Shielding Theory of Coaxial Cylindrical Structures

A theory on the shielding effectiveness of n coaxial metallic tubings which shield a coaxial cable has been developed based on electromagnetic (EM) field theory. The theory holds for all homogeneous, linear, and isotropic shields, magnetic or nonmagnetic, and covers essentially the entire frequency range. When a cable carries an evenly distributed axial current, the dominant mode of propagation is transverse magnetic (TM) and has only three field components, i.e., Ez, H?, and Er. The fields of the dominant mode leaking from the cable, with and without shields, have been determined rigorously from the solutions of Maxwell's equations and boundary conditions. The shielding effectiveness of the tubings, defined as the insertion loss, has thus been readily obtained. To simplify the obtained expressions to a certain degree such that numerical calculations are manageable, various approximations have been introduced and precisely justified. The limitations imposed on the simplified expression due to the approximations have been clearly listed. It has been shown that Schelkunoff's shielding theory is merely a special case of the present work. As an example, the shielding effectiveness of a single copper tubing surrounding an RG-8/U cable has been considered. The data measured from a carefully designed experimental setup show that at high frequencies, i.e., above 10 kHz, the curve predicted by the present work is about 1 dB above the empirical curve, while the curve due to Schelkunoff is about 5 dB below the empirical curve. At low frequencies, i.e.     

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.     

Cable Shielding Effectiveness Testing

This paper discusses an improved method of measuring the effectiveness of cable shielding and describes the results of tests on single- and multi-branched cables. Effects of significant shielding parameters of cables are also reported. These are the the effect of number of shield braid layers, braid material, braid angle, optical coverage, cable length, and wire size. The test method permits measurement of long specimens using high currents with a uniform current distribution along the cable shield. Measurements were made in the frequency range 0.5 to 100 MHz. The method is offered as a standard technique for measuring the shielding effectiveness* of shielded cables.     

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.     

Theory and analysis of leaky coaxial cables with periodic slots

Frequency band and coupling loss are the two important parameters of leaky coaxial cables with periodic slots. The frequency band can be predicted by analyzing the arrangement of the slots on the outer shield of the cable, but the coupling loss is not so easy to determine by classical methods. In this paper, the finite-difference time-domain (FDTD) method is used to calculate the electric field distribution in the slot cut in the outer conductor of the coaxial cable. The dyadic Green's function is then used to calculate the radiation field of the equivalent surface magnetic current densities. By these two methods, the coupling losses of the leaky coaxial cables with different periods, sizes and shapes of the slots can be accurately obtained. Some results in this paper were verified by the experimental results of leaky coaxial cables designed for railway mobile communications with a frequency band of 100-500 MHz     

Transmission Modes in a Braided Coaxial Cable and Coupling to a Tunnel Environment

Radio frequency transmission in a semicircular tunnel containing a braided coaxial cable is considered. The general formulation accounts for both the ohmic losses in the tunnel wall and a thin lossy film layer on the outer surface of the dielectric jacket of the cable. Using a quasi-static approximation, it is found that the propagation constants of the low-frequency transmission line modes are obtained through the solution of a cubic equation. However, for the special case when the conductivity thickness product of the Iossy film layer vanishes, this cubic equation reduces to a quadratic. The spatially dispersive form of the braid transfer impedance is also accounted for. It is shown that the quasistatic theory is well justified for frequencies as high as 100 MHz for typical tunnel geometries. Finally, special characteristic impedances are derived for the various modes of the equivalent multiconductor transmission line.     

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.     

射频同轴电缆电压驻波比影响因素综合分析

电压驻波比(VSWR)是射频同轴电缆重要的传输特性,采用场-路结合的方法建立了射频同轴电缆的三维有限元分析模型,分析了影响射频同轴电缆VSWR的因素,深入研究了内外导体直径偏差、绝缘层质量、内外导体的偏心度和电导率对射频同轴电缆VSWR因素的影响。     

Modélisation de l’impédance de transfert des câbles coaxiaux à blindages multiples

The shielding effectiveness of multishielded coaxial cables is determinated through the concept of the equivalent transfer impedance. The transfer impedance is computed from the main parameters of the coaxial structure. We describe in this paper the theoretical formulation to evaluate the amplitude of the disturbing voltage at the end of the cable flowed by the disturbing current. This result is used for the computation of the equivalent transfer impedance when the cable is made of various shields for exemple: homogeneous screens or braids. A comparison with the experimental results is also described.     

