IEC PAS-62338:2002 pdf download Screened balanced cables – Coupling attenuation measurement, triaxial method
4.4Procedure
The the diferential and common mode of the pair under test is terminated at the far end byist nominal values of the characteristic impedance according to fiqure 2. The sample is thencentered in the tube and fed by a generator in the differential mode via a balun.
The quotient of the voltages at the output of the outer circuit and the input of the cable ismeasured, either directly by a network analyser or with a calibrated step attenuatorassuming that the receiver has the same input impedance as the output impedance of thesignal generator (R =Z)] which is inserted as an alternative to the triaxial apparatus.
Only the peak values of the maximum of the voltage ratio or the minimum of the attenuationmust be measured and recorded as a function of the frequency in order to determine theenvelope curve.
Attenuation introduced by the inclusion of adapters, instead of direct connection, must betaken into account when calibrating the triaxial apparatus.
The voltage ratio measured is not dependent on the diameter of the outer tube of the triaxiatest set-up or on the characteristic impedance Z2 of the outer system, provided that Zz islarger than the input impedance of the receiver.
4.6Measurement Precautions
The cable under test shall be positioned as concentric as possible in the outer tube to obtainhomogeneous wave propagation.
The balun and the remaining cable length including the matching resistors, each shall bepositioned in a well screened box to avoid disturbances from outside into the test set-up aswell as to avoid radiation from the test set-up.
It is important to set the ferrite rings as near as possible to the receiver side of the tube toabsorb interfering,backward travelling waves.
5 Expression of results
The attenuation of the balun shall be subtracted from the measuring results.
The coupling attenuation a C has to be calculated with the normalised value Z S = 150 Ω:
where
a c is the coupling attenuation related to the radiating impedance of 150 Ω in dB.
a m,min is the attenuation recorded as minimum envelope curve of the measured values in dB.
a z is the additional attenuation of an eventually inserted adapter, if not otherwise eliminated e.g. by the calibration, in dB.
U 1 is the input voltage of the primary circuit formed by the cable in V.
U 2 is the output voltage of the secondary circuit in V.
Z 1 is the (differential mode) characteristic impedance of the cable under test in Ohms.
6 Requirement
The results of minimum coupling attenuation shall comply with the value indicated in the relevant cable specification.
If a limiting value of the radiating power is specified for a cable system operated with a defined power level, the difference between the power level and the limit of radiating power shall not be greater than the coupling attenuation of the cable provided for the system.

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