Forward scattering from cylinders of triangular cross section

The induced-field ratios (IFR) of conducting cylinders of a triangular cross section have been calculated. When the width of the cross-section is in the range of 1-2 wavelengths, theE-wave IFR's are substantially less than the corresponding values for square or circular cross sections. From the standpoint of RF aperture blocking of a reflector antenna, the triangular cross section thus offers an advantageous cross section for feed-support struts.     

The RCS of a cylindrical array antenna coated with a dielectric layer

The scattering properties of dielectric coated waveguide aperture antennas mounted on circular cylinders are investigated. Both the single element antenna and the array case are treated. The array antenna consists of 4 /spl times/ 32 rectangular apertures placed in a rectangular grid on the surface of an infinitely long circular cylinder. The problem is formulated in terms of an integral equation for the aperture fields which is solved with the method of moments using rectangular waveguide modes as basis and test functions. An efficient uniform asymptotic technique is used to calculate the excitation vector and the backscattered far-field. The asymptotic solution is valid for large cylinders coated with thin dielectric layers away from the paraxial (i.e. near axial) region. A similar asymptotic solution is used to calculate the mutual coupling in the nonparaxial region. For the self coupling terms and for the mutual coupling in the paraxial region a planar approximation is used with a corresponding spectral domain technique. Numerical results are presented as a function of frequency, angle of incidence, cylinder radius, and electrical thickness of the coating.     

Three-dimensional imaging of objects in accumulated snow usingmultifrequency holography

The three-dimensional (3-D) imaging of objects buried in snow using multifrequency holography is discussed. It is pointed out that the cross-sectional images obtained by radar used for target detection in soils, sand, and snow do not meet expectations in identifying the shapes of targets. A 3-D imaging reconstruction technique using multifrequency holography is proposed that is applied to recognize the shapes of the objects buried in snow. An experimental system using X-band microwaves was constructed and image reconstruction was accomplished mathematically by computer, with the image displayed on a CRT using a specially designed circuit. Field trials are reported in which it was possible to obtain 3-D images of metallic cylinders and a mannequin buried in snow     

Scattering of TM excitation by coupled and partially buried cylinders at the interface between two media

An analysis of scattering from coupled conducting cylinders near the planar interface between two semi-infinite, homogeneous halfspaces of different electromagnetic properties and from partially buried conducting cylinders is presented. The perfectly conducting cylinders of general cross sections are of infinite extent and the excitation is transverse magnetic to the cylinder axes. Coupled integral equations for the unknown current induced on the cylinders are derived and a numerical method for solving them is described. In addition, a simple technique is employed to determine the far-zone scattered field from knowledge of the cylinder current. Data displaying the distribution of the induced current and the scattered field patterns for cylinders of interest are presented and discussed.     

High efficiency and low sidelobe design for a large aperture offset reflector antenna

A design method giving high efficiency and low sidelobes is discussed for large aperture offset reflector antennas. A new reflector shaping technique using the subreflector and the beam waveguide reflector with the parabolic main reflector is proposed to simplify the main reflector manufacturing process. The effectiveness of the technique is confirmed by the model experiments. One problem with this reflector shaping technique is that the subreflector edge level cannot be controlled independently of the main reflector edge level. By investigating the relation between the gain reduction and the subreflector edge level, which affects the wide-angle sidelobe levels, the realizable characteristics of antennas are studied. In order to decrease the subreflector edge level without reducing the aperture efficiency, a technique using an extended reflector is also proposed. Its effectiveness is shown by theoretical and experimental investigations.     

