3.2 Attenuation


Attenuation is the loss or reduction in the amplitude (strength) of a signal as it passes through a medium. As a signal travels through a copper wire some of the signal will be absorbed by the medium (this is called heat dissipation). The effect of attenuation is one of the reasons why there are cable standards that specify the maximum length of cable run for particular types of cable, eg: 10BaseT maximum recommended run length is 100 metres.

Attenuation is measured in decibels (dB).

A general rule exists with regards to decibels and power:

Increase of 3 dB = Double the power

Decrease of 3 dB = Half the power

Increase of 10 dB = Ten times the power

Decrease of 10 dB = One-tenth the power

Note: You can think of attenuation as being the opposite of amplification.

Various factors can reduce the power of a signal as it passes through the copper wires used in UTP cables.

A cable tester can measure the reduction in power of a signal received from a device known as a signal injector – a small box, approximately the size of a deck of playing cards, attached to the far end of a cable.

Note: Fibre optic cable can also suffer from attenuation.