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     

A low-frequency model for wedge or pyramid absorber arrays-I:theory

The interaction of electromagnetic waves with an array of absorbing wedges or pyramid cones is studied in the low-frequency limit; i.e., when the period of the array is small compared with wavelength. A theoretical model is obtained using the method of homogenization, which replaces the transversely periodic structure with a transversely uniform medium possessing a certain (generally anisotropic) effective permittivity and permeability. Plane-wave reflection from such structures can then be modeled using well-known techniques for one-dimensionally inhomogeneous media; a Riccati equation for the reflection coefficient is used in this work. This model is appropriate for use with absorbers found in anechoic chambers used for electromagnetic compatibility and electromagnetic interference (EMC/EMI) measurements over the frequency range of 30-1000 MHz     

Design and construction of a multipurpose wideband anechoic chamber

An electromagnetic anechoic chamber has been constructed at the Multimedia University. It is designed to operate over a very wide frequency range from 30 MHz through 18 GHz. It can be used for electromagnetic compatibility (EMC) tests, antenna measurements, radar cross section (RCS) measurements, testing RF transceivers, calibration of scatterometers, and other electromagnetic research experiments. The geometry of the chamber is asymmetrical, consisting of a combination of rectangular and tapered volumes. The size of the chamber is about 64 ft /spl times/ 32 ft /spl times/ 24 ft high. Ease of construction and ease of lining of the absorbers were preserved so that the actual quietness is comparable to the theoretical simulation result. The transmitting source is placed at one end of the chamber, and the receiving antenna or object under test is placed in a designated quiet zone at the other end. The walls and ceilings are configured such that no first-order and or second-order reflections (except those reflected from the floor) propagate into the quiet zone. Hence, less-expensive absorbers can be used to achieve the required wideband performance. Reflection from the floor is required by the international EMC standards for radiated electromagnetic interference (EMI) emission tests; therefore, the chamber is designed for this capability. However, absorbing material can be placed on the floor to convert a semi-anechoic chamber into a fully anechoic chamber for radar and antenna measurements.     

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.     

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.     

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 doubly periodic moment method solution for the analysis anddesign of an absorber covered wall

A periodic moment-method solution for scattering from a doubly periodic array of lossy dielectric bodies is developed. The purpose is to design electromagnetic wedge and pyramidal absorbers for low reflectivity so that one can improve the performance of anechoic chamber measurements. The spectral-domain formulation and the moment-method volume polarization current approach are used to obtain the expressions for determining the scattering from a doubly periodic array of lossy dielectric bodies. Some wedge and pyramidal absorber configurations that have been designed, fabricated, and tested in the OSU/ESL compact range measurement facility are presented. By taking into account the complexity of real-world material structures, good agreement between calculations and measurements has been obtained     

New Electromagnetic Absorbers Composed of Left-handed and Right-handed Materials

New double-layered electromagnetic absorbers are presented in this paper. The new absorbers composed of one lossy left-handed material absorbing layer and one impedance matching layer consisted of lossless right-handed material. It is indicated that the reflection loss of below ?20dB can be obtained in the frequency range 7GHz–13GHz. Power attenuation achieving ?50dB of narrow frequency band electromagnetic absorbers can also be obtained by modulate permittivity of right-handed material. Furthermore, the thickness of the whole absorbing structure is only 2mm, which is particularly helpful in some practical applications. The presented results are of reference significance for accurate design of the new electromagnetic absorbers and of practical prospects for stealth technology.     

频率选择表面结构吸波体的电磁特性研究

采用时域有限差分法(FDTD)计算了蝶形和方环形FSS单元图形的反射与传输特性以及反射信号与入射信号的相位关系,并对计算结果进行了分析.结果表明电磁波的干涉相消是FSS吸波体吸波的主要原因,并且当反射电磁波满足2nπ的相位关系时才能达到最佳吸波效果;FSS吸波体的中心频率随介质层厚度的增加而下降;介质层厚度为5 mm时...     

Electromagnetic wave absorber with wide-band frequencycharacteristics using exponentially tapered ferrite

As a countermeasure of EMI or EMC, various types of electromagnetic wave absorbers are used. A wide-band design method of an electromagnetic wave absorber with using exponentially tapered ferrite, which has very wide-band frequency characteristics, is proposed and discussed. The wide-band electromagnetic wave absorber can be designed under some approximations by the theoretical model using the equivalent material constants (equivalent complex permittivity and permeability) method for the regions varying spatially in the shape of ferrite. Based on the model, wide-band electromagnetic wave absorbers with taper, which have not only excellent reflectivity frequency characteristics but also a bandwidth of 30 MHz to 2150 MHz or 2430 MHz under the tolerance limits of -20 dB reflectivity, were designed     

