Hybrid solutions at internal resonances

The use of hybrid solutions for integral equation (IE) formulations in electromagnetics is illustrated at frequencies where a perfectly conducting scatterer exhibits internal resonances. Hybrid solutions, incorporating the Fock theory and physical optics Ansatzes, and the Galerkin representation, are compared with the method of moments (MM) solutions of the electric, magnetic, and combined field formulations at such frequencies. Numerical results are presented for spheres and a right circular cylinder.     

A polarimetric model for the recovery of the high-frequency scattering centers from bistatic-monostatic scattering matrix data

The objective of this investigation is to develop multistatic electromagnetic identification/discrimination algorithms using the complete polarimetric scattering data. At high frequencies the electromagnetic scattering from a complex object is modeled by certain scattering centers. The high-frequency (physical optics) bistatic and monostatic scattering matrix properties of a flat plate model of such a scattering center are developed in detail. For the complex target representations, the single scattering center results can be extended to two and three scattering center models. It is suggested that the knowledge of the locations and the local geometries of these scattering centers can be useful in developing identification and pattern recognition algorithms.     

Lateral wave contribution to the radiation from a dielectric halfmedium

Two methods are presented for the computation of the radiation at microwave frequencies in a thin dielectric medium from a source located in a dense dielectric medium. One is based on geometrical optics and the other on physical optics. The geometrical optics approach encounters some difficulties in the evaluation of the radiation in the thin dielectric medium near grazing-incidence condition, due to excitation of a lateral wave, which is not taken in consideration by this approach. The physical optics method, which considers the lateral wave contribution, approaches this problem successfully and gives a better approximation of the radiated field near grazing. Numerical results, which compare the two methods, and experimental data are offered to validate the theoretical approach     

The geometric optics contribution to the scattering from a large dense dielectric sphere

An analysis is presented for calculating the backscattered fields of an electromagnetic plane wave by lossless dielectric spheres of arbitrary density. This method involves the Watson transformation which serves to split the exact Mie solution, given as an infinite series, into the geometrical optics fields and the diffracted fields. The former comes from the illuminated region of the sphere and may be obtained from the geometrical optics method. The latter comes from the shadow region and consists of two different types of surface waves. One is a "creeping wave" analogous to that of perfectly conducting spheres. The other is a wave which enters the sphere and emerges as a surface wave in the shadow region. This wave is unique to dielectric spheres and is the stronger of the two surface waves. In the widely used geometric optics methods it is assumed that the optics fields are the dominant contributors even though stationary rays which are not in the direction of backscatter must be added in to give a degree of agreement with the exact Mie series results. In this paper we derive the optics fields and show that they differ in some respects from those obtained by the geometric optics method. They are smaller than heretofore assumed and contribute negligibly to the backscatter in this particular range ofka(4-20). Using our rigorous approach we can show the diffracted fields to be the major contributors to the total backscatter. Numerical results for the backscattering cross sections using diffracted and optics fields, and optics fields alone will be presented for relative index of refraction of 1.6. The agreement between our results (diffracted and optics) and exact results from the Mie series is excellent. A subsequent paper will be concerned with the diffracted fields.     

Modeling of wind direction signals in polarimetric sea surfacebrightness temperatures

There has been an increasing interest in the applications of polarimetric microwave radiometers for ocean wind remote sensing. Aircraft and spaceborne radiometers have found a few Kelvins wind direction signals in sea surface brightness temperatures, in addition to their sensitivities to wind speeds. However, it was not clear what physical scattering mechanisms produced the observed brightness dependence on wind direction. To this end, polarimetric microwave emissions from wind-generated sea surfaces are investigated with a polarimetric two-scale scattering model, which relates the directional wind-wave spectrum to passive microwave signatures of sea surfaces. Theoretical azimuthal modulations are found to agree well with experimental observations for all Stokes parameters from near nadir to 65° incidence angles. The upwind and downwind asymmetries of brightness temperatures were interpreted using the hydrodynamic modulation. The contributions of Bragg scattering by short waves, geometric optics scattering by long waves and sea foam are examined. The geometric optics scattering mechanism underestimates the directional signals in the first three Stokes parameters, and predicts no signals in the fourth Stokes parameter (V). In contrast, the Bragg scattering was found to dominate the wind direction signals from the two-scale model and correctly predicted the phase changes of the upwind and crosswind asymmetries in T_υ and U from middle to high incidence angles. The phase changes predicted by the Bragg scattering theory for radiometric emission from water ripples is corroborated by the numerical Monte Carlo simulation of rough surface scattering. This theoretical interpretation indicates the potential use of polarimetric brightness temperatures for retrieving the directional wave spectrum of short gravity and capillary waves     

Scattering characteristics of non-linear loaded two-sided corner reflector

Method of integral equations of sideband component of surface currents and physical optics method can be used for calculation of non-linear corner reflector. Efficient equivalent areas of backward scattering at frequencies of incident field and sideband frequencies are calculated.     

