A 100-1000 MHz field strength standard for an anechoic chamber

A 100-1000 MHz range field strength standard using a short broadband dipole antenna has been developed for use in an anechoic chamber. The broad bandwidth of the standard antenna reduces calibration time and reduces the calibration differences which occur when the standard antennas must be changed for broadband calibration. The enclosure of the system in the anechoic chamber eliminates the interaction of the radiation with the neighboring environment, and ensures that the measurements are unaffected by weather. Error, due to reflection waves from each of the walls, the floor, and the ceiling of the chamber, was reduced by measuring the insertion loss with very closely spaced antennas, and by compensating the near-field effect theoretically. This frequency range is covered by only two standard antennas, and the field strength standard has been established with one-sigma uncertainty within ±1.6%     

NIM球面近场法天线校准装置的测量不确定度评定

介绍了中国计量科学研究院（NIM）开发的一套精密球面近场扫描法天线校准装置(400 MHz～110 GHz)在测量天线归一化方向图（限于幅度）时的不确定度评定。以“自我比较法”为基础,简介了探头、安装对准、数据采样和射频系统等引入的18个不确定度分量的评定结果,重点针对天线暗室反射引入的不确定度不易评估的难题,基于射线追踪法提出了一种分析、测量和计算相结合的“干扰法”。评估结果表明该套近场系统在-40 dB副瓣电平内,重复性优于0.1 dB (26 GHz)、在忽略探头极化误差时的扩展不确定度优于2.7 dB。最后分析了球面近场天线测量的不确定度特点。     

Assessment of a TD-Based Method for Characterization of Antennas

Antenna-characterization measurements are traditionally performed in the frequency domain (FD) through a vector network analyzer (VNA) in an anechoic chamber. Nevertheless, the high cost of the required setup strongly limits the possibility of using this approach. Starting from these considerations, a time-domain (TD)-based approach for characterizing antennas without using an anechoic chamber is assessed. As a matter of fact, instruments operating in TD are usually less expensive than VNAs; nevertheless, with appropriate data processing, they provide as much information. Particularly, it is demonstrated that the selection of an optimal time windowing is the main factor that guarantees a high accuracy level in the corresponding FD. The proposed approach leads to the accurate evaluation of the reflection scattering parameter S ₁₁(f) from time-domain reflectometry (TDR) data. The experimental validation is tested on a commercial radio-frequency identification (RFID) reader antenna, and the results are compared with reference VNA measurements performed in an anechoic chamber. The ultimate goal of this paper is to demonstrate that, through calibrated TDR measurements, along with an optimal time windowing, an accurate antenna characterization can be achieved.     

Requirements imposed on the anechoic factor of anechoic chambers when measuring backscattering patterns

The quantitative requirements imposed on the echo suppression of an anechoic chamber when measuring the scattering indicatrix are obtained using a four-ray model of the interaction between the object being measured and the anechoic chamber. It is shown that, depending on the bistatic scattering, these requirements may be stricter or less rigorous than the similar requirements for antenna measurements. Translated from Izmeritel'naya Tekhnika, No. 1, pp. 43–44, January, 1996.     

Radar cross section measurements using near-field radar imaging

In this paper a technique to obtain the far-field scattering signature of bodies, using near-field measurements, is proposed. The method is based on near-field radar imaging techniques. The backscattered field data are collected in a controlled environment over a large frequency band and aspect angle using a near-field antenna. A focused radar image of the body is generated. Probe correction to compensate for the radiation pattern of the interrogating antenna is conducted during the two-dimensional imaging of the object. The contribution from each scattering center to the total backscattered far-field is obtained from the radar image. The proposed technique is applied to obtain the far-field radar cross section (RCS) for an object from near-field measurements conducted in an anechoic chamber at the University of Pretoria, South Africa     

Channel capacity evaluation for a multiple-inputmultiple-output terminal in the presence of user?s hand

The impact of the user's hand holding a multiple-input-multiple-output (MIMO) terminal on system performance and specifically on channel capacity is investigated. A 4 times 4 MIMO system is considered with a personal digital assistant terminal equipped with a compact array of 4 patch elements. The proposed methodology is based on the use of radiation patterns of the antenna elements in the presence of the other elements and in the presence of a user's hand. Radiation patterns are obtained by means of simulations as well as of measurements conducted in an anechoic chamber. Two MIMO channel models are considered, namely a correlation-based model and a geometry-based model, where the antenna patterns are incorporated. The evaluation of MIMO channel capacity demonstrates the impact of the specific terminal antenna array. Moreover, significant degradation of channel capacity is proved because of the presence of the user's hand.     

