新型微波暗室吸波材料——AXB型吸波材料

介绍了新型微波暗室吸波材料--AXB型吸波材料,该材料同常规吸波材料相比吸波性能相当,同时,具有功率承载能力强、安全环保、洁净卫生、结构稳定等显著特点。自1999年定型以来已成功建造各类微波暗室70多座,被广泛应用于雷达、卫星、无线通信、天线测量和EMC测试领域。     

FDTD analysis of wave propagation in nonlinear absorbing and gainmedia

An explicit finite-difference time-domain (FDTD) scheme for wave propagation in certain kinds of nonlinear media such as saturable absorbers and gain layers in lasers is proposed here. This scheme is an extension of the auxiliary differential equation FDTD approach and incorporates rate equations that govern the time-domain dynamics of the atomic populations in the medium. For small signal intensities and slowly varying pulses, this method gives the same results as frequency-domain methods using the linear susceptibility function. Population dynamics for large signal intensities and the transient response for rapidly varying pulses in two-level (absorber) and four-level (gain) atomic media are calculated to demonstrate the advantages of this approach     

Modeling of ground-penetrating-radar antennas with shields andsimulated absorbers

A three-dimensional (3-D) finite-difference time-domain (FDTD) scheme is employed to simulate ground-penetrating radars. Conducting shield walls and absorbers are used to reduce the direct coupling to the receiver. Perfectly matched layer (PML) absorbing boundary conditions are used for matching the multilayered media and simulating physical absorbers inside the FDTD computational domain. Targets are modeled by rectangular prisms of arbitrary permittivity and conductivity. The ground is modeled by homogeneous and lossless dielectric media     

Simulations of ground-penetrating radars over lossy andheterogeneous grounds

The versatility of the three-dimensional (3D) finite-difference time-domain (FDTD) method to model arbitrarily inhomogeneous geometries is exploited to simulate realistic ground-penetrating radar (GPR) scenarios for the purpose of assisting the subsequent designs of high-performance GPR hardware and software. The buried targets are modeled by conducting and dielectric prisms and disks. The ground model is implemented as lossy with surface roughness, and containing numerous inhomogeneities of arbitrary permittivities, conductivities, sizes, and locations. The impact of such an inhomogeneous ground model on the GPR signal is demonstrated. A simple detection algorithm is introduced and used to process these GPR signals. In addition to the transmitting and receiving antennas, the GPR unit is modeled with conducting and absorbing shield walls, which are employed to reduce the direct coupling to the receiver. Perfectly matched layer absorbing boundary condition is used for both simulating the physical absorbers inside the FDTD computational domain and terminating the lossy and layered background medium at the borders     

A time-domain method for characterizing the reflection coefficientof absorbing materials from 30 to 1000 MHz

A wideband time-domain reflectometer is used to evaluate the reflection characteristics of RF/microwave absorbers. The reflectometer uses an array of two identical broadband antennas (both transmitting and receiving). The two antennas are used in a difference mode to remove the undesired signals and enhance the small reflections being measured. Using this technique, one can separate the target surface reflection from those generated outside the target area. The bandwidth of the pulses is 30 to 1000 MHz, and the reflection coefficient is measured over this range. The method has been used to characterize the reflectivity of three different types of absorber placed in an anechoic chamber. The results are reported together with a discussion of the main sources of errors     

Electromagnetic scattering by pyramidal and wedge absorber

Electromagnetic scattering from pyramidal and wedge absorbers used to line the walls of modern anechoic chambers is measured and compared with theoretically predicted values. The theoretical performance for various angles of incidence is studied. It is shown that a pyramidal absorber scatters electromagnetic energy more as a random rough surface does. The apparent reflection coefficient from an absorber wall illuminated by a plane wave can be much less than the normal absorber specifications quoted by the manufacturer. For angles near grazing incidence, pyramidal absorbers give a large backscattered field from the pyramid side-faces or edges. The wedge absorber was found to give small backscattered fields for nuclear-grazing incidence. Based on this study, some new guidelines for the design of anechoic chambers are advocated because the specular scattering models used at present do not appear valid for pyramids that are large compared to the wavelength     

Human proximity effects on circular polarized handset antennas inpersonal satellite communications

