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.     

细同轴天线的宽频带技术

运用同轴天线的原理,引入附加阻抗,通过同一传输线变压器的不同阻抗变换作用,改善天线的阻抗特性,展宽细同轴天线的频带,同时使天线尺寸大为减小。     

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.     

新型外导体结构柔软低损耗同轴电缆的研制

介绍了一种扁铜带束绞+铝塑复合带绕包+镀锡铜线编织外导体结构的柔软低损耗同轴电缆的研制,旨在减小电缆的传输损耗和电压驻波比,提高电缆的阻抗均匀性以及屏蔽特性。通过合理的结构设计和有效的工艺措施,使本柔软低损耗同轴电缆在电性能和机械性能方面获得令人满意的提高。     

Fast and Rigorous Analysis of EMC/EMI Phenomena on Electrically Large and Complex Cable-Loaded Structures

A fast and comprehensive time-domain method for analyzing electromagnetic compatibility (EMC) and electromagnetic interference (EMI) phenomena on complex structures that involve electrically large platforms (e.g., vehicle shells) along with cable-interconnected antennas, shielding enclosures, and printed circuit boards is proposed. To efficiently simulate field interactions with such structures, three different solvers are hybridized: (1) a time-domain integral-equation (TDIE)-based field solver that computes fields on the exterior structure comprising platforms, antennas, enclosures, boards, and cable shields (external fields); (2) a modified nodal-analysis (MNA)-based circuit solver that computes currents and voltages on lumped circuits approximating cable connectors/loads; and (3) a TDIE-based transmission line solver that computes transmission line voltages and currents at cable terminations (guided fields). These three solvers are rigorously interfaced at the cable connectors/loads and along the cable shields; the resulting coupled system of equations is solved simultaneously at each time step. Computation of the external and guided fields, which constitutes the computational bottleneck of this approach, is accelerated using fast Fourier transform-based algorithms. Further acceleration is achieved by parallelizing the computation of external fields. The resulting hybrid solver permits the analysis of electrically large and geometrically intricate structures loaded with coaxial cables. The accuracy, efficiency, and versatility of the proposed solver are demonstrated by analyzing several EMC/EMI problems including interference between a log-periodic monopole array trailing an aircraft's wing and a monopole antenna mounted on its fuselage, coupling into coaxial cables connecting shielded printed circuit boards located inside a cockpit, and coupling into coaxial cables from a cell phone antenna located inside a fuselage.     

Radio frequency coupling between an antenna and two unshieldedparallel wires above a metal sheet-measurement precautions

An unshielded 1 m transmission line, made of two parallel wires suspended 0.1 m above a plane metal sheet, is illuminated in the VHF and UHF bands by a log-periodic dipole array (LPDA). A network analyzer is used to measure the voltage induced at one end of the line. The measured signal is compared to the predictions of a moment method numerical analysis, using the NEC code. The measured and predicted coupling are in reasonable agreement only once two measurement precautions are taken. First, the illuminating antenna has to be well-balanced to suppress common mode radiation from the coaxial cable connecting the transmitter to the antenna. Second, an output balun must be inserted between the unshielded transmission line and the coaxial cable to the network analyzer. This is needed to prevent common mode signals on the coaxial cable from contaminating the measurements. Besides these practical steps, it is also necessary to use physical insight and high frequency circuit theory to develop a good equivalent circuit of the output balun for use in the computational model     

On the electromagnetic field of a dielectric coated coaxial cable with an interrupted shield

A boundary value treatment is used to derive field expressions for a dielectric coated coaxial cable with a gap in the shield. Specific results are given for an incident TEM mode in the interior coaxial region. For example, it is shown that, for the VHF region, as much as 18 percent of the incident power is radiated into the external region via a single circumferential gap in the shield. Comparable amounts of power are transferred to a reflected TEM mode in the cable and into surface waves in the dielectric jacket.     

