Electromagnetic fields: the jury's still out. 2. Societalreverberations

The debate over setting standards for exposure to power-frequency electromagnetic fields when science has not yet determined what levels, if any, are dangerous is examined. Regulatory agencies, standards bodies and utilities are struggling to make policy in the face of two opposing views: one camp believes the evidence of health effects is enough to warrant precautionary action to limit exposure, while the other insists that research must present proof of a harmful effect before practical limits (i.e. limits that can be achieved at a reasonable cost to society) can be determined. Burgeoning litigation over high-voltage transmission lines is making the task of providing power increasingly difficult for the utilities. Almost all utilities provide field measurements to customers upon request, and many have programs for characterizing magnetic fields in various environments. The IEEE's standards activity, which is the focus of voluntary efforts to establish EM (electromagnetic) fields standards in the United States is discussed, as well as national electric and magnetic field exposure standards for ELF (extremely low frequency) fields that have already been adopted by some countries. Some solutions for transmission lines, and the technological and economic factors entailed, are explored. The debate over the risk of electrical appliances is examined     

ELF Coupling to Spherical Models of Man and Animals

The induced fields, currents, and power absorbed by spherical models of humans or animals when exposed to extremely low frequency (ELF) electromagnetic fields are calculated. Spherical models can be expected to provide values of the right order of magnitude. The induced fields are a sum of the quasistatic solutions for a sphere irradiated by a uniform electric and magnetic field. Calculations are provided for ELF fields emanating from the proposed Navy's Sanguine antenna and extra high voltage (EHV)/ultra high voltage (UHV) transmission lines proposed by the utility industry. The question of biological hazards from exposure to fields of these systems is also considered.     

低频脉冲磁场对屏蔽体耦合特性的试验研究

低频脉冲磁场对屏蔽体的耦合效应与低频磁场自身参数、屏蔽体尺寸、材料特性及屏蔽体结构有关,作用过程比较复杂,实际耦合强度和规律应以试验研究结果确定。目前关于时变磁场的研究成果主要集中在单频磁场、快沿磁场等方面,而且效应试验研究较少。针对此种情况,文章选取钢板、纯铁板和铜板等几种常用的金属材料,制作了相应的屏蔽体缩比模型,利用低频脉冲磁场模拟试验系统研究了两者的耦合规律,分析了电磁参数等因素对耦合特性的影响,在一定程度上弥补了目前研究成果的不足。     

Temperature-independent low-voltage wide-bandwidth optical modulators

Temperature-independent electro-optic modulators have been constructed using lithium niobate. A frequency response extending to 2 GHz has been achieved with a relatively low drive power. The modulators have been used with a 0.63 ?m laser and are easily aligned on a standard optical bench.     

Demonstration of 3 pT/√(Hz) at 10 Hz in a fibre-opticmagnetometer

Detection of low frequency magnetic fields with interferometric fibre optic sensors has been limited by the presence of a residual signal at the carrier frequency, which upconverts low frequency noise. The authors present a technique which eliminates the residual signal and associated sideband noise, and demonstrate minimum detectable fields of ≈3 pT/√(Hz) at 10 Hz     

Nighttime Variations of Extremely Low Frequency (ELF) Signal Strengths in Connecticut

A limited amount of extremely low frequency (ELF) horizontal magnetic field strength measurements have been taken in Connecticut during the past 3 years. The transmission source for these 1.6 Mm range measurements was the U. S. Navy ELF Wisconsin Test Facility. The principal result obtained from these measurements is that there are considerable variations in the ELF nighttime propagation parameters.     

A spatio-temporal dipole simulation of gastrointestinal magnetic fields

We have developed a simulation of magnetic fields from gastrointestinal (GI) smooth muscle. Current sources are modeled as depolarization dipoles at the leading edge of the isopotential ring of electrical control activity (ECA) that is driven by coupled cells in the GI musculature. The dipole moment resulting from the known transmembrane potential distribution varies in frequency and phase depending on location in the GI tract. Magnetic fields in a homogeneous volume conductor are computed using the law of Biot-Savart and characterized by their spatial and temporal variation. The model predicts that the natural ECA frequency gradient may be detected by magnetic field detectors outside the abdomen. It also shows that propagation of the ECA in the gastric musculature results in propagating magnetic field patterns. Uncoupling of gastric smooth muscle cells disrupts the normal magnetic field propagation pattern. Intestinal ischemia, which has been experimentally characterized by lower-than-normal ECA frequencies, also produces external magnetic fields with lower ECA frequencies.     

