Periodic Dyadic Green’s Function for Field Analysis in EMC Chamber

Herein a novel Dyadic Green＇s Function （DGF） is presented to calculate the field in ElectroMagnetic Compatibility （EMC） chamber. Due to the difficulty of simulating the whole chamber environment, the analysis combines the DGF formulation and the FEM method, with the latter deals with the reflection from absorbers. With DGF formulation for infinite periodic array structures, this paper investigates electromagnetic field in chamber with truncated arrays. The reflection from the absorber serves as the virtual source contributing to the total field. Hence the whole chamber field calculation can be separated from the work of absorber model set-up. Practically the field homogeneity test and Normal Site Attenuation （NSA） test are carried out to evaluate the chamber performance. Based on the method in this paper, the simulation results agree well with the test, and predict successfully the victim frequency points of the chamber.     

基于镜像射线跟踪法的电波暗室仿真设计

为提升暗室吸波工程的设计能力,运用镜像射线跟踪法对10 m 法电波暗室进行仿真设计,通过合理布置吸波材料,考虑一次反射和二次反射对静区的影响,计算了不同极化方式下静区的反射电平、归一化场地衰减和场均匀性。结果表明垂直极化和水平极化时,场地电压驻波比仿真与测试偏差小于2.3 dB;归一化场地衰减仿真与测试误差小于3 dB,场均匀性误差小于2 dB。仿真与测试结果吻合较好,各项指标均满足设计要求,验证了吸波材料布局的合理性及算法的可靠性。该仿真方法可应用于暗室静区性能评估,优化吸波材料布局,进而有效缩短设计周期。     

电波暗室吸波墙壁反射电平测试技术

介绍了测试电波暗室墙壁反射电平的两种方法,既电压驻波比法（VSWR）和基于矢量网络分析仪的改进电压驻波比法（AVSWR）,并对这两种方法进行了比较,给出了每种方法的优缺点。     

Effects of External PIM Sources on Antenna PIM Measurements

Antenna Passive Intermodulation (PIM) level measurement results are rarely credited to be due to external signal receiving characteristics of the antennas or serious effects from external PIM sources, such as the anechoic chamber absorber and antenna tower. This paper presents an antenna PIM model for a reflected PIM measurement method. Based on the findings of null point generation and the behavior of the third order PIM values obtained by theoretical predictions and experimental results, we concluded that the results of the antenna PIM level test were influenced by the external PIM sources generated by the anechoic chamber absorber and the path differences of PIM signals coming into the antenna.     

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     

A new error reduction technique for reflection coefficient measurements for use in quick laboratory tests

ABSTRACT

Reflection coefficient measurement is one of those fundamental methods for determining the performance of radar absorber materials, and reflections due to these measurements are originated from the discontinuities in the measurement system and they result in error. In this paper, a new technique for the calibration of the scalar reflection coefficient measurements in the frequency range of 3–18 GHz in a portable two-sectioned metallic chamber is being proposed for use in quick laboratory tests with reasonable error (redacted error). Calibration measurements are performed by two calibration standards in frequency domain and transformed to time domain for further calculation. Proposed technique exhibits a good agreement with the theoretical values for especially in compact chambers where plane-wave conditions are not fully satisfied, and it is not in need of any complex time gating process. Furthermore, it does not require considering the phase differences which occurs at the measurement plane, providing a cheap and faster solution, reducing time and complexity in characterization of radar absorber materials. This technique with the mentioned test set-up could supply scalar reflection coefficient measurements with overall 0.55 dB error level and useful for practical applications where high level of accuracy is not required.     

A periodic moment method solution for TM scattering from lossydielectric bodies with application to wedge absorber

The periodic moment method (PMM) solution for the scattering from two-dimensional lossy dielectric bodies is developed. The purpose is to design a microwave wedge absorber for low reflectivity so that one can improve the performance of anechoic chamber measurements. With PMM, the reflection and transmission coefficients of periodically distributed bodies illuminated by a plane wave have been accurately calculated using a Cray Y-MP supercomputer. Through these studies, some wedge absorber configurations have been designed, fabricated, and then tested in the OSU/ESL compact range measurement facility. Two 8-in commercial wedges, a curved wedge, and a four-layer wedge, were studied. In all cases, good agreement between calculations and measurements was obtained     

Extensions to the hybrid method of moments/uniform GTD formulationfor sources located close to a smooth convex surface