Analysis of linear coaxial antennas

Two types of linear coaxial antennas, coaxial-colinear antennas, and slotted coaxial antennas are studied to check the possibility of using them as the base-station antenna in personal communication systems. The slot voltages and input impedance of linear coaxial antennas are obtained by using a transmission-line analysis where the radiation effect is accounted by a shunt and a serial admittance, respectively. The current distribution is obtained by solving an integral equation using the method of moments. The radiation pattern and directivity are then obtained from the current distribution and the reflection coefficient inside the coaxial cable. Factors analyzed include frequency, coaxial filling permittivity and segment number     

Time-Domain Investigation on Cable-Induced Transient Coupling Into Metallic Enclosures

A hybrid time-domain method is proposed for characterizing electromagnetic interference (EMI) signals coupled into some composite structures with metallic enclosures, braided shielded cable, printed circuit boards, and even lumped active devices. In order to rapidly capture the induced interior EMI, the finite-difference time-domain, modified node analysis, and multiconductor transmission lines methods are combined together and implemented successfully. Numerical investigation is carried out to demonstrate the frequency-dependent transfer impedance of the coaxial cable, the induced voltage at the place of active loaded element in the transmission line network, and the enclosure shielding effectiveness of these composite enclosures. The captured transient response information is useful for further designing electromagnetic protection of the inner circuits against the impact of voltage or current surge caused by nonintentional as well as intentional electromagnetic interference.      

An efficient numerical approach for modeling microstrip-typeantennas

An efficient numerical approach to model antennas that include a microstrip element radiating in the presence of material layers is developed. The class of antennas considered is fed through the ground plane by a coaxial transmission line. The reaction integral equation is formulated by treating the coaxial aperture as part of the antenna. The substrate thickness can be arbitrary, making this numerical technique suitable for high-frequency applications. The effects of the substrate are also included in the analysis. Numerical results are obtained for the current distribution and input impedance. The algorithm is validated with experimental results     

New method for measuring transfer impedance and transfer admittanceof shields using a triaxial cell

Two performance parameters of a cable or connector shield are its surface transfer impedance Z_T and its surface transfer admittance Y_T. A new method for measuring these properties is presented. The use of two different terminations for the cable or conductor under test (CUT) allows one to determine both Z_T and Y_T. Through characterization of the inner and outer transmission lines of the triaxial cell, using time domain reflectrometry, Z_T and Y_T can be determined in amplitude as well as in phase. The phase is obtained by de-embedding the measured S-parameters up to the CUT. The de-embedding of the measurements also allows one to extend the frequency range up to 3 GHz. To illustrate this method a solid shield with a circular aperture and a coaxial cable with a braided shield have been measured and compared, respectively, with theoretical predictions and published results     

Calculated Channel Characteristics of a Braided Coaxial Cable in a Mine Tunnel

The braided coaxial cable is studied as a communication scheme in a mine tunnel. A simplified rectangular waveguide model is adopted for the tunnel, and the shield of the cable is assumed to behave as a single inductive transfer impedance. Specific results on the attenuation of the monofilar and bifilar (or coaxial) modes of propagation, taking into account the possible existence of a thin lossy film on the cable, are presented. In order to estimate the maximum possible range of communication, we consider the coupling factors of these modes to transmitting and receiving dipoles inside the tunnel, and we present results on these factors for various cable parameters and over a wide range of frequencies.     

Coupling Impedance of Cylindrical Tubes

The concept of a coupling (transfer) impedance as developed by Schelkunoff for cylindrical tubes is applied in the determination of the mutual coupling to a grounded coaxial cable. Formulas are developed utilizing the coupling impedance concept when the exciting, or interfering, circuit is either a single-wire line or another coaxial cable over a ground plane.     

On the Effective Transfer Impedance of Thin Coaxial Cable Shields

It is shown that the effective transfer impedance per unit length of a thin coaxial cable shield is given by Zs h2/Ys, where ZS and Ys are, respectively, the cable's inductive transfer impedance per unit length and capacitive transfer admittance per unit length, and h is the axial propagation constant. This general result is illustrated by consideration of a specific shield model, the M-filar filamentary helix.     