Losses, sidelobes, and cross polarization caused by feed-supportstruts in reflector antennas: design curves

In a symmetrical reflector antenna the feed system and its support struts block the aperture and thereby deteriorate the radiation characteristics. Simple design curves are presented for the efficiency reduction, the sidelobe levels, and the cross polarization caused by strut blockage. The results are obtained from an analytical study that includes the induced field ratio (IFR) of the struts. The most significant IFR values for struts with circular cross section are calculated and plotted in a way which makes them easy to use as design curves. The use of the design curves is demonstrated by an example     

Backscatter RCS for TE and TM excitations of dielectric-filledcavity-backed apertures in two-dimensional bodies

Transverse electric (TE) and transverse magnetic (TM) scattering from dielectric-filled, cavity-backed apertures in two-dimensional bodies are treated using the method of moments technique to solve a set of combined-field integral equations for the equivalent induced electric and magnetic currents on the exterior of the scattering body and on the associated aperture. Results are presented for the backscatter radar cross section (RCS) versus the electrical size of the scatterer for two different dielectric-filled cavity-backed geometries. The first geometry is a circular cylinder of infinite length which has an infinite length slot aperture along one side. The cavity inside the cylinder is dielectric filled and is also of circular cross section. The two cylinders (external and internal) are of different radii and their respective longitudinal axes are parallel but not collocated. The second is a square cylinder of infinite length which has an infinite length slot aperture along one side. The cavity inside the square cylinder is dielectric-filled and is also of square cross section     

Analytic design of conformal slot arrays

A completely analytic design process has been developed for small slot arrays which accounts for the varying effect of mutual coupling as a function of element position. A previously developed theory for the design of small arrays has been extended to include conformal dielectric-filled waveguide slot arrays. Computer software has been assembled which enables calculation of the slot geometries required to implement a specified aperture distribution and input impedance condition. The slot self- and mutual admittances are calculated numerically, thus eliminating the traditional measured slot data from the design process. This design technique has been applied to conformalX-band slot arrays on cylinders of a few wavelengths diameter. The arrays consist of multiple dielectric-filled waveguides, each of which is a narrow-band standing-wave linear array of longitudinal shunt slots. The computerized design process adjusts the length and offset of each slot in the total array until the desired aperture distribution and impedance match are achieved. A flow diagram of the design program and test results from experimental arrays are presented.     

电磁窗桁架阻挡比对天线口径场影响分析

基于感应电流率理论, 提出了将阻挡面积与口径场分布相结合的阻挡比计算方法, 修正了传统算法存在的入射场分布不均匀问题, 分析了金属桁架电磁窗中金属桁杆的散射效应, 研究了桁架阻挡比在特定天线口径场分布下对天线传输特性的影响, 以卡塞格伦天线为例计算了平行极化波入射桁杆阵列时的插入损耗, 并将计算结果与FEKO软件中矩量法仿真结果进行了对比验证.结果表明:考虑天线口径场的阻挡比算法相比传统算法在计算精度上有了较大提高, 最大误差不超过0.1 dB; 金属桁架电磁窗的插入损耗不仅受阻挡面积与感应电流率的影响, 还与天线口径场能流密度分布有关; 减小阻挡比, 是设计低损耗电磁窗的有效途径.     

Analysis of a wide radiating slot in the ground plane of amicrostrip line

An analysis of a wide rectangular radiating slot excited by a microstrip line is described. Coupled integral equations are formulated to find the electric current distribution on the feed line and the electric field in the aperture. The solution is based on the method of moments and using the space domain Sommerfeld-type Green's function. The information about the input impedance or reflection coefficient is extracted from the electric current distribution on the microstrip line utilizing the matrix pencil technique. The theoretical analysis is described and data are presented and compared with other theoretical and experimental results     

Scattering from parallel metallic cylinders with arbitrary cross sections

The integral equations for scattering by a set of parallel metallic cylinders, each cylinder of arbitrary cross section, are solved directly by means of a digital computer program giving the current distribution induced on the scatterer surfaces, the scattering cross section vs azimuthal angle, and the induced field ratio (IFR) for both parallel and perpendicularly polarized incident waves. The present program is suitable for cylinders up to25lambdato40lambdacircumference. The validity of the computer program has been verified by comparing results 1) with exact calculations for circular cylinders, 2) with measured values for more than 40 rectangles and rectangles with feathered and rounded faces, and 3) by interchanging the direction of incidence and the direction of observation, whereby, according to the reciprocity theorem, the observed scattered field must remain the same.     