Propagation Theory and Calculations at Lower Extremely Low Frequencies (ELF)

Propagation theory along the lines of Budden's terrestrial waveguide formalism is reviewed. Special emphasis is placed on its application to the lower extremely low frequency (ELF) range for which a number of calculations have been made to demonstrate the relationship between propagation parameters and the physical constants of the ionosphere. Calculations performed with diffuse anisotropic ionospheres yield reasonable agreement with ELF test results.     

Actuation and transduction by giant magnetostrictive alloys

Since the discovery and understanding of the magnetic properties of the rare earth elements, the transduction properties of magnetostrictive alloys has improved significantly over earlier nickel based alloys. Alongside the advances in permanent magnet technology, novel compounds containing terbium, dysprosium and iron are now available in commerical quantities. Given giant magnetostrictive strains (∼1200 ppm) coupled with a high transduction efficiency (>50%) and a large load bearing capability, all of which can be realised in modest magnetic fields, these new materials are now being employed in a variety of applications. High frequency generation is still dominated by piezoelectrics. However, the new materials have an excellent low frequency response, operate at low voltages and can withstand stress cycling. Non-linear and hysteretic behaviour has been controlled by state of the art digital control techniques and algorithms to such an extent that active vibration cancellation has been successfully demonstrated.     

Magnetic-field pickup for low-frequency radio-interference measuring sets

A magnetic-field pickup has been developed utilizing the Hall effect in intermetallic semiconductors. Unlike a loop pickup, the sensor responds to magnetic flux density and thus is independent of frequency. Due to its extremely small size, it makes possible the measurement of magnetic fields in constricted regions. When used in conjunction with ferrite flux collectors, its sensitivity is 10^-7gauss in the range of 30 cps to 15 kc. Details of design and construction are given.     

A 1.8-GHz low-phase-noise CMOS VCO using optimized hollow spiralinductors

A completely integrated 1.8-GHz low-phase-noise voltage-controlled oscillator (VCO) has been realized in a standard silicon digital CMOS process. The design relies heavily on the integrated spiral inductors which have been realized with only two metal layers and without etching. The effects of high-frequency magnetic fields and losses in the heavily doped substrate have been simulated and modeled with finite-element analysis. The achieved phase noise is as low as -116 dBc/Hz at an offset frequency of 600 kHz, at a power consumption of only 6 mW. The VCO is tuned with standard available junction capacitances, resulting in a 250-MHz tuning range     

光纤陀螺在直流及交变磁场中的磁敏感性研究

介绍了光纤陀螺的磁敏感性机理并进行了实验研究。利用亥姆霍兹线圈装置和光纤陀螺测试平台,研究了光纤陀螺在直流磁场中的磁敏感性,得到光纤陀螺数据输出与磁感应强度基本呈线性关系,以及轴向磁场和径向磁场影响系数的大小和光纤陀螺磁敏感轴的方向。主要针对50Hz的交流磁场研究交流磁场对光纤陀螺零偏和零漂的影响及其交流磁敏感性,得到交流磁场对零偏的影响可以忽略,但磁感应强度与陀螺零漂值的大小基本呈线性关系。研究了不同频率交流磁场对光纤陀螺输出的影响,说明了光纤陀螺数据输出主要与瞬态的磁感应强度成正比,不同频率交流磁场对光纤陀螺零漂值的影响不同,存在一个频率(或频率范围)对光纤陀螺零漂的影响较大,其频率范围与光纤陀螺输出带宽有关。     

Radar cross section of axisymmetric objects

A method for computing the resonant region scattering by slender axisymmetric objects with arbitrary surface impedance is outlined. This method employs matching of the electric and magnetic fields on axis. Results obtained by this method have been used to generate contour maps of radar cross section (RCS) as a function of frequency and aspect. Maps for three conducting cones and a cone-spheroid covering the diameter-to-wavelength range of 0.05 to 0.70 are shown. The relationship of the high and low frequency asymptotic behavior to the mapped results is discussed. Computed and measured RCS patterns are compared.     