Satellite-based systems are the next step in mobile communications. Several low and medium Earth orbit mobile communication satellite systems have been proposed and are currently being deployed. For all these systems, high-performance circularly polarized antennas for the mobile terminals are of importance. Although considerable material is available on circularly polarized antennas, there is an absence of information on how the human's close proximity to the antenna affects the circular polarization purity of the radiated field. This paper presents an analysis of representative circularly polarized helical handset antennas. The helix is used because of its wide bandwidth properties and capability to provide both circular and linear polarization. Thus, this element could be used for handsets that are planned to work with both the terrestrial-based communications systems of today and the satellite-based communication systems of tomorrow. The intent is to characterize the effects the close proximity of a human head model has on the computed performance of the circular polarized antenna. The method of moments (MoM) and finite-difference time-domain (FDTD) numerical techniques are used to study various helix structures on top of a small box representing a handset. In order to be able to effectively apply the FDTD, a novel square helix structure is introduced. Results computed with these two techniques are compared to illustrate the accuracy of each implementation. The results indicate significant polarization purity degradation caused by the presence of a human head. For the particular geometry simulated, the presence of a head model degraded the average axial ratio within a 50° vertical cone from 2.9 to 9.1 dB. This significant increase in axial ratio can have profound effects on link budgets     

具有屏蔽腔和吸波材料的探地雷达天线的FDTD分析

天线是探地雷达系统的关键部件之一,系统要求天线具有较好的辐射波形以及一定的隔离度.给出了一种可用于探地雷达系统的分布电阻加载的变形TEM喇叭天线,并且采用三维FDTD方法分析了屏蔽腔和吸收材料对其辐射特性的影响.计算结果表明,通过加入屏蔽腔和吸收材料,可以减小收发天线之间的直耦信号,使目标回波信号更加明显,从而提高了系统的动态范围.     

Absorbing surface impedances of lossy layers in the finite difference time-domain method

In this paper the time-domain surface impedances of an homogeneous absorber layer, are given for the vertical and horizontal polarizations, or respectively for the electric field perpendicular or parallel to the incidence plane. It turns out that the application of the concept in finite difference time-domain (FDTD) in absorbing surface impedances boundary conditions, gives results in good agreement with analytical Fresnel reflection coefficients.     

Design of cavity-backed slot antennas using the finite-differencetime-domain technique

A cavity-backed slot antenna is thought to be one of the most suitable elements for the wireless transmission of microwave energy. A design technique is developed for the cavity-backed slot antenna using the finite-difference time-domain (FDTD) method. The technique is effective in characterizing antenna performance such as the input impedance and the far-field pattern since it takes into account the geometry of the feeder as well as the cavity. We present a method that overcomes difficulties when the FDTD method is used to design the antenna. Moreover, we discuss how to determine the calculation parameters used in the FDTD analysis. Several numerical results are presented, along with measured data, which demonstrate the validity, efficiency, and capability of the techniques. The paper proposes a new prediction method for the frequency characteristics of the cavity-backed slot antenna, which applies computational windows to time-sequence data. It is emphasized that windowing the slow decaying signal enables the extraction of accurate antenna characteristics. We also discuss how to estimate the antenna patterns when we use a sinusoidal voltage excitation     

Analytical study on change of temperature and absorption characteristics of a single-Layer radiowave absorber under irradiation electric power

Radiowave absorbers are frequently used in high power applications, such as radar sites, and the rise of temperature of the absorber has often become a matter of concern. In this paper, the finite-difference time-domain (FDTD) method combined with the heat transport equation (HTE), also known as the FDTD-HTE method, which has been widely used in the analysis of microwave heating or temperature increase in the human head due to portable phones, is used in the analysis of the wave absorption characteristics of a single-layer wave absorber. The complex permittivity of a sample made of epoxy resin, measured by a cavity-resonator method when the sample is irradiated by a high power field for different irradiation times, is used in the analysis, and the wave absorption characteristics of the absorber under investigation highly depend on the input power and irradiation time of the high power field.     

机载光电跟踪设备减震系统设计

减震系统采用\     

Modeling semi-anechoic electromagnetic measurement chambers

Previous studies developed a model to predict theoretically the low-frequency plane-wave reflection coefficient of an array of pyramid cone absorbers such as those used to line anechoic electromagnetic measurement chambers. The present authors apply this model in a geometrical optics approach to predict the electromagnetic field in a chamber lined with cone absorbers in the frequency range of 30-300 MHz. The results are compared with site attenuation measurements for two actual semi-anechoic chambers     

一种新型微隙突波吸收器

采用半导体技术和Ｓｉ半导体材料研制出一种新型微隙突波吸收器。它具有固有电容小、电流吸收容量大、响应速度快、体积小、成本低，易实现大规模生产等特点。     

FDTD analysis of resistor-loaded bow-tie antennas covered withferrite-coated conducting cavity for subsurface radar