Design and Analysis of a 3-Arm Spiral Antenna

A novel 3-arm spiral antenna structure is presented in this paper. This antenna similar to traditional two-arm or four-arm spiral antennas exhibits wideband radiation characteristic and circular polarization. Advantages offered by the new design are two fold. Unlike the traditional spiral antennas the three-arm spiral can be fed by an unbalanced transmission line, such as a coaxial line or coplanar waveguide, and therefore an external balun is not needed at the feed point. Also by proper choice of arms' dimensions the antenna can be directly matched to any practical transmission line characteristic impedance and therefore external matching networks are not required. This is accomplished by feeding the antenna at the outer radius by a coplanar waveguide (CPW) transmission line and tapering it towards the center. The antenna can also be fed from the center using a coaxial or CPW line perpendicular to the plane of the spiral antenna. A full-wave numerical simulation tool is used to optimize the geometry of the proposed 3-arm spiral to achieve a compact size, wide bandwidth operation, and low axial ratio. The antenna is also designed over a ground plane to achieve a unidirectional radiation and center loading is examined that improves the axial ratio. Simulated results like return loss, radiation pattern, gain, and axial ratio are compared with those obtained from measurements and good agreements are shown. Because of its unique feed structure and compact size, application of the proposed 3-arm spiral antenna for wideband array applications is demonstrated     

Linear Dummy Loads

The need for linear dummy loads is established. It is indicated that nonlinear dummy loads generate predominantly oddorder harmnonics. A theoretical analysis is presented which demonstrates that an ?infinite? coaxial cable with an infinite load (open circuit)at the output has an input impedance equivalent to the cable's characteristic impedance Z0. An equation is derived relating the voltage standing-wave ratio S, cable length h, and the cable attenuation constant ?. The linearity of a cable load is compared with the linearity of ?typical? commercially available loads and the commercial loads tested are found to generate 20 to 80 dB more third harmonic. Cable selection guidelines for a linear coaxial cable load are presented.     

Propagation Along a Coaxial Cable with a Helical Shield

A leaky coaxial cable is modelled by a dielectric coated conductor shielded by a finite number of unidirectional helical wires. A modal equation is derived and soIved numerically for the propagation constants of both the monofilar and bifilar modes. Numerical results are also presented for the effective surface transfer impedance of the shield. This parameter is found to depend, in general, on the propagation constant.     

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.     

同轴电缆串扰的仿真计算与测试

鉴于互连系统中电缆的电磁干扰问题,首先论述了由传输线理论对同轴电缆串扰的计算仿真,进而根据此模型自制了同轴电缆电磁耦合测试装置,测试若干结构参数对同轴电缆串扰电压的影响,得出了同轴电缆的串扰电压大小随电缆长度、干扰源电压和频率的增大而增大,最后比较仿真结果和测试结果,基本上达到了很好的一致性,此测试方法和结论对电气工程师测试和解决电缆串扰问题具有一定的实用价值。     

Superconductive Coaxial Cable as a Communication Medium with Enormous Capacity

Superconductive coaxial cable has been investigated to survey its feasibility as a communication medium with enormous capacity. The measurements of the frequency and the temperature dependences of the attenuation of coaxial pairs show that dielectric loss is dominant at 4.2 K, but conductor loss increases and gets the attenuation sensitive to temperature as 5 K is exceeded at a few gigahertz. The effect of periodic impedance lrregularities, which is the greatest problem in the transmission characteristics, has been solved by the improvement of the manufacturing process based on spectral analysis of the dimensional variations of the materials, and it is suggested that superconductive coaxial line is applicable to wide-band communication. A thermal insulation pipe for the cable is manufactured and its insulation efficiency is measured. The feasibility of superconductive coaxial cable Will be increased not only by developing the refrigeration system, but also by applying superconductive materials with low surface resistance and critical temperatures higher than lead.