Nondestructive determination of electromagnetic parameters ofdielectric materials at X-band frequencies using a waveguide probesystem

An accurate technique used to measure complex permittivity and permeability of isotropic materials simultaneously has been developed by employing a flanged open-ended rectangular waveguide probe over a frequency range of 8-12 GHz. Two coupled integral equations for the aperture electric field are formulated and solved numerically using Galerkin's method. A series of experiments has been conducted, and the calibration of the probe system using an adjustable shorter is explained. The inverse results on the electromagnetic (EM) properties of various materials (including solid and liquid materials) based on the measured reflection coefficients of the incident dominant mode are presented. It is also shown that the EM parameters of isotropic materials having low complex permittivities can be determined accurately, while those with higher complex permittivities cause larger measurement errors     

微波波导五端口测量网络设计

介绍了工作于9～10 GHz频带的对称五端口波导网络的设计方法.为了使五端口波导网络能较好地工作于六端口反射计中,要求各端口反射系数的绝对值小于0.1.这里结合了频移法、渐变线分割研究法和数值解法等理论对五端口波导测量网络进行设计.     

Measurement of the reflection and transmission properties of conducting fabrics at milli-metric wave frequencies

There is increasing interest in conducting fabrics and their uses at radio frequencies and microwave frequencies. The reflection and transmission measurements of bobbinet and knitted materials from around 8 GHz into the milli-metric frequency range 110 GHz, where the material geometry is comparable to the wavelength of the wave are considered. Bobbinet materials were found to behave like lossy dielectrics and may be useful in the construction of thin lightweight screening and absorption planes. While the knitted materials, with very small mesh geometry, gave a reflection coefficient which was comparable to a metal foil     

Experimental Investigations of Radio-Absorbing Coatings over a Very Broad Frequency Band

A radar measuring system which enables the modulus of the reflection coefficient of radio-absorbing coatings to be determined over a very broad frequency band (2–18 GHz) is described and the systematic features of the measurements are pointed out. The results are presented of a comparison of the frequency dependence of the power reflection coefficient of the coating, measured using the system described, with data obtained using narrow-band panoramic measuring equipment.     

Dielectric characteristics of ore minerals in a 10–40 GHz frequency range

A new approach to investigation of the complex dielectric permittivity of both nonmetallic and ore minerals in the microwave frequency range is proposed. Using this approach, data on the complex permittivity of sphalerite, magnetite, and labradorite in a 10–40 GHz frequency range have been obtained for the first time. A method is proposed for calculating the complex permittivity from experimentally measured frequency dependences of the reflection and transmission coefficients of a plane-parallel plate of a given mineral. Approximate expressions that can be used for calculations of the complex refractive index and permittivity of minerals are presented.     

沙质沉积物反射系数的宽带测量方法

海底作为海洋波导的重要边界,其反射系数对声传播起着至关重要的作用。为了快速而准确地获取海底反射系数,提出了一种基于脉冲压缩技术的测量方法,对接收信号进行压缩来提取反射波。该方法不仅可以克服实验过程中经常遇到的多途干扰,而且测量过程简单,可以同时获得测量频带内所有频点的反射系数,即实现了对反射系数的宽带测量。在实验室环境条件下,针对广泛覆盖于大陆架及沿海地区的沙质沉积物反射系数进行了测量,通过20~60 kHz频率范围内窄带和宽带测量结果的比较发现,宽带测量结果在频带边缘存在一定的偏差和起伏,但可以采用增大发射信号带宽提高压缩效果或用适当的窗函数压低旁瓣的方法来得到频率范围更宽、精度更高的测量结果。     

A Computerized System for the Measurement of Attenuation in a Helix Waveguide

A computer controlled system for fast and accurate measurements of the TEo11, TEo12 mode attenuations in a helix waveguide over the frequency range of 85-110 GHz is described. Measurement is done by monitoring the change in amplitude of a 10-ns wide pulse propagating inside the helix, as a function of the position of a moving short circuiting piston. Data are acquired and processed by a mini-computer facility. A brief discussion on the generation of the desired waveguide modes and the results of some typical measurements are also given.     

PAn/Fe3O4网状纳米复合物的合成表征及吸波性能

在十六烷基三甲基溴化胺（CTAB）存在下,采用原位化学氧化聚合法制备了聚苯胺／Fe3O4网状磁性纳米复合材料,通过改变Fe3O4纳米粒子在聚苯胺（PAn）中的含量获得了电磁性能可调的纳米复合物,采用FT—IR、XRD、SEM、TEM、电导和磁性能测试对复合物进行了表征,通过矢量网络分析仪获得了试样在2—18GHz范围的复介电常数和复磁导率,经计算获得微波反射损耗曲线,发现当样品中Fe3O4的含量为15．8wt％时,在9．0GHz处具有最大的反射损耗-17．1dB,损耗起．过-10dB的频宽为1GHz。     

