A Feasibility Study to Monitor Soil Moisture Content Using Microwave Signals

A buried leaky coaxial cable is used as a sensor to continuously monitor the average moisture content of irrigated soil at a desired depth. The two-phase feasibility study consists of an analytical and an experimental investigation. The modal equation in the cable is expressed in terms of the surface impedance at the inner conductor boundary and an effective surface impedance at the outer leaky conductor boundary. The effective surface impedance is related to the soil moisture content through its measured attenuation. The results of the experimental work condncted in the field are in good agreement with the analytical results. It is shown that the recorded phase difference between the test signal from the buried cable and the reference signal can be used to monitor the soil moisture content.     

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     

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.     

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.     

Surface transfer impedance measurement: a comparison betweencurrent probe and pull-on braid methods for coaxial cables

Theoretical models to compute the surface transfer impedance of cables often rely on simplifying assumptions. This, together with the fact that surface transfer impedance can vary considerably between cable samples of the same type, means that measurements become necessary. In this way an average performance may be determined. Many transfer impedance measurement methods have been proposed over the years and each has its own relative strengths. Two frequency-domain measurement methods are compared: the current probe method and the pull-on braid method. Both methods are inexpensive and can be set up very quickly without expensive cable preparation. Moreover, they operate over a broad frequency range with high accuracy. This is shown by the good agreement obtained between measurements carried out with the two methods     

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.     

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.     

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。     

50Ω皱纹铝管外导体射频同轴电缆的研制

简述了50Ω皱纹铝管外导体射频同轴电缆的研制,着重介绍了其生产工艺难点。该电缆的实测性能表明,皱纹铝管外导体射频同轴电缆可替代同规格的皱纹铜管外导体射频同轴电缆用于移动通信市场。     

基于自适应遗传算法的寄生电流抑制结构设计与研究

为了抑制同轴线外导体表面存在的寄生电流，设计并研究了基于自适应遗传算法的寄生电流抑制结构.该结构是一个与同轴线外导体相连且两侧有五对对称I型开槽的金属板，在开槽处加载集总电容以实现小型化要求.该结构工作原理可等效为一组串联LC谐振电路，当有外部因素导致同轴线外导体表面有寄生电流存在时，该等效电路呈现的高阻抗特性会抑制寄生电流的流经.为了选取合适的电容，提出了一种自适应遗传算法来优化计算加载电容的大小，该遗传算法实现了交叉率和变异率的非线性自适应调整，与标准算法相比，该算法有更优的收敛速度以及鲁棒性.利用此算法完成了相应的计算和分析，结果表明该结构能够在5 250~5 925 MHz范围内对寄生电流实现-10 dB以上的抑制效果.本文方法能够指导寄生电流抑制结构设计，并进行寄生电流抑制效果的定量分析.     

基于电容加载的宽带小型化扼流结构的设计与研究

设计并研究了基于电容加载的宽带小型化扼流结构来抑制同轴线外导体表面的寄生电流。提出了能定量分析其扼流效果的通用性方法。此扼流结构与同轴线外导体相连,由两侧对称I型开缝的金属板组成,并在缝隙处加载电容使其实现小型化。其工作原理等效于一组串联的LC电路,当同轴线外导体外表面存在有寄生电流时,扼流结构的高阻抗特性会在一个宽频带内抑制电流的传导。该文对扼流结构的特性原理及理论基础进行了研究,结果表明此扼流结构能在5250~5925MHz范围内对寄生电流的传输特性有-12d B的抑制性能。     

大长度低损耗半硬同轴电缆的研制

设计了一种采用膨体聚四氟乙烯作为绝缘,连续激光焊接铜管作为外导体的新型低损耗半硬同轴电缆,以打破传统的半硬同轴电缆制造长度的局限性,同时对电缆工艺的可行性进行了探讨。膨体聚四氟乙烯绝缘的采用降低了电缆的介电常数,减小了半硬同轴电缆高频段的传输损耗。连续焊接铜管外导体的采用不但增加电缆的制造长度,并且保留传统半硬同轴电缆良好的阻抗均匀性和屏蔽特性,同时也提高了生产效率,降低了材料消耗。     

射频功率放大器的宽带匹配设计

多倍频程功率放大器具有显著的优点,同轴电缆阻抗变换器能实现射频功率放大器有效的宽带匹配。在给出同轴电缆阻抗变换器方案设计的基础上,详细介绍了其基本原理以及1∶1和1∶4同轴变换器的具体结构及等效电路。针对工程需要,以同轴电缆阻抗变换器为宽带匹配网络的核心,设计了一款超宽带匹配功率放大器,经软件仿真优化及测试验证,阻抗匹配准确,测试结果达到了技术指标要求。     

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     

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.     

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     

Time-domain study of a shielding cable system with a nonlinear load

An impulse current of several kiloamperes was injected to the shield of a shielded cable, which was terminated by a varistor. The induced voltage on the inner conductor caused by this impulse current reaches an amplitude in excess of the varistor's threshold level. The clamped voltage across the varistor and the injected current have been studied for different termination conditions of the shielding cable. Furthermore, this paper also presents the use of a lumped circuit to simulate the transfer impedance of an “electrically short” shielded cable in the time domain. In combination with the varistor circuit model, the shielded cable with the nonlinear load, a varistor, was also simulated straightforwardly in the time domain. Good agreement was found between the measured voltage and current oscillograms and the calculated waveforms. It is thereby demonstrated the validity of the developed lumped circuit model for the transfer impedance of a shielded cable     

Transient analysis of a cable with low-conducting layers by a finite-difference time-domain method

The impedance and admittance formulas of a cable with low-conducting layers have not yet been derived, and thus a transient analysis considering the layers cannot be carried out sufficiently by an existing transient analysis program such as the Electro-Magnetic Transients Program. The present brief has analyzed a transient response of a power cable including low-conducting layers using a finite-difference time-domain method. Transient current waveforms at both ends of the cable are distorted depending on the conductivity of the low-conducting layers. Also, the propagation velocity of a surge current is dependent on it. When the conductivity of low-conducting layers is around 10/sup -3/ S/m, the shunt admittance of the cable dominates the above phenomena. On the other hand, they are ascribable to the series impedance when the conductivity is about 10 S/m.     

Frequency- and time-domain modeling of the transfer impedance and distributed longitudinal induced voltage by means of a SPICE equivalent circuit

Frequency- and time-domain expressions for the transfer impedance of single conductor shielded cables are proposed. The time-domain convolution needed for the evaluation of the distributed longitudinal voltage induced on the internal conductor of the cable is directly evaluated by means of an equivalent SPICE circuit that can be incorporated in already existing shielded coaxial cables circuit models.     

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。