We present an extension to the uniform geometrical theory of diffraction (GTD) for reflection from smooth curved surfaces. This approach allows the source to be much closer to the reflecting surface than the conventional uniform GTD formulation and does not require a Hertzian dipole source. In essence, the field point is mirrored in the plane tangential to the specular (reflection) point; the incident field is then calculated at the mirror point and the uniform GTD reflection coefficients are used to mirror this field to the original field point. This formulation reduces exactly to the conventional uniform GTD if the incident field is ray optical. The application to a hybrid method of moments (MoM)/GTD code is outlined and results computed using this code are presented for a dipole radiating in the vicinity of a cylinder     

A general expression of dyadic Green's functions in radiallymultilayered chiral media

A general expression of spectral-domain dyadic Green's function (DGF) is presented for defining the electromagnetic radiation fields in spherically arbitrary multilayered and chiral media. Without any loss of the generality, each of the radial multilayers could be the chiral layer with different permittivity, permeability, and chirality admittance, while both distribution and location of current sources are assumed to be arbitrary. The DGF is composed of the unbounded DGF and the scattering DGF, based on the method of scattering superposition. The scattering DGF in each layer is constructed in terms of the modified and normalized spherical vector wave functions. The coefficients of the scattering DGFs are derived and expressed in terms of the equivalent reflection and transmission coefficients, by applying boundary conditions satisfied by the coefficient matrices     

顶角设置QRD散射体的混响室内场均匀性分析

提出了混响室内设置散射体的一种新方法。用FDTD方法对该结构混响室内的场进行了仿真分析,并与无搅拌器及散射体结构的混响室内的场均匀性、散射体位于混响室内6个壁面时混响室内的场均匀性进行了分析比较。结果表明:本文所提出的8个顶角处均设置QRD散射体的方法可有效提高混响室内的场均匀性,实现了进一步提高混响室内场均匀性的目的。     

基于AFA型吸波材料的微波暗室静区性能仿真

基于射线追踪法开发的静区电平仿真软件应用于AFA型吸波材料(难燃型吸波材料)铺设的微波暗室静区性能仿真。软件以国内暗室性能检测机构的喇叭天线数据为依据设定发射天线的辐射特性模拟函数,得到发射天线的直射波方向图。以AFA 型吸波材料的实测数据为依据设定吸波材料的吸波特性模拟函数,计算微波暗室各面墙壁反射电场在静区内叠加后的总反射电场值,并据此得到静区内的反射电平分布。仿真与实测结果表明,1 GHz 以下静区反射电平仿真结果下限值与测试结果偏差相对较大;1 GHz 以上两者偏差小于1 dB,吻合较好。验证了该仿真软件能够快速并较准确地用于微波暗室吸波工程的设计与评估。     

Electromagnetic field analysis of a four-wire anechoic chamber

The electromagnetic field characteristics and plane-wave generating ability of a parallel four-wire anechoic chamber which is to be operated in the VHF range is analyzed. The analysis considers three cases: the first is a four-wire chamber when only a thin layer of absorber material covers the chamber walls, the second is a four-wire chamber when only free space surrounds the four wires of the system, and the third is a four-wire chamber when a thick layer of absorber material covers the anechoic chamber walls. The three cases are analyzed by a variety of mathematical methods including the method of moments, waveguide theory, and a finite-difference time-domain method. Numerical results for the propagation constants and electromagnetic fields that are generated in the chamber in the three cases are presented     

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.     

THE DYADIC GREEN''''S FUNCTION IN A LOADED RECTANGULAR WAVEGUIDE AND ITS APPLICATION

This paper presents a method to derive the Dyadic Green's Function(DGF)ofa loaded rectangular waveguide by using the image method.In the calculation of the DGF,we use the integral transformation and replace the multi-infinite summation by a single one;thus it greatly simplifies the calculation and saves computer time.As an example of the DGF'sapplication,we give the moment method's scattering field calculation of a metal sphere resting onthe broad wall of the loaded rectangular waveguide.Results of our calculations well agree withboth data of experiments performed in our laboratory and those are published.It is easy to seethat the method used in this paper can be expanded to other related waveguide problems.     