Nondestructive Measurement of Materials Using a Waveguide-Fed Series Slot Array

The reflection from an array of narrow series slots on the broad face of a rectangular waveguide is used to determine the permittivity of lossy materials. The theoretical study of the electromagnetic field in the vicinity of the slots yields a system of integral equations which takes into account the internal and external coupling between slots. The numerical solution is discussed and an optimization procedure for the array is presented, which provides a simple relationship linking the reflection coefficient to the complex permittivity over a specified range. Computed results are compared to experimental values for air and a lossy dielectric at 10 GHz, showing good agreement. Interpolation from computed data is provided by a polynomial expansion: this allows one to determine the complex permittivity once the reflection factor has been measured.     

Time-Domain Measurements for Determination of Dielectric Properties of Agricultural Materials

A time-domain reflectometer (TDR) system was used to obtain data on the frequency dependence of the dielectric properties of a few agricultural materials. Values for the complex relative permittivity in the frequency range from about 30 MHz to 1 or 2 GHz were obtained from the reflection coefficient, which was calculated from the ratio of the Fourier transforms of the incident and reflected waveforms from the dielectric sample in a coaxial line. With the TDR system, broad dielectric dispersion and absorption regions, similar to those found by frequency-domain measurements, were determined with significant reductions in time and effort.     

A Microwave Interferometer for Wide-Band Measurements of Complex Transmission Coefficients

A wide-band (8.2-12 GHz) microwave interferometer was developed to measure the complex transmission coefficient of samples inserted in a waveguide system. Amplitude A and phase shift ? of a signal are presented either as A sin ?, A cos ? versus frequency, or in polar coordinate form Aei? as frequency is swept. The attenuation range of measurement is 0-50 dB; phase shifts between n·2? and (n + 1)·2? are given as ? - n·2? (n = 0, 1, 2,···). The unique feature of this system is its ability to measure broad frequency bands in a simple way. The system was tested with components with known characteristics and used to measure transmission of a sheet of water.     

WR-42功率基准系统有效效率不确定度评定

WR-42功率基准是一套微量热计系统,利用直流替代及量热技术对被测热敏电阻型功率座的有效效率进行定标。对WR-42功率基准系统中有效效率不确定度评定的过程进行了介绍,并给出了有效效率的测量结果与不确定度的评定结果。该基准的研制成功,填补了18～26.5GHz频段的空白,结合其它功率基准,使我国射频与微波功率计量基准频段从10MHz连续覆盖到了220GHz。     

International comparison GT-RF 75 A 4: reflection coefficient in 14mm/50 Ω coaxial line at 0.5 GHz, 3 GHz and 7 GHz

The results are presented of an international comparison of measurements of the complex reflection coefficient in the G 900-14 mm/50 Ω coaxial line technique at 0.5 GHz, 3 GHz and 7 GHz in which eleven national laboratories of metrology participated. The standards showed very small drifts of the reflection coefficient over a period of nine years. With few exceptions, good agreement between the mean values reported by the various participants is observed     

Microwave absorption properties of FSS-impacted composites as a broadband microwave absorber

The development of a cost-effective microwave absorber with wide bandwidth corresponding to reflection loss (RL)?≤??10 dB is still a very challenging task. A sugarcane bagasse-based agricultural waste composite has been analyzed for its elemental contents. The combination of elements is suitable for its possible usage as a cost-effective microwave absorbing material. Therefore, this composite has been subjected to morphological and electromagnetic studies to analyze its microwave absorbing behavior. The frequency dependent complex dielectric permittivity and complex magnetic permeability values were obtained using a transmission/reflection waveguide approach in the X-band. Furthermore, the effect of the Minkowski loop frequency selective surface (FSS) was studied over the absorption capability of the composite. It was found that the application of FSS leads to a reduction in thickness up to 2.9 mm and an enhancement in absorption bandwidth up to 3.6 GHz. The FSS patterned composite shows a remarkable performance with peak RL of ?28.4 dB at 10.7 GHz and absorption bandwidth of 3.6 GHz.     

Automatic material characterization at microwave frequencies

An instrument was assembled to measure complex permittivity at microwave frequencies in an efficient, inexpensive manner. It bridges the gap between traditional, tedious methods of characterization and expensive vector network analyzer techniques. A microcomputer is used to control the operation of a scalar network analyzer. Measurements of the magnitude of the reflection and transmission coefficients are automatically made as a function of frequency and temperature over a range of 2-18 GHz. A theory is developed covering the different regimes of operation: the thin sample, the multiple reflecting sample, and the high insertion loss sample. Sample thickness criteria for accurate measurements are reported. Results are presented for dielectric and semiconductor samples     

Radio frequency controlled synthetic wavelength sweep for absolute distance measurement by optical interferometry