Cardiac and respiratory related electrical impedance changes in thehuman thorax

Electrical impedance measurements have been made from the human trunk over the frequency range 9.6 kHz to 614 kHz. Measurements have been made from 12 normal subjects and the amplitude of the impedance changes associated with the cardiac and respiratory cycles have been recorded. It was found that the real part of the impedance fell to 64.0% of its low frequency value over the measured range of frequencies and that the changes associated with respiration fell in a similar manner. However, the cardiac related changes fell more rapidly with increasing frequency to 28.2% of the low frequency value. The origin of the measured changes is discussed with a view to understanding why the cardiac related changes fall more rapidly. It is not possible to relate in any simple way the frequency dispersion of a single component to that of the whole trunk. However, the results are consistent with the lungs being the major origin of both the cardiac and respiratory related components. The origin of the cardiac related impedance changes could be the pulsatile volume changes in the pulmonary tree. These could be shunted by nonpulsatile lung tissue that has decreasing impedance at high frequency and thus decreases the relative magnitude of the cardiac related changes. This hypothesis needs to be tested using localized measurements from the thorax and 3D modeling of the trunk     

Superconducting microwave bandpass filter using spiral resonatorswith no air bridges

A very compact superconducting bandpass filter using spiral resonators has been designed and demonstrated. No air bridges are required, an important feature for superconductors. The electric and magnetic fields are such that errors in track width have relatively little effect on resonant frequency and resonator coupling     

基于三相感应电机的小型化超低频发射天线研究

针对低频电磁传输中天线尺寸过大的问题，设计了一种基于三相感应电机的小型化超低频发射天线.电机工作时产生了时变低频电磁场，可作为超低频发射天线.为有效分析其电磁场分布，文章建立旋转磁偶极子数学模型以等效电机内部旋转时产生的磁场.首先，对电机内部磁场和旋转磁偶极子的关系进行阐述，利用麦克斯韦方程组，得到旋转磁偶极子的电磁场分布.其次，通过电磁仿真软件验证旋转磁偶极子的近场分布特性和远场辐射特性.最后，通过实验对电机近场的磁场分布进行验证.仿真和实测结果表明，三相感应电机具有超低频天线的辐射特性.     

Effects of Electromagnetic Radiation From Digital Wireless Telephone Frequencies on a Double Relaxation Oscillation SQUID

In this paper, the effects of electromagnetic fields from the frequency band of digital wireless telephones on a superconducting quantum interference device (SQUID) were experimentally investigated. A first-order planar gradiometer based on the double relaxation oscillation SQUID (DROS) was operated without low-frequency magnetic shielding, and was exposed to radiated electromagnetic fields in the frequency range of 300 MHz–2 GHz. The experiments have been performed in a fully anechoic chamber to minimize reflected waves in the chamber. The flux-to-voltage transfer and the field sensitivity of the gradiometer have been observed under the irradiation of fields. Results show that the performance of the gradiometer was significantly affected at a typical power level of the digital wireless telephones for both horizontally and vertically polarized electromagnetic waves. The noise level of the DROS output voltage also increases as the strength of the external fields becomes higher.      

Theory and Numerical Modeling of a Compact Low-Field High-Frequency Gyrotron

The electron-cyclotron maser interaction provides an extremely efficient means of generating high-power radiation in the millimeter and submillimeter regimes. For devices where both high frequencies and low magnetic fields are required, high cyclotron-harmonic interactions must be considered. We present here a finear and nonlinear analysis of a TE/sub m11/ whispering-gallery-mode gyrotron. Resonances at the mth and (m +- 1)th cyclotron harmonic are found. The start oscillation condition is calculated from linear theory for a wide range of parameters. Maximum efficiency for different beam and cavity conditions is calculated with a fully relativistic numerical simulation code. High efficiencies, >35 percent, have been found at the mth cyclotron harmonic. The effect on the efficiency of an initial velocity spread in the electron beam has also been considered.     

管道内流体固有频率的计算和测量研究

泵和压缩机广泛用于电力、石油、化工、钢铁和冶金等行业.管道振动会危及泵和压缩机的安全运转,依据平面波动理论,利用Fortran语言编写了计算程序,获得管路各阶流体固有频率.搭建了管道系统试验台,并对系统内流体的各阶固有频率进行了测量研究.     

Magnetic and photomagnetic properties of polycrystalline wide-gap semiconductor Cd1-xMnxTe thin films

The magnetic and photomagnetic properties of polycrystalline Cd_1-xMn_xTe thin films prepared by radio frequency (rf) sputtering have been investigated. Magnetization measurements were carried out using a rf superconducting quantum interference device magnetometer in the temperature range of 1.8∼300K at various magnetic fields up to 5.5 T. For temperatures above 40K, Curie-Weiss behavior was observed with a negative Weiss constant indicating predominantly antiferromagnetic interactions. Spin-glass transitions occurred at low temperatures. Photoinduced magnetization has been observed when the sample was illuminated by unpolarized light from an optical fiber.