环形波导微波场分布的计算

提出了环形波导场分布的计算模型 ,计算了环形波导内的微波场分布。结果表明即使在环形腔直径大于微波波长时 ,微波能量也能传到真空室中心附近 ,这为实现大面积均匀等离子体提供了理论依据     

Space-time measurement of indoor radio propagation

Most existing techniques for indoor radio propagation measurement do not resolve the angles from which signal components arrive at the receiving antenna. Knowledge of the angle-of-arrival is required for evaluation of evolving systems that employ smart antenna technology to provide features such as geolocation, interference cancellation, and space-division multiplexing. This paper presents a novel technique for the joint measurement of the angles, times and complex amplitudes of discrete path arrivals in an indoor propagation environment. A data acquisition system, based upon a vector network analyzer and multichannel antenna array is described, together with its use to collect channel measurement matrices. The inherent error sources present in these measurement matrices are investigated using a compact indoor anechoic range. Two signal processing algorithms are presented whereby the channel parameters may be estimated from raw measurements. In the first approach, an optimum beamformer is derived which compensates for systematic errors in the data acquisition system. This approach features very low computational complexity, and delivers modest resolution of path components. The second algorithm is based upon the maximum likelihood criterion, using the measured calibration matrices as space-time basis functions. This algorithm provides super-resolution of all path parameters, at the cost of increased computation. Several example measurements are given, and future directions of our research are indicated     

Calibration of specific absorption rate (SAR) probes in waveguideat 900 MHz

The radiation safety tests for hand-held mobile phones require precise calibration of the small electric field probes used for the measurement of specific absorption rate (SAR) in phantoms simulating the human body. In this study, a calibration system based on a rectangular waveguide was developed for SAR calibrations at 900 MHz. The cross-sectional dimensions of the waveguide are a=190 mm and b=140 mm. The waveguide is loaded with a rectangular liquid slab where the dielectric parameters of the medium simulate human muscle and brain. The precise SAR reference is derived from the temperature rise during a short-term (10-15 s) microwave heating of the lossy slab by measuring with sensitive thermistor-type probes equipped with highly resistive lines. The thermistor probes are calibrated against a calibrated mercury thermometer. To improve the uniformity of the electric field at the calibration position, the thickness of the tissue equivalent slab was varied to adjust the standing wave pattern. This resulted in an almost threefold reduction in the positioning error of the E-field probe. The absolute uncertainty of the calibration is estimated to be ±5% (2σ) not including the uncertainty of the conductivity. The difference between the thermally measured SAR and a value computed with the FDTD method was well within this limit of uncertainty. This kind of closed waveguide system is more compact and requires less microwave power than open field calibration systems. Moreover, no radio-frequency interference is generated     

之江实验室声学实验室设计和性能测试

之江实验室声学实验室包括消声室、半消声室、混响室和听音室,结合实验楼整体布局,确定了声学实验室内部结构尺寸,提出了声学设计性能要求,为类似声学实验室的设计和规划提供参考。建成后,按照JJF 1147、JJF 1143和GB/T6881.1-2002分别对消声室、半消声室、混响室和听音室声学性能进行了测试,结果表明,该声学实验室达到了预期设计目标,可提供高精度的声学环境,为之江实验室开展相应的声学研究提供了基础环境条件支撑。     

A Method of Reducing the Effect of Residual Reflections in an Anechoic Chamber on the Measurement Precision of the Group Delay in an Antenna

Measurement Techniques - It has been shown that methods currently being used for measuring group delay in an antenna in an anechoic chamber are limited in precision due to the effect of residual...     

