Measuring Connector Shielding Effectiveness During Vibration

This paper describes the equipment and technique used to measure shielding effectiveness* of threaded electrical connectors during vibration. The vibration fixture is a modified coaxial trough of which the connector is a part. The connector is mechanically loaded with a short section of cable. A known current is applied on the shield of this short cable section and across the receptacle/plug interfaces. RF leakage into the interior of the cable is determined by forming the interior wiring of the cable into a closed loop and measuring the current in this loop. The logarithmic ratio of the current in the shield to the current in the interior loop is the shielding effectiveness of the cable-connector combination. The cable shield is composed of brass convolute with a braid covering and provides, by itself, 100 dB isolation. This isolation was demonstrated by testing the shielding effectiveness (S.E.) of a base-line specimen which substituted a brass fitting for the connector. It is demonstrated that this method of testing connectors can be used to determine S.E. under vibration. The method is proposed as a standard method of qualification testing of connectors.     

Current flow in coaxial braided cable shields

In the last few years, much effort has been made to describe the behavior of shielded cables. Many researchers have attempted to understand how an electromagnetic field couples into a braided coaxial cable. There are some important publications on this topic. Nevertheless, up to now, it has not been possible to predict analytically the coupling through a braid shield. An electromagnetic field outside a cable induces a disturbance current in the cable shield. The coupling from the current in the shield into the cable can be described by the transfer impedance. How the current flows in the cable shield is an important quantity in this coupling process. Therefore, to understand the coupling mechanism into a cable, it is necessary to understand the behavior of the current flow in such a braided shield. The paper discusses the current flow in a braided cable shield. The assumption often made in the literature, that a braided shield behaves like a homogeneous tube with apertures, is shown to be inaccurate. It is also shown that the standard braid of the shield used had the same properties as a braid made with insulated wires.     

Magnetic Field Pickup by Flexible Braid Coaxial Cables

Cabling systems have long been known to be a source of unwanted pickup of radio-frequency interference. A typical source of pickup occurs when the outer sheath of a coaxial cable system forms part of a ground return for potentially interfering electromagnetic energy. Another source of penetration arises from a magnetic field which is perpendicular to the axis of the coaxial cable. A previous study demonstrated that the field emanating from a coaxial cable could be characterized as an equivalent open-wire pair. This particular effect is caused by the eccentricity between the centers of the outer and the inner conductors or because of other asymmetrical characteristics. The susceptibility of a coaxial cabling system to perpendicular magnetic fields can also be characterized in terms of an equivalent open-wire pair for short runs. The purpose of this investigation was to measure the shielding effectiveness of a typical flexible braid coaxial cable to magnetic fields perpendicular to its axis and to demonstrate that a coaxial cable may be considered as an equivalent open-wire pair in a cylindrical shield. A test setup employed a parallel plane transmission line to generate a uniform magnetic field perpendicular to the axis of the coaxial cable. The shielding effectiveness of the braid was measured for several classes of braid conditions. Then, using the same braid, a coaxial cable with an experimentally induced known eccentricity was employed and the overall pickup measured. The results confirmed the objectives of the investigation.     

Effect of Pigtails on Crosstalk to Braided-Shield Cables

An investigation of the effect of pigtail sections (terminal segments of braided-shield cables in which the shield braid has been stripped back exposing the interior wire) on the electromagnetic coupling to braided-shield cables from adjacent wires is presented. Experimental and computed data indicate that, even though the lengths of these pigtail sections may be only a very small portion of the total cable length, they may, over certain frequency ranges, constitute the dominant coupling mechanism for the braided-shield cable. For situations in which pigtail coupling is dominant, the shield simply serves to reduce the exposed section of the interior wire from what it would be if no shield were present. Thus the shield provides some reduction in coupling, but the effectiveness of the shield in reducing crosstalk is shown to be as much as 30 dB (over certain frequency ranges) less than it would be if the pigtail sections were eliminated. A low-frequency model which explains this phenomenon is given. The effect of various shield grounding configurations is also investigated.     

Shielding Performance of Triply Shielded Coaxial Cables

We determine the shielding effectiveness of a triply shielded coaxial cable and compare it to the performance of a singly shielded cable. Then we apply well-known transmission-line theory to make a parametric study of the shielding performance as a function of the geometrical and electrical parameters of the braids. We suppose that the three braids are homogeneous, in order to use exact expressions relating the transfer impedance to the shield characteristics. We study the influence of the intersheath space on screening effectiveness and on the intersheath resonances. To damp these resonances, a material having a low conductivity could be used between the braids. To improve the shielding at low frequencies, an intermediate copper braid can be replaced by a ferromagnetic material. We take into account the variation of the permeability as a function of frequency and we point out, in this case, the effect of the intersheath space.     

