Standard probes for electromagnetic field measurements

Discusses various standard antennas for measuring radio-frequency electric and magnetic fields. A theoretical analysis of each antenna's receiving characteristics is summarized and referenced. The standard probes described are an electrically short dipole, a resistively-loaded dipole, a half-wave dipole, an electrically small loop, and a resistively-loaded loop. A single-turn loop designed for simultaneous measurement of the electric and magnetic components of near-fields and other complex electromagnetic environments is also described. Each type of antenna demonstrates a different compromise between broadband frequency response and sensitivity     

Power line filter design for conducted electromagnetic interference using time-domain measurements

A time-domain technique for the design of passive power line conducted electromagnetic interference (EMI) filters in the frequency range 150 kHz-30 MHz is described. A digital storage oscilloscope (DSO) with adequate sampling, storing and processing features is sufficient for the design using the proposed technique. Accordingly, Agilent's Infiniium Oscilloscope (Model 54810A) has been used. The signals from LISN are directly fed into the two channels of the DSO where they are added and subtracted to separate the CM and DM components, thereby eliminating the need for common-mode-differential mode (CM-DM) separator. These components are stored in the DSO. A specially designed filter design software (FDS), residing in the DSO, estimates the noise spectrum by computing the Bartlett and Welch periodograms. It also computes the filter component values. Thus, the sampling of the conducted noise, separation of CM and DM components, signal processing, and filter value computations are all done using one DSO. A spectrum analyzer is not required. Bartlett periodograms have been preferred over Welch periodograms due to low memory storage requirements of the former. The proposed technique has been applied to the design of power line filter for a switched mode power supply (SMPS), and satisfactory results have been obtained. The proposed measurement scheme is compact, economical, and convenient. All the details of this work are presented.     

Pastes for electromagnetic interference shielding

A paste has been developed for electromagnetic interference shielding by mixing stainless steel fibers (8-μm diameter) and graphite flakes (5 μm) in a water-based graphite colloid (containing submicron graphite flakes and a binder) in a volume ratio 0.5:20:100. The resulting coating of thickness 0.1–0.2 mm after drying exhibits shielding effectiveness of 34 dB at 1 GHz, primarily due to reflection. The paste is superior to those containing graphite flakes or carbon filaments (0.1-μm diameter), due to the higher shielding effectiveness of the steel fibers. The graphite flakes in the paste serves to help suspend the steel fibers, in addition to contributing to shielding.     

电磁波激励下天线的响应特性分析

应用互易定理推导出了电磁波激励下天线的耦合长度和耦合面积的表达式,这两个物理量可以表征天线的响应特性.针对对称振子天线,在不同来波照射方向和端接不同带宽的接收机这两种情况下,计算获得了耦合长度和耦合面积.计算结果表明,工作频率为2GHz的对称振子天线的耦合长度随着接收机带宽的变化在频率f=2 4n (GHz) (n=0,1,2...)或f=4n (GHz) (n=1,2...)附近出现谐振峰;而耦合面积随来波入射角度的增大而增大,随接收机带宽的减小而减小.     

Standard linear antennas, 30 to 1000 MHz

It is demonstrated that the insertion loss between pairs of thin, linear antennas may be calculated using fairly simple equations that are generally considered to be good engineering approximations. Although the insertion loss calculation does not involve antenna gain directly (some measurements are actually made in the near-field where gain is not defined), the result is precisely the quantity obtained using the antenna gains in Friis's transmission formula, assuming the mismatch losses are zero. Therefore, the antenna gain product is implicit in the more general insertion loss equations. The particular measurement of insertion loss used here yields a quantity called site attenuation by electromagnetic compatibility engineers. A close agreement between measured and calculated data provides confidence in the site attenuation calculations when the site is essentially perfect, and provides confidence in the gain product of the antenna pair calculated using basically the same equations as those used for insertion loss. It is assumed that one-half of the mean value of the difference between the calculated and measured data is a good estimate of individual antenna performance. For the antennas described here, this measure of performance is typicallylE0.05dB and on the outside,iE0.42dB.     