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     

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。     

Broadband electromagnetic absorber with ferrite/ferrite hybridstructure

A new broadband electromagnetic absorber has been constructed by using the NiZn/NiZn ferrite hybrid structure and analysed using the FEM (finite element method). The structure is only 55.5 mm high, and has a reflection loss of <-20 dB in the frequency range 30 MHz-3 GHz. These characteristics are superior to those of conventional absorbers which have been used in anechoic chambers     

微波暗室静区性能仿真预测方法

为了有效预测微波暗室的静区性能,使用FEKO软件进行任意入射角度条件下角锥吸波材料的前向反射率仿真,并以某矩形暗室结构为例,利用吸波材料反射率仿真结果,采用射线追踪法计算了发射天线位于天线阵面上典型位置时暗室静区内反射电平的分布情况.经比较,仿真结果与实际测量结果一致性较好,该方法为微波暗室性能预测提供了有效手段.     

Generation of accurate rational models of lossy systems using the Cauchy method

A method to generate an accurate rational model of lossy systems from either measurements or an electromagnetic analysis is presented. The Cauchy method has been used to achieve this goal. This formulation is valid either for lossless or lossy system responses. Thus, it provides an improvement over the conventional Cauchy method and takes into account the relationship between the transmission and reflection coefficients of the system which in our case is a filter. The resulting model can be used to extract the coupling structure of the filter. Two examples have been presented. One deals with measured data and the other one uses numerical simulation data from an electromagnetic analysis.     

超宽带宽角极化不敏感的电路模拟吸波材料设计

传统的电路模拟吸波材料设计只考虑正入射时的吸波性能,当入射角较大,尤其是大于30°时,雷达吸波器的吸波效果明显恶化。随着现代双站雷达探测技术的发展,雷达探测电磁波可能来自不同的空间方向,这就要求雷达吸波材料不仅在电磁波正入射时具有较高的吸波性能,在斜入射时同样实现良好的隐身特性。为此,该文提出了一种新型的宽带吸波材料。...     

暗室墙面反射对雷达告警系统测向的影响

微波暗室为电磁波的定性分析或定量测试提供一个近似无反射的环境,当微波暗室内存在的反射不能忽略时,暗室墙面反射引起的多路径传输就会影响对测试结果的评定。在阐述比幅单脉冲测向原理的基础上,研究了由于暗室静区反射电平对雷达告警系统测向精确度的影响,建立了暗室静区反射电平与雷达告警接收机测向误差的关系模型,并且结合四元阵、八元阵、十六元阵说明了静区反射电平、来波方向与测向误差范围的关系。     

Reverberating chamber electromagnetic field in presence of anunstirred component

A theoretical model for the electromagnetic field within a reverberating chamber in presence of an unstirred component is presented. Due to the stochastic nature of the electromagnetic field within the chamber, a statistical electromagnetic approach is developed and tested versus measurements. On the theoretical side, such a physical model is representative of many complex electromagnetic environmental situations, e.g., urban environments, and, therefore, it is of great relevance in the EMC field. On the applicative side, the model can be furthered in order to characterize a reverberating chamber whether it is well operated or not and to set up an environment with a specific amount of direct coupling in conjunction with a stochastic field     

Low-Frequency Model for Rectangular Semi-Anechoic Chambers

An efficient and accurate method to compute the electromagnetic field inside a rectangular semi-anechoic chamber from 30 to 100 MHz is provided. A modal analysis method is used to develop the homogeneous and inhomogeneous solution. They are then combined with a generalized circuital analysis to take into account the effect of the several layers of ferrite and dielectrics on the walls. Several numerical results of the normalized site attenuation (NSA) of a semi-anechoic chamber are provided and compared with measurements, showing a very good agreement     

Transient reflection properties of a dispersive and lossy bi-isotropic slab with an anisotropic laminated composite backing

This paper presents a detailed analysis of the transient reflection from a general bi-isotropic slab backed by a laminated composite. Either a nonreciprocally Tellegen medium or a reciprocally dispersive chiral medium with losses is considered as the bi-isotropic slab. The laminated composite is a graphite-fiber-reinforced-plastic composite (GFRPC), which has been widely used as a replacement for metals and may be treated as a multilayered anisotropic medium from the electromagnetic point of view. For the analysis, a theoretical model is developed based on a state-equation approach. On the basis of this model, both the time-domain and the frequency-domain plane-wave reflection results are obtained and studied. The factors affecting the results, such as the material parameters of the bi-isotropic slab and the fiber-orientation pattern of the GFRPC, are investigated in detail. Some special reflection properties, such as the twist-polarizer reflection and the antireflection, are observed based on this investigation. The properties may lead to a novel transient absorber technology for electromagnetic compatibility (EMC) applications.