This paper presents a full-wave analysis of a ground penetrating radar (GPR) using the finite-difference time-domain (FDTD) method. The antenna treated here consists of a resistor-loaded bow-tie antenna, which is covered with a rectangular conducting cavity of which inner walls are coated partially or fully with ferrite absorber. Some techniques are introduced into the FDTD analysis to obtain the accurate results and to save the computer resources. The validity of the FDTD analysis is confirmed experimentally. Furthermore, the effects of the ferrite absorber on the GPR characteristics are theoretically investigated in detail. The FDTD results indicate that the remarkable improvement of the antenna characteristics for the GPR system cannot be attained by the ferrite absorber     

Low-frequency reflectivity approximation for two- andthree-dimensional EM absorbers

Large-size electromagnetic absorbers are mainly used in anechoic and semi-anechoic chambers for electromagnetic compatibility testing. Therefore, the determination of the reflectivities in the low-frequency range (30-300 MHz) are of paramount importance in the performance evaluation of the absorber and, finally, in a “dark room” design. We here present a low-frequency approximation of the reflectivity based on a boundary and surface integral equation technique. This approach makes it possible to compare the approximation to the rigorous integral equation approach and to other approximations in the literature. The validity of the new low-frequency approximation is discussed based on reflectivity calculations of representative two- (2-D) and three-dimensional (3-D) absorber structures     

Optimal design of PML absorbing boundary condition for improving wide-angle reflection performance

A method to optimise the angle reflection performance of Berenger's perfectly matched layer (PML) absorbing boundary condition for finite-difference time-domain (FDTD) method is described. By specifying the conductivity in each of the PML sublayers, the wide-angle reflection performance can be improved. To improve the angle reflection performance as well as to reduce the discretisation error characteristic due to the FDTD approximation, a two-step conductivity profile is used in PML. To obtain the optimal value for each conductivity profile, the micro-genetic algorithm method is employed. Using this method, good wide-angle reflection performance of PML is achieved.     

一款具有双方环结构的有源电路模拟吸波材料

简单阐述电路模拟吸波材料的吸波原理,提出使用CST电磁仿真软件,通过电抗加载的方式对有源电路模拟吸波材料进行分析的方法。设计一款具有双方环结构的有源电路模拟吸波材料,并使用该方法对其进行分析。仿真结果表明这款吸波材料具有良好的双频吸波特性,通过改变两个方环的加载电阻(即改变PIN管的偏置电流),可改变吸波材料的吸波频率。     

近距离观察电磁兼容暗室的场均匀性——电磁兼容暗室的有限元分析

采用有限元法(FEM)对电磁兼容暗室的测试面场均匀性(FU)进行了三维仿真分析,目的是比较地面铺设不同吸波材料(仅限于聚氨酯角锥和铁氧体瓦)以及四周墙面安装的吸波材料距离地面20 cm(即与地面相接的四周金属墙面有20 cm裸露在外)时FU的变化情况。计算结果表明:在其他条件相同情况下,仅改进地板铺设的吸波材料性能,可以显著改善FU;而四周墙面铺设的吸波材料离地面有20 cm时,FU将变差。此外,还提供了测试面中的电场三维分布图,使测试面的FU可视化;根据FU分布图分析了FU在较低频和较高频时的不同变化情况。对暗室的三维仿真分析,使得在实际建造暗室之前,就能对暗室的场均匀性有比较准确和具体的了解,从而可以及时发现和解决问题。     

Uncertainty and Ambiguity in Terahertz Parameter Extraction and Data Analysis

Phase sensitive measurement techniques, such as THz time-domain spectroscopy or dispersive Fourier transform spectroscopy, are very useful tools to obtain a complete set of optical material parameters. When recording the electric field as a function of time delay between THz and optical pulse, the absorption coefficient and the index of refraction can be extracted. However, the analysis shows ambiguity. Here, we describe an analysis which yields a complete set of mathematical solutions and show how the physically relevant can be deduced. We present a comprehensive mathematical survey for parameter extraction. We have recorded the THz spectra of anthracene and the fatty acid capric acid as examples for weakly absorbing solid samples, and an ionic liquid as an example for a strongly absorbing liquid sample. Finally, we discuss the uncertainty of the obtained optical parameters using error propagation of the Fourier transformation with a simple model and a rigorous mathematical procedure.