Modeling of the Reverberation Chamber Method for Determining the Shielding Properties of a Coaxial Cable

The paper considers the reverberation chamber (RC) method for the measurement of the shielding effectiveness (SE) of coaxial cables with braided shields. In particular, the voltage at the cable termination is numerically computed and compared to that measured in an RC. The RC field is represented by a finite summation of random plane waves, and a finite-difference time-domain (FDTD) code is used to calculate the outer shield current induced by the RC field. The knowledge of the shield current distribution allows the determination of the voltage at the cable termination's internal circuit after a proper numerical averaging. It is then compared to the measured voltage averaged over stirrer rotations. The method is applied to a commercially available cable model RG58, and using the nominal value for the transfer impedance of this cable type gives results in a satisfactory agreement with the measurements. Finally, the possibility of recovering the transfer impedance from the measured SE of the RC is discussed.     

Electromagnetic coupling to and from shielded cables-optimizingfactors

Judging the shielding effectiveness of shielded cables often means in practice that only the transfer impedance is considered. The transfer impedance essentially characterizes the coupling via the magnetic field; the coupling via the electric field, the transfer admittance, is mostly neglected. This may be correct for shields with high optical coverage but for optimized single braided shields (coverage ≈0.8 . . . 0.9), the transfer admittance has to be taken into account. In practice, the cable shields are mostly grounded or open-ended at the line ends. With regard to the shield connections, the electromagnetic coupling to a cable by a plane wave and coupling from a cable are investigated. From the results, optimizing factors for the coupling parameters of shielded cables are deduced. By means of these optimizing factors the coupling to and from a cable can be minimized in certain applications     

Screening efficiency of triaxial cables with optimum braidedshields

An investigation into the shielding effectiveness of triaxial cables employing wire shields with optimum braid constructions is described. Several triaxial configurations with different braid designs for inner and outer cables were simulated. It is shown that crosstalk performance of the triaxial cables are affected by both absolute values and polarities of the surface transfer impedances of the shields. The effect of electric field coupling on the overall crosstalk within the triaxial system has also been studied in both frequency and time domains. For triaxial cables with shorted braids, the effects of electric field coupling are limited to high frequencies, and their effects at low frequencies are small     

High frequency cable connector for twisted pair cables

A new cable connector for twisted pair cable usable for high frequency applications is presented in this paper. An elastic conductive matrix as an interface between cable and printed wiring board (PWB) is pressed against the ends of the copper wires of the cable, and a land grid array on the PWB, thereby making the connections. The shielding braid of the cable is lengthened by a tube structure up to the same plane as the end of the copper wires, where the shielding is connected to an earth plane on the PWB. This not only gives a sound basis for good electromagnetic interference (EMI) behavior, but can also serve as an adequate structure for a dc barrier of common-mode currents in the shielding of twisted pair cables. A washer-formed capacitor between the earth of the PWB and the shielding tube structure would probably be the only addition needed. Measurements, performed on two connectors and the connected twisted pair cable, confirmed the hypothesis of how the performance of the new cable would be improved in the high frequency range compared to the SOFIX cable connector.     

Resonance Properties of the Shield of a Coaxial Cable over a Ground Plane

In situations where several high-power transmitters and their antennas are to be used near one another, a certain amount of mutual interference can be expected. An instance of particular interest is that of high-intensity radiation inducing standing waves between the shields of nearby coaxial cables and a metal deck of ground plane. Standing waves induced may cause high potentials and possible breakdown at the ends of the cable, damaging connectors and antennas. There may also be some reduction of the shielding effectiveness of the coaxial cable when high-voltage standing waves are present in the shield. It has been common practice to eliminate such standing waves by periodic grounding of the outer conductor of the coaxial cable. This, however, requires penetration of the insulation material on the cable and formation of metal-to-metal joints on the shield. This is not only an inconvenient method of installation, but is also undesirable around salt water. Copper shielding will corrode, and corrosion at the joint of the dissimilar metal can cause nonlinear interference effects. The standing waves induced in the transmission system formed by the cylindrical shield of a coaxial cable and a conducting plane are examined theoretically and experimentally as a function of the shield-to-ground impedance at the end points only (Z1 and Z2 of Fig. 1). Ordinarily, standing waves are eliminated by terminating a guiding system in its characteristic impedance. In this situation, however, the exciting source (i.e., incident radiation) is distributed along the length of the transmission system.     