Analysis and measurements for improved crank-line antennas

This paper describes the radiation characteristics of crank-line antennas radiating a circularly polarized wave. First, the radiation efficiency versus substrate permittivity is evaluated. Second, a 12-cell crank-line antenna of substrate permittivity ε_r=1 and antenna height B=λ_11.85 is investigated as a reference antenna, where λ_11.85 is the wavelength at a frequency of 11.85 GHz. It is found that the main beam direction of the reference antenna varies 7° over a frequency range of approximately 6%, with an axial ratio of less than 3 dB and a gain of approximately 21 dB. Third, attention is paid to the gain behavior versus the antenna height. A way to increase the gain by modifying the antenna height is proposed. An increase of 1.5 dB from the gain of the reference antenna is demonstrated. Finally, the axial ratio, gain, and decoupling factor for crank-line antenna arrays are presented and discussed     

New approach to compact measurements on reflector antennas

The letter describes a novel scheme for very compact measurements on reflector antennas. The measurement configuration involves a plane reflector located just beyond (and parallel to) the aperture plane, so that radiated power is reflected and refocused back into the feed. Measurement of return loss can then give information on antenna efficiency and gain. The basic concept is described and its performance confirmed by means of experiments on a 10 ft (3.048 m) diameter reflector. Possible methods of extending the performance of the scheme are briefly discussed.     

Cosite interference between wire antennas on helicopter structuresand rotor modulation effects: FDTD versus measurements

As telecommunication systems become more complex and more antennas are placed on the same structure (e.g., helicopter airframe) the problem of interference becomes significant for the performance of the systems. The finite-difference time-domain (FDTD) method is used to calculate coupling between wire elements; e.g., monopoles and loops mounted on ground planes and helicopter airframes. Also, rotor modulation effects on coupling are investigated. All the numerical results obtained by FDTD are validated by comparison with measurements     

电磁脉冲下通信偶极天线的时域响应快速计算

针对电磁脉冲作用下通信系统受扰的场路结合计算过程较为复杂的问题,提出了基于等效电路的偶极子天线端口时域响应的计算方法.通过分别建立天线的等效电路模型以及电磁脉冲照射下的等效干扰源模型,快速求解天线端口的时域电流响应.选取电磁辐照条件下两个不同尺寸的偶极子天线进行验证,并与采用全波电磁数值软件计算的结果进行了对比.结果表明：提出的等效电路方法不仅速度快而且元件参数易调整,等效电路模型还可以直接应用于通信系统内部电路的电磁受扰评估计算.     

近场测量相控阵天线的全息成像方法

研究了一种利用近场测量技术全息成像相控阵天线口径幅相的方法。该方法是把近场测量获得的方向图函数与由单元形式及幅相分布表示的方向图函数进行比较,采用FFT算法和空间域的Fourier重构法,可以快速、精确地成像出相控阵天线口径的“全息图”,进而诊断出阵中单元幅相的奇异程度。通过仿真实验,检验了该方法的成像分辨率和精度,并考察了不同口径区域的成像误差对辐射方向图的影响程度,说明该具有非常好的应用前景。     

Multipath interference for in-flight antenna measurements

Formulations are presented to predict the pattern of interference (multipath) between direct rays and those reflected from the surface of the earth. These formulations can be used to assess the performance of ground-to-air, air-to-ground, ground-to-ground, and air-to-air communication and antenna measuring systems. Methods are also introduced to determine accurately the point of reflection and the path phase difference between the direct and reflected waves. Various divergence factors, used to account for energy spreading from a curved surface, are presented and compared. The formulations introduced can account for any polarization state (linear, circular, elliptical), sense of rotation (right hand, left hand), and tilt angle of the transmitting and receiving elements, and for polarization changes due to reflection.     