TEM室法环天线校准系统建立及测量结果不确定度评定

横电磁波室法是9kHz~30MHz频段环天线校准中广泛采用的一种方法。环天线分为有源环天线和无源环天线两类,这两种天线的磁场天线系数在量值范围上差别大,对系统动态范围要求不同。根据这一特点,对两类环天线分别建立横电磁波室作为标准场的环天线校准系统,基于网络分析仪开展扫频测量。针对电磁兼容领域中常用的直径60cm以下有源和无源环天线分别开展校准方法研究,给出磁场天线系数测量结果和测量结果的不确定度评定。该环天线校准系统可以扫频完成常用环天线在9kHz~30MHz频段内的校准,解决大动态范围问题,环天线校准效率也得到提高。     

Radiometric Evaluation of Antenna-Feed Component Losses

Low-noise antenna systems are frequently used in conjunction with measurements of atmospheric and cosmic background noise at microwave frequencies. The input transmission line losses of these receiving systems must be precisely calibrated to insure proper identification of the portion of operating noise temperature attributable to the external environment. Although most components in an antenna line can be calibrated by conventional insertion-loss measurements, many feed component losses must be evaluated by means of nonstandard techniques. This paper describes a radiometric method for calibrating the loss of multimode antenna-feed components in which the field is linearly or circularly polarized. The method consists of measuring operating noise temperature, first with the components under evaluation installed and again after substitution by a waveguide section of known loss. Calibration and error analysis equations are derived and discussed. Application of the radiometric method, for the calibration of a mode-generator and quarter-wave plate polarizing section, resulted in a loss measurement of (0.0069 ±0.0016 pe)/dB.     

E-plane sectoral horn power divider

E-plane sectoral horn power divider design method for high-power antenna array applications is presented. In this method, power division is proportional to the cross section areas of the waveguide channels at the aperture of an E-plane sectoral horn antenna. Two different power dividers are designed and produced to be used with an X-band antenna array having 30 dB Taylor distribution. The first one employs a simple E-plane sectoral horn antenna. This straightforward application of the method might be limited with production tolerances. The second one incorporates a corrugated E-plane sectoral horn antenna, whose design is relatively complex but less sensitive to production tolerances. The produced dividers are compared based on simulations and measurements and good agreement with the theoretical expectations is observed for both types.     

A NOVEL QUICK-CLOSING PROBE FOR MEASURING LOCAL PARTICULATE PHASE DENSITY IN GAS-PARTICLE SUSPENSIONS

An innovative quick-closing probe system was designed, built, and tested for measuring the local particulate phase density of gas-particle suspensions. Experiments with glass beads and powder coals were conducted in a horizontal channel with an aspect ratio of 11 to 1, and also in a 0·25 m I.D. bench-scale vortex chamber. Results of particle density distributions in these two systems indicated the ability of the probe to perform in highly turbulent, swirling, dilute and intermediate phase gas-particle suspension flows. The probe was connected with a microcomputer-assisted particle weighing system for data collection and reduction. Comparison of measured data by this probe with calculated results was assessed. Since this probe requires no calibration curve nor empirical equation, it serves as a primary standard for particle density measurements. The design features, working principle, component characteristics, overall performance, and test results are discussed in this paper.     

A NOVEL QUICK-CLOSING PROBE FOR MEASURING LOCAL PARTICULATE PHASE DENSITY IN GAS-PARTICLE SUSPENSIONS

ABSTRACT

An innovative quick-closing probe system was designed, built, and tested for measuring the local particulate phase density of gas-particle suspensions. Experiments with glass beads and powder coals were conducted in a horizontal channel with an aspect ratio of 11 to 1, and also in a 0·25 m I.D. bench-scale vortex chamber. Results of particle density distributions in these two systems indicated the ability of the probe to perform in highly turbulent, swirling, dilute and intermediate phase gas-particle suspension flows. The probe was connected with a microcomputer-assisted particle weighing system for data collection and reduction. Comparison of measured data by this probe with calculated results was assessed. Since this probe requires no calibration curve nor empirical equation, it serves as a primary standard for particle density measurements. The design features, working principle, component characteristics, overall performance, and test results are discussed in this paper.     

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     

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.     

Precision Determination of the Dielectric Properties of Nonmagnetic High-Loss Microwave Materials

New tests by the author to determine the, dielectric properties of high-loss nonmagnetic microwave materials used for waveguide absorbers and anechoic chamber linings reveal two significant results: dielectric parameters may be calculated from simple formulas without solving transcendental equations, and accuracy is limited only by the homogeneity of the sample and the precision of its fit in the waveguide. The technique used is based upon prior work by other researchers but is refined for materials with loss tangents exceeding 0.1. These previously have received little attention in the literature. Results of tests in the S-band yield loss tangents between 0.4 and 0.8, and dielectric constants from 1.5 to 2.0 for samples completely filling the cross section of WR284 waveguide. The author reviews other methods, analyzes error sources, and describes his laboratory work.