Radar imaging from a distorted array: The radio camera algorithm and experiments

High angular resolution radar imaging may be achieved with a large-aperture antenna even if the aperture is distorted, provided that adaptive signal processing compensates for the distortion. The radio camera is an instrument designed for this purpose. Its algorithm for imaging ground-based targets is described and experimental results are given for a 3 cm wavelength demonstration system using a distorted 27-m random sparse array. The measured beamwidth of 1 mrad conformed to theory, confirming the validity of the technique. Extension of the algorithm to accommodate isolated targets such as aircraft and ships also is discussed.     

Electromagnetic diffraction from a dielectric-filled slit-cylinderenclosing a cylinder of arbitrary cross section: TM case

A simple moment solution to the problem of the diffraction of a TM plane wave from an infinite, perfectly conducting slotted cylinder of an arbitrary cross section is summarized. The slit cylinder encloses a smaller perfectly conducting cylinder of an arbitrary cross section, and the space between the cylinders is filled with a dielectric material. The equivalence principle is used to obtain a set of coupled integral equations for the induced/equivalent surface currents on the cylinders, and the method of moments is used to solve numerically the integral equations. The electric field integral equation formulation is used. The advantages and the limitations of the method are discussed. Sample results for the induced current, aperture field, internal field, and scattering cross sections are given. These are in good agreement with some of the available published data     

DESIGN AND ANALYSIS OF MULTI-STEP AMPLITUDE QUANTIZATION WEIGHTED 2-D SOLID-STATE ACTIVE PHASED ARRAY ANTENNAS

An aperture design technique using multi-step amplitude quantization for two-dimensional solid-state active phased arrays to achieve low sidelobe is described. It can be applied to antennas with arbitrary complex aperture. Also, the gain drop and sidelobe degradation due to random amplitude and phase errors and element (or T/R module) failures are investigated.     

多阶振幅量化加权二维固态有源相控阵天线的设计与分析

本文论述了一种多阶振幅量化二维固态有源相控阵天线获得低副瓣的口径设计方法,这种方法可适用于任意复杂口径天线。另外,对于由随机幅相误差和单元(或T/R组件)失效而引起的副瓣电平恶化和增益下降也进行了分析。     

Application of the Backscattering Technique to the Determination of Parameter Fluctuations in Multimode Optical Fibers

An experimental study of backscattering traces from fibers with random variations of core diameter and numerical aperture has been carried out. The "mode-filtering" technique, which is employed in the measurements, is shown to be a powerful means of separating the parameter fluctuation effects from the actual exponential power decay. The experimental results are then found to be in excellent agreement with the predictions of a recent backscattering theory. It is shown that backscattering measurements can easily be used to place upper limits on the magnitudes of parameter fluctuations in present day fibers.     

A comparison between near-field shielding-effectivenessmeasurements based on coaxial dipoles and electrically small apertures

The near-field shielding effectiveness (SE) of a material can be measured by placing it between two closely spaced dipoles (electric or magnetic) and noting the resulting insertion loss. An alternative approach is to cover an electrically small aperture with the test material and measure the resulting loaded aperture polarizability (electric or magnetic), as is done in a dual TEM (transverse electromagnetic) cell. The author develops approximations leading to simple closed-form SE expressions by developing expressions that relate these two configurations. Small-aperture theory is discussed first, followed by a development of approximate dipole coupling expressions. The results are then compared with an experimental example     

Sizing Trunk Bundles Which Can Be Seized at Both Ends with Different Grade of Service

In this paper, a technique is established and prepared tables are furnished for the sizing of trunk bundles which can be seized at both ends, but where different grade of service must be rendered at each end. An approximate approach is also discussed. These solutions find applications in the engineering of foreign exchange voice systems serving a customer base (low blocking) and interdepartmental communications (high blocking).     

Pattern distortion of aperture antennas radiating in the presence of conducting platforms

An analytically simple and numerically efficient technique for calculating the pattern distortion of aperture antennas radiating in the presence of conducting platforms located in the near or far field of the antenna is presented. The technique presented, based on uniform geometrical theory of diffraction (UGTD), is also applicable for large aperture antennas (aperture area> 15 lambda^{2}). An excellent agreement between the calculated and measured results obtained for a typical aperture antenna mounted on a conducting platform confirms the validity of the analytical technique developed.