A physical interpretation of the equivalence theorem

A physical interpretation of the equivalence theorem is presented. A simple example is used to show that the external and internal equivalence cases are analogous to incident, reflected, and transmitted wave problems in the geometrical optics sense.     

The optical crossbar switch for signal routeing on board communication satellites

A switch matrix operating on baseband or microwave signals is a critical element of communications satellites employing multiple beam antennas and on-board switching. Optical switching by spatial light modulators (SLMs) offers a means of implementing large and highly flexible switch arrays capable of routeing signals at baseband or microwave frequencies. This approach offers potential mass, power and size advantages compared to alternative technologies. The paper reviews the essential features of optical crossbar switch architectures based on SLMs and discusses options for the lasers, SLMs, interface optics and photodetectors. Proof-of-concept demonstrators for optical crossbar switches operating on both baseband and microwave signals are described. Finally, an outline design for a compact switch module is described and the critical component developments needed to realize this are identified.     

A time-domain physical optics heuristic approach to passiveintermodulation scattering

Passive intermodulation (PIM) is a phenomenon which often arises at junctions between different materials. This may be a major issue in tightly packed antenna farms as those typically present on communication satellites. Here, an heuristic nonlinear extension to the time-domain physical optics (TD-PO) is proposed, to take into account electromagnetic scattering at intermodulated frequencies when such a junction is illuminated by two impinging electromagnetic fields at different frequencies. Simulation results are compared with measures to validate the model     

一种新型红外/毫米波共口径目标模拟器电磁特性的优化设计方法

为了减小反射红外、透射毫米波的介质对红外/毫米波共口径目标模拟器的电场相位分布的影响,提高毫米波信号与红外信号的指向一致性,将几何光学法(GO)和遗传算法(GA)相结合,对红外/毫米波共口径目标模拟器中的反射红外并且透射毫米电磁波的介质表面形状进行优化,且得出了函数表达式.利用电磁仿真软件对优化后的目标模拟器指定距离处电场幅度和相位的分布进行仿真.仿真结果表明:优化后的共口径目标模拟器电磁波相位分布变化很小,毫米波信号与红外信号指向具有良好的一致性.     

Introduction a l’optique métaxiale

Metaxial optics appear first as a continuation of classical paraxial optics by studying the light transfer operated by an optical system, but here for any position of the screen. This is done in a higher order approximation where transfer surfaces are represented by their osculating spheres. In opposition to geometrical axiomatics, only diffractionprocesses are used to describe light transfer; this is done with the help of spacetime operators acting on an essentially polychromatic random electromagnetic field and simultaneously on its statistical properties, mainly coherence. A discontinuity in the medium does not affect the essential aspect of diffraction in homogeneous medium concerning the general transfer, as represented by the trilogy: Fourier transform, spatial filtering and curvature transparency, wich appear successively. It’s specificity consist in inserting in addition a coherent image of the object in the output medium. Such an image behaves like a secondary source for any screen located in this medium. An undiaphragmed centered system shows the same properties: coherent and aplanetic imaging on an image sphere, structural trilogy for any screen. Those different aspects, as well as their implications for fieldcoherence duality, are studied in detail in the general framework of a partially coherent object and emphasized for the specific case of spatial incoherence. At last, it is consequent to note that the metaxial theory keeps his validity strictly, when applied to any electromagnetic systems, especially radioelectrical ones, and even to acoustical systems.     

Water-Immersed Microwave Antennas and Their Application to Microwave Interrogation of Biological Targets

This paper describes a method of significantly improving the resolution of systems used for interrogating the spatial variation of permittivity of biosystems at S band. The basic principle employed is to contract the wavelength of the interrogating radiation and to reduce the physical aperture of the interrogating probes by immersing the transmitting antenna, receiving antenna, and the target into a material with a high dielectric constant, namely water. The antenna design is described, and line scans employing transmitted and reflected energy are presented.     