Propagation Along a Braided Coaxial Cable in a Circular Tunnel

The modes of propagation along a coaxial structure contained within a circular tunnel are considered. The primary objective is to develop an approximate impedance boundary condition at the outer surface of the shielded cable that can be used in previously developed formalisms for axial conductors in tunnels. It is assumed that the metal braid can be characterized by a surface-transfer impedance. We also account for the possibility that a Iossy film exists on the outer surface of the dielectric jacket of the cable.     

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.     

Analysis of the Shielding Characteristics of Saturable Ferromagnetic Cable Shields

A numerical technique is developed that may be used to analyze the shielding characteristics of a cable with a saturable ferromagnetic outer sheath in an intense low-frequency environment. The solution relates the center conductor current to the total current exciting a coaxial cable with a ferromagnetic sheath. Several examples are given to illustrate how saturation effects the shielding properties of a ferromagnetic sheath.     

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.     

多芯电缆屏蔽效能测试方法研究

首先介绍了屏蔽效能的定义,然后根据多芯电缆和同轴电缆的区别与联系,提出了适用于测量多芯屏蔽电缆效能的注入线测试方法,并且比较了测试结果和仿真结果,为测试人员合理选择有效的测试方法提供了依据。     

An Equivalent Surface Source Method for Computation of the Magnetic Field Reduction of Metal Shields

A numerical method for computation of the resultant quasi-static magnetic field in the vicinity of parallel wires and metal shields is presented. The primary magnetic field source is time-harmonic currents in wires. This field is modified by conducting magnetic and/or nonmagnetic shields. The material is assumed to be linear under the applied source field. The shielding effectiveness can be estimated by a comparison between the primary and the resultant field. The reaction magnetic field is expressed by a sum of fields caused by equivalent single- and double-layer sources distributed on the shield surface. Integral equations for unknown distributions of these equivalent sources are derived from the Green's second identity implemented inside and outside the shields. These equations are coupled integral equations, and are solved by the moment method. Numerical results of the resultant (shielded) magnetic field obtained with the proposed method are compared with the results of: 1) analytically solvable problems; 2) measurements; and 3) two different numerical methods.      

毫米波半柔软射频同轴电缆的研制

介绍了一种毫米波半柔软射频同轴电缆的结构设计与制造工艺。该电缆可传输毫米波射频信号,具有传输频率高(40GHz)、插入损耗小、弯曲成型能力强等性能特点。微孔聚四氟乙烯绝缘和导电金属层+镀锡铜线编织+热浸锡整体外导体结构是该电缆的主要创新点。     

Transferred surface currents in braided coaxial cables

It is shown that the transfer of longitudinal current to the outer surface of a long length of braided coaxial cable, such as might be used in a `leaky-feeder? radiocommunication system, is a function of other parameters besides the surface-transfer impedance of the braid. The dielectric is particularly significant.     

Cheby?shev quarter-wave stepped balun transformer

The design of a balun transformer from coaxial to open-wire line is presented, in which conventional quarter-wave-matching theory is used to produce a compact device with optimal Cheby?shev performance over its passband. Included in the design are the evaluation and corrections for the junction capacitances caused by stepping the outer coaxial conductor.     

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.     

一种含同轴电缆的线束内部串扰高效分析方法

为了高效分析含同轴电缆的线束内部串扰,提出了一种新的简化电缆束模型的方法. 该方法把简化电缆束的过程分为两步:首先,为了解决同轴电缆编织屏蔽层难以仿真的问题,利用内外传输线转移矩阵简化同轴电缆,将含同轴电缆的电缆束转化为全由单芯线组成的三维电缆束模型;然后,使用等效线束法将全单芯线的电缆束进一步简化,减少需要分析的单芯线数量. 简化后的模型可用于任何三维电磁仿真软件中的电缆束内部串扰仿真分析,而无需考虑基于经典传输线理论的等效电路法的局限性. 将提出的方法与等效电路法分别应用于电缆束内部串扰的分析,两者的仿真结果一致性良好,说明文中提出的方法是准确可行的,能够实现含同轴电缆的线束内部串扰的高效分析.