Localization Of Leaky Coaxial Cable Distributed System Faults By Frequency Domain Reflectometry

Compared with traditional antenna,leaky coaxial cable is widely used to carry train controlling message in the condition of narrow wireless communication environment,such as tunnels,due to its quality to distribute signal evenly.However,leaky coaxial cable and its accessories are unavoidably broken owe to naturally aging and human factor.It is a great challenge to subway maintaining department to figure out how to diagnosis and eliminate faults in such a short time as the inspection period of subway fundamental facility is strictly limited.This paper takes polyethylene insulated leaky coaxial cable as studied object and focuses on the fundamental principles of double-end-detection,TDR(Time Domain Reflectometry)and FDR(Frequency Domain Reflectometry).The advantages and disadvantages on faults localization ability and Engineering implementation difficulty of the three methods above are analyzed.The main factors influencing the performance of FDR are signal bandwidth and number of points,whose relation with the performance of FDR is also analyzed.The ability of inspecting dual-faults of FDR is also researched through simulation experiment.The main work and results of this paper are as listed below:1.Establishing the equivalent model of leaky coaxial cable.Based on the physical structure of polyethylene insulated leaky coaxial cable,the equivalent model is established and its parameters,including resistance,inductance,capacitance and conductance,are computed.2.Formula on computing the distance between fault and signal is deduced.Fundamental principle of TDR and FDR is further studied to deduce the formula on computing the distance between fault and signal source.3.Simulations on the common faults of leaky coaxial cable.The leaky coaxial cable simulation circuit is established by PSpice.Simulation circuit is powered by a pulse signal and a sweep signal and fault types(including open,short,low-resistance,high-resistance)are changed,while the distance between the signal source and the fault is also changed within 1000 m,while the fault type is low-resistance or high-resistance,the fault resistance should also be adjusted.The simulation results show that TDR is able to detect all fault types except high-resistance and the localization error of TDR is within ±5m.The simulation results also show that TDR is unable to detect any faults,when the distance above is not bigger than half of the product of pulse width and signal speed.FDR whose localization error is within ±5m has the ability to detect all types of faults.4.Analysis on application of FDR.Aimed at the requirement that detecting device should meet the needs of the localization distance of at least 1500 m and the localization error of ±5m in the temperature of-25? to 55?.The real condition of multi-faults should also be seriously considered.Under the condition of 800 MHz band,open fault type and the distance within 2000 m,the bandwidth and point is changed to analyze the localization error and time,which shows that localization error is inversely proportional to bandwidth and detection points,while localization time grows inverse proportion to bandwidth but direct proportion to detection points.Localization error of ±5m and distance of 1500 m is well satisfied when detection points is 5000 and bandwidth is 40 MHz.FDR fully support the fault types of low-resistance & low-resistance,low-resistance & open,low-resistance & short,low-resistance & high-resistance,high-resistance & low-resistance,high-resistance &open,high-resistance & short,high-resistance & high-resistance.