Modeling of optical response in graded absorber layer detectors

Commercial and military applications for ir detectors continually push the performance envelope to achieve the highest signal-to-noise ratio at the longest possible wavelength. The highest performance is generally achieved when the detector cutoff wavelength is made as short as possible to minimize dark current while maintaining high uniform responsivity over the system optical passband. This requires an accurate prediction of the detector spectral shape near cutoff. As the incident wavelength approaches cutoff, the optical absorption coefficient decreases and internal reflections within the layer become important, causing constructive or destructive interference and consequently modulation of the detector spectral shape. The analysis is complicated since the absorber layer often includes compositional grading, particularly for very long-wavelength IR devices. This article describes an approach for analyzing the spectral response of backside-illuminated, compositionally graded detectors, and the results are compared with experimental data. The analysis shows that the spectral shape near cutoff is impacted by a weak resonance within the absorber created by a combination of a strong reflection from the detector front side and a weak reflection from the absorber-substrate backside interface. The analysis, which is an extension of the work of Rosenfeld et al., takes into account the weak resonance through computation of the optical field and generation rate by the Wetzel-Kramers-Brillioun (WKB) method. The method has been compared with experimental data for several cases using both parabolic and hyperbolic models for the absorption coefficient, and excellent agreement is achieved with the hyperbolic model.     

时域有限差分法分析混晌室中场的均匀性

针对目前仿真分析混响室内的场分布过程中计算量大、耗费时间长和对计算机配置要求高的现状,用在一定的约束条件下随机分布于球面上的一组等效源辐射的平面电磁波叠加而成的一维推进的重叠平面渡模拟搅拌器搅拌过程中混响室内的电磁波,并采用时域有限差分法(FDTD)法对基于上述模型的混响室内的场分布进行了仿真计算,得出了使混响室内场分...     

Chebyshev multilevel absorber design concept

Pyramidal- and wedge-absorber materials are used extensively in anechoic measurement chambers to attenuate stray signals. Typical absorber layouts result in large absorber walls in which the absorber tips and bases are roughly aligned in the same plane. Such a quasi-periodic configuration produces a strong coherent specular reflection which dominates the absorber scattered field. Based on the multisection impedance transformer concept, one can divide absorber elements into different levels (layers) so that this coherence can be destroyed to reduce the specular absorber scattering level. The synthesis of this desired behavior can be implemented by the Chebyshev transformer technique, which provides the largest bandwidth given a passband ripple threshold. The resulting reflected field is then the product of the original absorber response times the Chebyshev reduction factor, which is independent of polarization and absorber properties. Various measured results are used to show that more than a 10-dB improvement can be achieved at the critical low end of the frequency band using this approach. This improvement cannot be achieved using conventional design concepts unless the absorber size is doubled     

A low-frequency model for wedge or pyramid absorber arrays-II:computed and measured results

Based on the theoretical model developed in Part I of this two-part paper, we present numerical results for the plane-wave reflection coefficient from arrays of pyramid-cone absorbers when the period of the array is smaller than half a wavelength. Comparison is made with results of a moment-method calculation, as well as with experimental measurements. The model is then used to modify the design of some existing types of commercially available absorbers for reduced reflection in the frequency range of 30-300 MHz. These improved absorbers have been installed in an anechoic chamber used for electromagnetic compatibility and electromagnetic interference (EMC/EMI) measurements. Site attenuation measurements from this chamber are presented which show closer correspondence to an ideal open-field test site     

A Low‐Cost Method to Evaluate Absorber Reflectivity Using an Antenna with a Small Radiating Aperture and Frequency‐Domain Instrument

We propose a way to measure the absorber reflectivity at a low cost. Only one simple antenna with a small radiating aperture and a frequency‐domain instrument are utilized. The previously used equation for calculating the reflectivity of an absorber is inaccurate, and, therefore, a new equation is derived based on multiple reflection analysis and three test models. Notably, the reflection coefficient of the antenna is included in the derived equation. The accuracy of the proposed method is proven through simulation and measurements. It can be easily applied to a product examination by absorber manufacturers and customers owing to its advantages of simplicity, cost effectiveness, and non‐cutting examination.     

FDTD计算暗室静区指标

本文主要借用空间驻波比法测量暗室静区反射电平的概念,采用FDTD的方法研究了电波暗室工作于低频时的静区指标,较好地解决了射线追踪法无法计算电子战暗室低频时静区反射电平的问题.讨论了激励源选取,边界设置等诸多情况.最后就辐射单元在天线阵面不同位置、不同工作频段时静区反射电平的变化规律进行了研究.