一种线缆护套的屏蔽效能测试方法

设计了一种基于串扰理论的线缆护套屏蔽效能的测试方法,根据测试结果分析了屏蔽护套不同接地方式对线缆间感性耦合、容性耦合的影响,试验结果可为在实际工程中抑制线缆串扰提供依据。     

屏蔽电缆屏蔽特性测试方法的比较和分析

本文根据测量原理的不同将各种屏蔽电缆屏蔽特性测试的方法归纳为“场测试法”和“传输线测试法”两大类,并分别以混响室法和线注入法为代表比较分析了两类方法的异同和各自需要关注的一些问题。两种方法分别用屏蔽效能和表面转移阻抗来表征电缆的屏蔽效果,在实际运用中各有优劣,体现出很强的互补性。     

多芯电缆屏蔽效能原理及其仿真分析软件

在车载通信系统电磁兼容设计方面,多芯电缆的屏蔽效能是一个很重要的指标,采用屏蔽电缆是消除和抑制电磁干扰的有效手段.本文首先提出了屏蔽效能的定义,并根据多导体传输线理论论述了多芯电缆屏蔽效能的仿真计算方法,然后根据多芯电缆屏蔽效能的原理,给出设计开发基于VB和Borland C 语言的多芯电缆仿真分析软件的基本设计思路和实现过程,最后比较了仿真结果和测试结果,达到较好的一致性,为设计人员合理设计和选择电缆提供了依据.     

Transfer impedance of braided coaxial cables under applied longitudinal torsion

The surface transfer impedance ZT of several braided coaxial cables was measured under applied longitudinal torsion to determine the electromagnetic shielding characteristics of the cables under such conditions. The results cover the frequency range 100 kHz to 150 MHz with torsion angles varying between 0°, +360° m?1 and ?180°m?1. It was found that ZT increases with torsion angle and depending on the degree of optimisation in the braid, increases of up to 800% can occur at a torsion angle of 360° m?1. Further, when a cable is returned to its original position, after being subjected to torsion, the resulting value of ZT is higher than the original value.     

Shielding Effectiveness of Braided-Wire Shields

An analysis of the shielding effectiveness of braidedwire shields is made using the parameters of the woven wire and the theory of electromagnetic coupling through small irises. The coupling through the rhombic-shaped holes in the braid is approximated by using the electric and magnetic polarizabilities of elliptical holes of the same width and length as the rhombus. The analysis develops the transfer impedance and the mutual capacitance of the shield. The transfer impedance is calculated and plotted for shields of different optical coverages. The variation of mutual capacitance and mutual inductance with the weave angle of the braid are examined.     

Cable Trays in EMC: Measurement and Modeling to 30 MHz

Common mode (CM) currents are a major source of interference in electrical and electronic systems. Cable trays are often used to shield cables from unwanted CM electromagnetic interference, and their shielding characteristics are defined in terms of transfer impedance. We present the measurement and modeling of nonmagnetic U-shaped cable trays from 300 kHz to 30 MHz. A calibrated vector network analyzer in a screened environment is required for the high dynamic range measurements. We use method of moments simulations to determine the transfer impedance and mutual inductance within the interior region of a cable tray. We refined the modeling after detailed attention to the code. The computational and measured data are in good agreement. We propose the simulation as a means to predict the magnetic fields, mutual inductance, and transfer impedance associated with victim cable loops in the cross section of nonmagnetic cable trays to frequencies well beyond our studied range of 30 MHz.     

A Study on the EM Leakage Arising from Braided Shielding Cable

1 Introduction For its good shielding and flexibility properties ,braided shielding cable is widely used in modern com-munications[1 ~2]. Because there are diamond shapedholes onthe shield,the magnetic fieldthat generated bythe current onthe shield goesintothe cable throughtheholes. On the other hand,the magnetic filed that gen-erated by the inner conductor current penetrates theholesinto the outer space . The electromagnetic leakagewill occur due tothe diffusionthroughthe metal andthefield pe…     

多芯屏蔽电缆屏蔽效能的脉冲电流注入法测量

为了测量脉冲电流注入电缆的时域屏效,采用皮电流I0与短路芯电流Id峰峰值之比的分贝数求解电缆的屏蔽效能方法,给出了某百芯屏蔽电缆的等效电压源内阻和屏蔽效能值,并对外场辐射试验法中的某百芯初样电缆总的屏蔽效果进行了计算和讨论。     

A New Method of Analysis for Multigrounded Cable Shields Inductively And Conductively Coupled to Multigrounded-Neutral Power Lines

A configuration in which a communications cable with a multigrounded shield has a length that is inductively and conductively coupled to a multigrounded-neutral conductor of a power line is analyzed. A new formulation is given for the power-frequency voltages and currents that develop along the shield and core of the cable. A simplified network is derived for the coupled systems from which the variables of the formulation can be evaluated. There are no restrictionson the uniformity of the grounds, the spacing of bonds, or the terminations. The procedures aredirectly applicable to the evaluation of shielding effectiveness and to protection problems arising from power faults.     

多层平板屏效计算中的等效传输线法

讨论了多层平板屏效计算中的等效传输线法。定义了不同传输线间的反射系数与透射系数,求出了反射系数和透射系数表达式。文中透射系数的定义是对传线基本理论的有益补充。