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Measurement of Surface Transfer Impedance (Triaxial Method)

 At present there is still no domestic standard about the measurement of surface transfer impedance used for such cables as shielded symmetric cable and shielded multi-core cable, which are similar with  coaxial cable. 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. This context introduces the measuring method A in IEC 62153-4-3:2002. The measurement below 100 MHz using this method can be conducted needless in shielded room, and the measurement is easy to conduct.


1. The definition of transfer impedance, ZT


The transfer impedance, ZT, of a longitudinally isostatic shield with short electrical length is defined as the ratio of the longitudinal voltage, U2, induced in the matched outer circuit, which is formed by the measured shield and testing fixture in a unit length, and the current , I1, fed into the inner circuit. Vice versa.


2. The measurement method of transfer impedance


The transfer impedance of a cable is measured on the cable in a triaxial apparatus. The shield and inner conductor of the sample cable make up the inner circuit, and the cable shield and the testing fixture make up the outer circuit, furthermore, the inner circuit, i.e. the cable termination is matched, and the near end of the outer circuit, i.e. the near end of the shield is shorted, and the far end is connected to the receiver.


3. The preparation of cable sample and the test apparatus


The sample length should not exceed 50% of the coupling length.

The test apparatus is a triaxial measuring tube, the sample must suit to the test apparatus, and the cable’s shield forms the outer conductor of the inner circuit and the inner conductor of the outer circuit. The outer conductor of the outer circuit is a metallic (e.g. Brass, pure copper) good conducting tube which is not ferromagnetic, and the measuring tube is shorted with the shield at the feed-in end of the cable.

The measuring equipment may be a network analyzer or an independent signal generator and a frequency selection test receiver. To measure extremely low transfer impedance requires using low noise amplifier to amplify the received signal. When the characteristic impedance of the cable does not coincide with the signal generator, it is necessary to use an impedance transformer.

4. Impedance matching


If the reflection coefficient between the characteristic impedance of the cable and signal generator is more than 0.2, it is necessary to use an impedance transformer. It can be realized with a series resistor, Rs and a parallel resistor, Rp (the commercial adapter can be used to conduct some typical impedance combinations, for example, 50/75Ω).

The Information Transmission Lines Quality Inspect Test Center, M. I. I. (the Inspection and Test Center of No 23 Research Institute, CETC) has been engaged in the research and measurement of transfer impedance (triaxial method) since 2001, and provided this testing service to the industry, so that having met the increasing demands in measuring transfer impedance.

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