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Measurement of Shielding Effectiveness
Shielding effectiveness is an important parameter for evaluating Radio-Frequency coaxial cable's shielding performance, belonging to the category of EMC. Recently with the conducting of mandatory CE certification in European Union, the shielding effectiveness is paid more and more attentions.
 
The commonly used methods to measure shielding effectiveness of RF coaxial cable are triaxial method, absorbing clamp method, wire injection method, and GTEM cell method. Of which triaxial method can be used to measure the transfer impedance, coupling loss, screening attenuation, etc. of cables; absorbing clamp method can be used to measure the screening attenuation of cables; wire injection method can be used t measure the coupling loss of cables; and GTEM cell method can be used to measure the screening attenuation of cables.
 
The definitions for the parameters about shielding effectiveness are as follows:
1. Transfer impedanceis defined as the ratio of the voltage measured along the unit length of cable shield layer to the current passing through the inner conductor.
2. Coupling loss is defined as the logarithmic ratio of the input power of the cable to the coupling power, which got the maximum effect of either near or far end. (for coaxial cables, it equals to screening attenuation).
3. Screening attenuation is defined as the logarithmic ratio of the input power of the cable to the radiated maximum peak power, it can also be considered as a electrically long cable's shielding effectiveness.
 
The associated standards about the measurement of shielding effectiveness include:
 
IEC 62153-4-3 “Surface transfer impedance-Triaxial method”;IEC 62153-4-4 “Shielded screening attenuation, test method for measuring of the screening attenuation up to an above 3 GHz”;EN 50289-1-6 “Electrical test methods-Electromagnetic performance”;ANSI/SCTE 78 “Test Method for Transfer Impedance”;GB/T 17737 “Radio-frequency cables Part1:Generic specification-General, definitions, requirements and test methods”. IEC 62153-4-3:2002 “Electromagnetic compatibility-- Surface transfer impedance-Triaxial method
 
IEC 62153-4-3 “Surface transfer impedance-Triaxial method”;IEC 62153-4-4 “Shielded screening attenuation, test method for measuring of the screening attenuation up to an above 3 GHz”;EN 50289-1-6 “Electrical test methods-Electromagnetic performance”;ANSI/SCTE 78 “Test Method for Transfer Impedance”;GB/T 17737 “Radio-frequency cables Part1: Generic specification-General, definitions, requirements and test methods”. Of which IEC 62153-4-3:2002 “Electromagnetic compatibility (EMC)-surface transfer impedance (triaxial method)” and EN 50289-1-6:2002 “Transfer impedance (triaxial method)” are a testing method for transfer impedance which can either be used for measuring coaxial cables, or be used for measuring shielded symmetric cable and shielded multi-core cable, which are similar with coaxial cable. IEC 62153-4-3:2002 has two measuring methods: A (match-short circuit method) and B (short circuit-short circuit method), of which measuring method A is totally the same with that of EN 50289-1-6:2002.
 
The Information Transmission Lines Quality Inspect Test Center, M. I. I. has been engaged in the research on the methods and system for the measurement of cable shielding effectiveness since 1988, and developed the national standard about screening attenuation measurement (absorbing clamp method) as the main writer, since 2001 it has engaged in the research on the triaxial method for measuring cable’s shielding effectiveness. At present the laboratory has owned the testing capability based on the following international and domestic standards: IEC 62153-4-3 Surface transfer impedance-Triaxial method; IEC 62153-4-4 Shielded screening attenuation; EN 50289-1-6 Electrical test methods, Electromagnetic performance (absorbing clamp method/triaxial method); GB/T 17737 Radio-frequency cables Part1:Generic specification (absorbing clamp method/triaxial method); and ANSI/SCTE 78 Test Method for Transfer Impedance.
 
 
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