method of moments, the backscattering radar cross sections (RCS) of the turbulence structure are calculated. The scattering results can reflect the turbulent intensity of the wave profiles. For the wake turbulence with low rotating speed, the scattering intensity of HH polarization is much smaller than VV polarization at large incident angles. When the turbulence becomes stronger with high rotating speed, the scattering intensity of HH polarization also becomes stronger at large incident angles, which is almost the same with VV polarization. And also, the bistatic scattering of the turbulence structure has the similar situation. These scattering results indicate that the turbulence structure can also give rise to an anomaly compared with traditional sea surface. The study of electromagnetic (EM) scattering from turbulence structure generated by the propeller can help in better understanding of the scattering from different kinds of waves and provide more bases to explain the anomalies of EM scattering from sea surfaces.     

Application of the method of equivalent edge currents to composite scattering from the cone-cylinder above a dielectric rough sea surface

Compared with scattering from a rough surface only, composite scattering from a target above a rough surface has become so practical that it is a subject of great interest. At present, this problem has been solved by some numerical methods which will produce an enormous calculation amount. In order to overcome this shortcoming, the reciprocity theorem (RT) and the method of equivalent edge currents (MEC) are used in this paper. Due to the advantage of RT, the difficulty in computing the secondary scattered fields is reduced. Simultaneously, MEC, a high-frequency method with edge diffraction considered, is used to calculate the scattered field from the cone-cylinder target with a high accuracy and efficiency. The backscattered field and the polarization currents of the rough sea surface are evaluated by the Kirchhoff approximation (KA) method and physical optics (PO) method, respectively. The effects of the backscattering radar cross section (RCS) and the Doppler spectrum on the size of the target and the windspeed of the sea surface for different incident angles are analysed in detail.     

Correlation of Bragg scattering from the sea surface at different polarizations

The phase interference fading in Bragg backscattering from the sea surface at moderate incidence angles is considered using both moment method calculation of the scattering from measured water profiles and an implementation of the slightly rough, tilted facet model. The fading of the instantaneous scattering cross-section is shown to be independent of the instantaneous phase of the illuminating signal. The vertical and horizontal polarization fading responses are therefore strongly correlated when identical carrier frequencies and modulations are used at both polarizations since the electromagnetic energy is Bragg resonant with the same small-scale roughness in both cases, independent of whether the polarization channels are phase locked. Instantaneous horizontally polarized Bragg backscattering (HH) exceeding that at vertical polarization (VV) is extremely unlikely in this case. Use of an offset in the frequencies of monochromatic signals used at the two polarizations can lead to reduced correlation between the fading if the illumination footprint length is sufficiently large so that the frequency shift introduces a significant additional phase shift between the scattering from the leading and trailing edges of the footprint. The fading shows only a very weak correlation when this phase shift exceeds 400°. HH Bragg backscattering exceeding VV will be much more common under these specialized conditions.     

Electromagnetic backscattering from freak waves in(1+1)-dimensional deep-water

To study the electromagnetic (EM) backscatter characteristics of freak waves at moderate incidence angles, we establish an EM backscattering model for freak waves in (1+1)-dimensional deep water. The nonlinear interaction between freak waves and Bragg short waves is considered to be the basic hydrodynamic spectra modulation mechanism in the model. Numerical results suggest that the EM backscattering intensities of freak waves are less than those from the background sea surface at moderate incidence angles. The normalised radar cross sections (NRCSs) from freak waves are highly polarisation dependent, even at low incidence angles, which is different from the situation for normal sea waves; moreover, the NRCS of freak waves is more polarisation dependent than the background sea surface. NRCS discrepancies between freak waves and the background sea surface with using horizontal transmitting horizomtal (HH) polarisation are larger than those with using vertical transmitting vertical (VV) polarisation, at moderate incident angles. NRCS discrepancies between freak waves and background sea surface decreases with the increase of incidence angle, in both HH and VV polarisation radars. As an application, in the synthetic-aperture radar (SAR) imaging of freak waves, we suggest that freak waves should have extremely low backscatter NRCSs for the freak wave facet with the strongest slope. Compared with the background sea surface, the freak waves should be darker in HH polarisation echo images than in VV echo images, in SAR images. Freak waves can be more easily detected from the background sea surface in HH polarisation images than in VV polarisation images. The possibility of detection of freak waves at low incidence angles is much higher than at high incidence angles.     

The method of the boundary diffraction wave for impedance surfaces

The line integral of the boundary diffraction wave theory is extended for the diffraction process of waves by the impedance surfaces with edge discontinuities. With this aim, the exact diffraction field expression of Maliuzhinets is transformed into a line integral. The method is applied to the scattering problems of waves by a spherical reflector with edge discontinuity and the diffracted fields are evaluated asymptotically. The resultant expressions of the waves are examined numerically.     

Nonresonant mechanism of scattering of electromagnetic waves by sea surface: Scattering by steep sharpened waves

We develop an asymptotic theory of nonresonant backscattering of electromagnetic waves in the X-band by the ocean surface. Small-height (5÷20 cm) breaking surface waves with sharpened edges are assumed to be the main cause of nonresonant scattering. Using the methods of physical optics and geometrical theory of diffraction, we calculate the contribution of breaking sharpened waves to the scattering cross section for two orthogonal polarizations of electromagnetic scattering. It is shown that the main contribution to the backscattering is from the mirror reflection from the leading edge of such a wave, and the sharpness of the wave edge leads to the fact that the backscattering cross section of horizontally polarized radiation can exceed that of the vertically polarized radiation. Institute for Space Research, Russian Academy of Sciences, Moscow, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 3, pp. 240–254, March 1999.     

Physical optics theory for the scattering of waves by an impedance strip

The physical optics integral of the scattered waves by an impedance strip is derived by using the modified theory of physical optics. The surface currents of the physical optics integral, which was introduced for the scattered waves by an impedance half-plane, are taken into account. The uniform diffracted fields of the impedance strip are evaluated asymptotically. The second order diffraction terms are also obtained. The total scattered field and its subcomponents are plotted and the effect of the second order diffraction and strip width to the scattering is investigated numerically.     

The physical optics integral on the scatterer's unlit surface

The scattering integrals of the modified theory of physical optics are redefined according to the illuminated and unlit surfaces of the scattering object. With this aim the canonical problem of wedge diffraction is taken into account. It is shown that the new scattering integral contain two geometrical optics and diffracted fields. One of the geometrical optics waves is the reflected field component that propagates in the real space. The other one transmits to an imaginary space through the scattering surface and does not have any influence in the real space. The diffracted waves exist in the real space and satisfy the related boundary condition on the scattering surfaces. The resultant field expressions are compared with the exact series solution of the problem numerically.