Leaky coaxial cable is also called radiation coaxial cable, which is a type of coaxial communications cable having incompletely closed outer conductor, and it is widely used in the tunnels of railways and highways, subways, elevators as the communications media. Leaky coaxial cable utilizes its radiation characteristics of electro-magnetic coupling to play its role for wireless communications in tunnels, subways, and so on, where wireless communications are unable to realize with wireless transmission. With the rapid development of China’s railway and urban construction leaky coaxial cable products will meet the rare development opportunities in the next few years.

Coupling loss is an important parameter of leaky coaxial cables which is distinguished from those of general communications cables. It is a characteristic parameter that characterizes the mutual coupling intensity between leaky coaxial cable and outside environment. The measurement methods are clearly defined in IEC 61196－4 and GB/T 17737.4 “Coaxial communication cables - Part 4: Sectional specification for radiating cable，and the definition is as follows:

* L*c=10lg(*P*t/*P*r) (1)

Where* L*c is coupling loss, dB; *P*t is the transmission power in the leaky cable, W; *P*r is the receiving power of the standard dipole antenna, W.

Since the transmission power at a location of the leaky cable equals to the cable input power minus the power attenuation from the input end to that location, the formula for calculating local coupling loss, *α**c*(*z*) is as follows:

* α*c(*z*)=*N*e-(*α*×*z*)-*N*r(*z*) (2)

Where *α*c(*z*) is local coupling loss, dB; *N*e is the power level, dBm; *N*r(*z*) is the receiving level at the antenna, dBm; *α **is** **the* attenuation coefficient, dB/km;* z is the distance from the input end of the cable to the antenna, km.*

The measured coupling loss can be characterized by two typical values, *α*c50 and *α*c95, *α*c50 refers to that the receiving probability of 50%, i.e. 50% of the measured local coupling loss, is less than that value; *α*c95 refers to that the receiving probability of 95%, i.e. 95% of the measured local coupling loss, is less than that value

In IEC 61196 and GB/T 17737.4 standards, the cable length should be at least ten times of the wavelength at the measuring frequency, in the same time, in order to ensure the measurement to be effective it is necessary to meet the stipulation of enough position resolution, in the case of 95% receiving probability ten times of measurement should be carried out for each semi-wavelength so as to calculate the coupling loss. Therefore it is impossible to achieve the measurement of coupling loss by manpower, and it is necessary to rely on computer and automatic measurement system.

The Information Transmission Lines Quality Inspect Test Center, M. I. I. developed, relying on self effort, the automatic measurement system specially for coupling loss measurement of leaky coaxial cable (see the following Fig.), the system consists of track handcart, serve control system, signal source, receiver, antenna, computer program controlled system, etc. and is fully able to meet the requirements for the coupling loss measurement of leaky coaxial cables.