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Impedance

Impedance is the AC resistance of the link. Unlike DC resistance, impedance is a frequency-dependant parameter. Read on to find out more.

Impedance is the frequency-dependent AC resistance of a link. It is a measure of the resistance an electromagnetic wave (the data signal) experiences when travelling along the cable. Depending on the tester, impedance is measured only on one point of the cable (e.g. 4 m after the jack in the patch panel). Impedance is also called pair impedance.

 

Impedance should be as close as possible to the 100 Ohms specified by the cabling standards and should be the same for all four pairs.

Impedance testing is only rarely called for. Where impedance suddenly changes, e.g. where the wires are connected to a plug or jack or where there is a kink in the cable, a part of the data signal is reflected. This is recorded much better by return loss testing, so impedance testing is of small importance.

 

Trouble-shooting tips if impedance tests fail

 

If return loss tests also fail, start with the trouble-shooting of return loss. Very often this solves the impedance problems as well.

Typical cause of the problemThis might help
(without engagement)
Cable has become wetLet the cable dray and test agein, replace the cable
Maximum tensile load of the cable was exceeded when the cable was pulled inReplace the cable
The cable runs near a powerful heat or cold sourceRe-route the cable
Category of the cable is too lowSelect appropriate test program,
replace the cable
Cable defective (material defect)Replace the cable

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