DD IEC-PAS 62338:2002 pdf download Screened balanced cables — Coupling attenuation measurement, triaxial method
3.2.3Coupling attenuation ac
Balanced cables which are driven in the differential mode will radiate a part of the inputpower, due to irregularities in the cable symmetry. For unscreened balanced cables, thisradiation is depicted by the unbalance attenuation au. For screened balanced cables thedisturbing power from the pair is additional attenuated by the outer screen. The unbalancecauses a current in the screen which is then coupled by the transfer impedanceand capacitive coupling impedance into the outer circuit.
Consequently the effectiveness against electromagnetic disturbances of shielded balancedcables is the sum of the unbalance attenuation a of the pair and the screening attenuation a.of the screen. Since both quantities usually are given in a logarithmic ratio, they may simplybe added into the coupling attenuation ac:
a.=a +a
(13)
The logarithmic ratio of the feeding power P and the periodic maximum values of the power0r.max which may be radiated due to the peaks of voltage U in the outer circuit is termedcoupling attenuation ac:
a.=-10·1ogio
(14)
The relationship of the radiated power P, to the measured power P received on the inputimpedance R is:
max
(15)
There will be a variation of the voltage U, on the far end, caused by the electromagneticcoupling through the screen and superimposition of the partial waves caused by the surfacetransfer impedance Zr the capacitive coupling impedance Z: (travelling to the far and nearend) and the totally reflected waves from the near end.
4Measurement
4.1 Equipment
The measuring set-up is shown in Figure 2 and consists of:
A metallic non ferromagnetic tube with a length sufficient to produce a superimposition ofwaves in narrow frequency bands which enable the envelope curve to be drawn.
A networkanalyser. (A separate generator and receiver may also be used).
A balun for impedance matching of unbalanced generator output signal to thecharacteristic impedance of balanced cables (applicable only for symmetrical cables), seesubsection 4.2
Ferrite rings with an attenuation aFerrit > 10 dB in the measured frequency range.
Metallic boxes to shield the balun and the remaining cable length including the matchingresistors
DD IEC-PAS 62338:2002 pdf download
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