Attenuation and Capacitance Effects in Structured Cabling
Capacitance is the ability of the insulating material between conductors of a cable to store a charge when potential difference exists between the conductors. The lower the capacitance of a cable, the better the signal quality it transmits.
As capacitance is accumulated over the distance transmitted, the lower the capacitance that we started with, the longer the distance we can transmit between source and receiver. Capacitance is always present on a cable just like the resistance. There is nothing that can be done to eliminate it. Thus cables are designed to lessen the bad effects that it has.
It reduces the high frequencies of the signal more than the low frequencies. In a cable, as well as the transmit-to receive capacitance, there is also
capacitance between the transmit and surrounding conductors, and the received and surrounding conductors. The sum of these capacitances acting on one pair is called the mutual capacitances.
The mutual capacitance of a cable should be constant throughout its length. A low mutual capacitance allows the transmitted signal to travel a longer distance without degradation of the signal. The cable is designed to have these capacitances balanced to reduce noise.
The resistance and inductance of copper cable pairs are affected very little by the presence of moisture. However, the presence of water in a cable core has substantial effect on the mutual capacitance of the pairs.
Even a small amount of water, distributed over a length of cable, produces an increase of a few percent in mutual capacitance. The pairs in a section of cable, which is completely, filled with water exhibits values of mutual capacitance in excess of twice the nominal value. Water in the cable replaces the air, which was part of the insulation between the conductors.
Water in a cable which did not cause any noticeable transmission impairment at voice frequency could cause an unsatisfactory error rate in systems that require the use of Category 5 cables.
Attenuation refers to the reduction in transmitted signal as it travels along a line. It increases with capacitance and frequency. The lower the value of attenuation, the better. Therefore water damage can increase the attenuation of a cable.