Application of spherical near-field measurements to microwaveholographic diagnosis of antennas

Microwave diagnosis of antennas is considered as a viable tool for the determination of reflector surface distortions and location of defective radiating elements of array antennas. A hybrid technique based on the combination of the spherical near-field measurements and holographic metrology reconstruction is presented. The measured spherical near-field data are first used to construct the far-field amplitude and phase patterns of the antenna on specified regularized u-v coordinates. These data are then utilized in the surface profile reconstruction of the holographic technique using a fast-Fourier-transform (FFT)/iterative approach. Results of an experiment using a 156-cm reflector antenna measured at 11.3 GHz are presented for both the original antenna and the antenna with four attached bumps. Several contour and gray-scaled plots are presented for the reconstructed surface profiles of the measured antennas. The recovery effectiveness of the attached bumps has been demonstrated. The hybrid procedure presented is used to assess the achieved accuracy of the holographic reconstruction technique because of its ability to determine very accurate far-field amplitude and phase data from the spherical near-field measurements     

开关电源的电磁干扰及抑制技术

由于开关电源本身的工作原理,使得电磁干扰问题相当突出,其干扰信号经传导和辐射对电子设备、通信系统等产生电磁污染。本文介绍了电磁干扰的产生机理和传输方式：一种是传导传输方式,另一种则是辐射传输方式。本文从电磁干扰产生的机理入手,着重分析了开关电源电磁干扰产生的原因,总结了抑制电磁干扰的措施,比如滤波技术、屏蔽技术、软开关技术、扩频调制技术、PCB设计技术和接地技术等。     

An interference suppression scheme for UWB Signals using multiple receive antennas

In this letter a simple narrow band interference (NBI) mitigation scheme for ultra-wide bandwidth (UWB) signals using multiple receive antennas is examined. The low spatial fading characteristic of UWB signals is exploited to provide "interference diversity" by selecting the signal with the lowest received power among multiple antennas. The resulting distribution of the signal-to-interference ratio (SIR) at the receiver is derived and compared with results based on measurement data. The performance improvement of the scheme in mitigating NBI is also examined through BER simulations.     

Closed-form solution to electromagnetic fields in far zone radiated by toroidal antennas

In this paper, a single-wound toroidal helix antenna (SWTHA) and a contra-wound toroidal helix antenna (CWTHA) are analyzed in an analytical way. In this connection, the total winding length of a SHTA is estimated and a fairly accurate and closed-form expression of the length is obtained. Meanwhile, a closed form solution of the electromagnetic fields in far zone or their alternative vector potential components is obtained. The formulation is analytical, based on the assumption of uniform SWTHA and CWTHA currents and the large ratio a/b of the toroidal antenna radius a over the turn radius b. The solution is general enough for any number N of wound turns. The results are finally reduced to those existing results available elsewhere in literature by letting the radius b be zero (for infinitely thin wire and circular loop antennas) and the winding number N be zero (for the normal circular loop antennas). The reduced results of the potential components are the same as those in the textbooks. The analytical results are obtained for the first time, useful for antenna designers and also antenna engineering education.     

Effective interference cancellation scheme based on smart antennas

An effective interference cancellation scheme based on a new multipath model is introduced. A CM (constant modulus) equalisation approach based on an adaptive array with a spatial prefilter is derived. It has been proven by numerical simulation that this method achieves a better performance, even under serious multipath conditions     

FDTD simulations and measurements for cell phone with planar antennas

The Finite-Difference Time-Domain (fdtd) method is used to calculate the cellular phone far and near field and the specific absorption rate (sar) in the user’s head. The conventional λ/4 monopole and a simple planar antenna are simulated and measured. The simulated results are in good agreement with those measured for thevswr and for the radiation patterns on the horizontal and vertical planes. These results show that significant improvements in the antenna radiation efficiency and in the reduction of thesar in the head are obtained when planar antennas are used.     

A review of bistatic k-space imaging for electromagnetic predictioncodes for scattering and antennas

A bistatic k-space imaging concept for frequency-domain (FD) electromagnetic (EM) computer codes is presented. The concept enables the computation of images without the frequency sweep required for experimental images, resulting in a significant reduction in computational effort. This analytical imaging technique uses bistatic radiation computed from a generalized radiation integral. Images permit physical insight and understanding into how a radiation or scattering pattern is produced, by decomposition of the resultant radiation into contributions from localized scattering centers, or hot spots. Knowledge of these hot spots permits a user to understand and modify the structure to obtain desired features     

Limiting electromagnetic interference due to on-board devices from on-bench tests

Electromagnetic interference due to on-board devices may differ from what can be expected after on-bench tests. An approach, aimed at ensuring that the on-board device performance is satisfactory, is presented. It is based on the computation of new limits for on-bench tests and is illustrated in a very simple configuration.