We present a new technique applied to the variable optical synthetic wavelength generation in optical interferometry. It consists of a chain of optical injection locking among three lasers: first a distributed-feedback laser is used as a master to injection lock an intensity-modulated laser that is directly modulated around 15 GHz by a radio frequency generator on a sideband. A second distributed-feedback laser is injection locked on another sideband of the intensity-modulated laser. The variable synthetic wavelength for absolute distance measurement is simply generated by sweeping the radio frequency over a range of several hundred megahertz, which corresponds to the locking range of the two slave lasers. In this condition, the uncertainty of the variable synthetic wavelength is equivalent to the radio frequency uncertainty. This latter has a relative accuracy of 10(-7) or better, resulting in a resolution of +/-25 microm for distances exceeding tens of meters. The radio frequency generator produces a linear frequency sweep of 1 ms duration (i.e., exactly equal to one absolute distance measurement acquisition time), with frequency steps of about 1 MHz. Finally, results of absolute distance measurements for ranges up to 10 m are presented.     

WR-28功率基准系统中等效源反射系数校准研究

在WR-28功率基准系统中,等效源反射系数是确定量热计有效效率、分析不确定度与进行基准功率量值传递的重要参数,其只与接入量热计的定向耦合器有关,而与信号源无关。介绍了传统三端口测试法、直接校准法和最新提出的Shimaoka K法3种三端口器件等效源反射系数的测试方法。在频段26.5~40 GHz,运用Shimaoka K法对三端口器件（定向耦合器）进行等效源反射系数测量,并与其他两种方法实验结果比较,验证了Shimaoka K法的可行性与准确性,同时通过实验比较,由于Shimaoka K法具有的综合优势,使其成为WR-28功率基准中等效源反射系数的最佳测试方法,并以Shimaoka K法为例,对等效源反射系数不确定度进行了评定。     

Integrated optics magnetic sensor from 2 kHz to 9 GHz

A new type of integrated optical magnetic field sensor is presented in this paper. The proposed sensor consists of a Mach-Zehnder waveguide interferometer and a doubly loaded loop antenna. Such a structure can successfully avoid detection of the undesired electric field signal. The size of the sensor is 35 mm×6 mm×1 mm. The measurements show that the frequency response is from 2 kHz to 9 GHz, the dynamic range is 98 dB, and the minimum detectable magnetic field is 51.8 μA/m at 1 GHz. Therefore, this sensing system can be used in electromagnetic compatibility measurements.     

短切中空多孔碳纤维复合材料的吸波性能

以中空多孔聚丙烯腈(PAN)原丝为原料, 通过预氧化处理和碳化处理工艺制备了中空多孔碳纤维, 采用SEM和XRD对其微观结构和晶体结构进行了表征, 并对其吸波性能进行了分析. 研究结果表明, 中空多孔碳纤维是一种非石墨结构的电损耗型雷达波吸收剂; 随着短切中空多孔碳纤维体积分数的提高, 随机分布的纤维/石蜡复合吸波材料的介电常数随之增大; 用所得的电磁参数结果计算了不同厚度材料的反射率, 在2～18GHz频率范围内, 当体积分数为33.30%, 厚度为2mm时, 最低反射率为-21.36dB, 其中<-5dB的反射率带宽为5.17GHz, <-10dB的反射率带宽为2.88GHz.     

Microwave near-field reflection property analysis of concrete formaterial content determination

One of the most important parameters associated with concrete is its compressive strength. Currently, there is no reliable nondestructive testing technique that is capable of robust determination of this parameter. Concrete is a heterogeneous mixture composed of water, cement powder, sand (fine aggregate), rocks of various size or grade (coarse aggregate), and air (porosity). Water and cement powder chemically combine into a cement paste binder which, in due curing time, produces concrete with its specified compressive strength. Compressive strength of concrete is strongly influenced by its water-to-cement (w/c) ratio as well as its coarse aggregate-to-cement (ca/c) ratio. Therefore, if these two parameters are determined using a nondestructive testing technique, then they may be correlated to the compressive strength. Near-field microwave nondestructive testing techniques, employing open-ended rectangular waveguide probes, have shown tremendous potential for evaluating concrete constituent make-up. In this paper, the results of an extensive set of measurements, using these probes, are presented. The results demonstrate that the statistical distribution of the multiple measurements of the magnitude of reflection coefficient of concrete specimens with various constituent make-ups follows two well-known distributions as a function of frequency. It is shown that for the specimens investigated this distribution is Gaussian at 10 GHz and uniform at 3 GHz. Furthermore, the standard deviation of the measured magnitude of reflection coefficient at 10 GHz is shown to correlate well with ca/c ratio, whereas, the mean of this parameter at 3 GHz is correlated well with w/c ratio. Subsequently, these parameters may be used in conjunction with well established formulae or a look-up table to determine the compressive strength of a given concrete specimen