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αc(z)=Ne-(α×z)-Nr(z) (2)
Where αc(z) is local coupling loss,dB; Ne is the power level,dBm; Nr(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,αc50 and αc95,αc50 refers to that the receiving probability of 50%,i.e.50% of the measured local coupling loss,is less than that value; αc95 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 coupling loss measurement system of leaky coaxial cable (see the following Fig.) 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.