A fast physical optics (FPO) algorithm for double-bounce scattering

The fast physical optics (FPO) method for computing back-scattered fields over ranges of aspect angles and frequencies is extended to encompass double-bounce scattering. Computations are performed within the framework of the physical optics approximation appropriate in the high-frequency regime. The proposed algorithm is directly applicable to fixed angle bistatic configurations and a variety of double scattering settings. The method comprises two steps: 1) decomposition of the scatterer into subscatterers and computation of the scattering amplitudes of all pairs of subscatterers and 2) interpolation, phase correction, and aggregation of the scattering amplitude patterns of all subscatterer pairs into the pattern of the entire scatterer. The proposed method is especially suited for generating synthetic data for radar imaging simulations.     

分数傅里叶变换全息图及其再现像的解析性

将分数傅里叶变换用于全息图制作,针对各种记录方式,全面研究了分数傅里叶变换全息图无透镜再现像的共轭关系和放大率关系,确切完整地给出了分数傅里叶全息术傍轴几何光学理论的数学表达和物理解释。计算机模拟实验证明了结论的可靠与可行。     

双透镜分数傅里叶变换全息图

运用波前相因子判断法,分析了双透镜分数傅里叶变换全息图的成像特征,提供了再现像的中心坐标、共轭关系、放大率关系,完整地给出了再现过程傍轴几何光学理论的数学表达和物理解释,并在实验上进行了验证。     

Modified GTD for generating complex resonances for flat strips and disks

The complex resonance frequencies of a scatterer are important elements in target classification and identification. In the singularity expansion method (SEM), the resonances are defined by a homogeneous integral equation whose numerical solution is feasible in the low, but not in the high, frequency range. At high frequencies, the geometrical theory of diffraction (GTD) provides an attractive numerical alternative and, furthermore, incorporates an interpretation of the resonance generation process in terms of multiple wavefront (ray) traversals. Except for extremely simple scatterer configurations, the (damped) complex resonances are known to occupy an entire half of the complex frequency plane. Dominant and higher order creeping wave GTD applied to cylinders and spheres does indeed yield resonances arranged along a sequence of "layers" in that entire half-plane, but multiple edge diffracted GTD applied to flat strips and disks furnishes only a single (dominant) layer. By drawing analogies with higher order creeping waves on a smooth object, the conventional edge diffracted GTD field is here augmented by higher order ray fields undergoing higher order "slope diffraction." Each of these higher order ray fields can be made to satisfy its own resonance equation, which is now found to provide the missing layers, with remarkably accurate values for the resonances when compared, where available, with those calculated numerically by the moment and T-matrix methods. The success of higher order ray diffraction in predicting the complex resonance structure suggests that this mechanism may play a corrective role also in other edge dominated scattering phenomena.     

Complex ray interpretation of reflection from concave-convexsurfaces

Reflection from a smooth target with inflection points, when investigated by the physical optics method, reveals far-zone contributions arising from real and complex stationary points. The former represent conventional specularly reflected real-ray fields whereas the latter, which are nonspecular in real space, can be interpreted as complex-ray fields reflected specularly from the complex extension of the scatterer surface. To explain the nonspecular contributions, the complex stationary point fields are regarded as specular reflections of complex incident rays from the analytic extension of the boundary into a complex coordinate space. It is verified that this construction using complex geometrical optics is in complete agreement with physical optics asymptotics and that the complex reflection points for far-zone fields lie near the concave-to-convex transitions on the physical contour. Numerical results supporting the validity of this assertion are provided. It is concluded, therefore, that a complete ray theory of reflection from the illuminated portion of a smooth object with inflection points must include specularly reflected real as well as complex rays, with the latter originating from the complex extension of the surface contour     

Development of a dual-reflector feed for the Arecibo radiotelescope: an overview

The 305-m spherical-reflector antenna of the Arecibo Observatory will be equipped with a dual-reflector feed. This reflector system is often referred to as the Gregorian. An overview of the work that led to this design is given, and the basic ideas behind the design are described. The methods and algorithms that have been developed to synthesize the shapes of the two reflectors and to analyze them are reviewed. Results are given of analysis by forward ray tracing, using geometrical optics with edge-diffraction corrections included. At low frequencies, analysis is performed by physical optics (PO) integration. A `mini-Gregorian' that has been constructed, built, and tested to verify the dual-reflector feed concept is described     

一种用于卫星电视接收的FZP天线

本文用物理光学法对ＦＺＰ反射器的聚焦场、聚焦效率及偏轴扫描特性等进行初步分析，并由此提出一种使用ＦＺＰ天线实现多颗卫星信号同时接收的技术方案。原理性样机实际接收结果证实了其可行性