Shielded Twisted Pair Cable (STP)

Twisted-pair cables that contain a metal shield to reduce the potential for electromagnetic interference (EMI) are called Shielded Twisted Pair (STP) cables. EMI is caused by signals from other sources such as electric motors, power lines, high-power radio and radar signals in the vicinity that may cause disruptions or interference, called noise. At first glance, it may appear that because STP cable is physically encased in a shield, all outside interference is automatically blocked; however, this is not true.

Just like a wire, the shield acts as an antenna, converting received noise into current flowing in the shield when it has been properly grounded. This current, in turn, induces equal and opposite currents in the twisted pairs. As long as the two currents are symmetrical, they cancel each other out and deliver no net noise to the receiver. However, any discontinuity in the shield or other asymmetry between the current in the shield and the current in the twisted pairs is interpreted as noise. STP cable is only effective at preventing radiation or blocking interference as long as the entire end-to-end link is shielded and properly grounded.

STP cable also has drawbacks; for example, its attenuation may increase at high frequencies, and its balance (or longitudinal conversion loss) may decrease if the effects of the shield are not compensated for, which leads to crosstalk and signal noise. The shielding effectiveness depends on the material of the shield, its thickness, the type of EMI noise field, its frequency, the distance from the noise source to the shield, any shield discontinuity, and the grounding structure used. Nor can it always be guaranteed that the shield itself will contain no imperfections.

Some STP cables use a thick braided shield others use a relatively thin overall outer foil shield. These cables, called screened twisted-pair (ScTP) cables or foil twisted-pair (FTP) cables, are thinner and less expensive than braided STP cable. However, they are not easier to install, the minimum bending radius and maximum pulling tension force must be rigidly observed when these cables are installed; otherwise, the shield may experience a tear.