We know that EMI (electromagnetic interference) is a disturbance in twisted pair cables. It affects the performance of an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction. The primary way to combat EMI in cables is through the use of shielding. The shielding can act on EMI in two ways: first, it can reflect the energy; second, it can prevent the noise and conduct it to ground. With the help of cable shielding, cables can be immune to the disturbance and keep a stable connection. And this article will present some knowledge about STP (shielded twisted pair) cable.
First of all, let’s have a quick overview of STP cable. As the name suggests, STP cable has a metal shield around each twisted pair which is composed of copper tape, a layer of conducting polymer or a braid (made of copper or aluminum mostly). And the shield can be applied to each pair of the cable or to all pairs together. Typically, STP cable is more expensive than UTP (unshielded twisted pair) cable. And due to the fragile and rigid shield of STP cable, it is necessary to pay much attention to the installation and maintenance of STP cable.
There are mainly two types of shields: braided shield and foiled shield. Braided shield is made up of woven mesh of bare or tinned copper wires. The braid makes it much easier for cable termination by crimping or soldering when attaching a connector. However, braided shield does not provide 100% coverage. It typically provides 70% to 95% coverage according to the tightness of weave. But as a matter of fact, 70% coverage is always sufficient if cables are fixed. Another shielding is foiled shield. This type of shielding uses a thin layer of aluminum. Because it is thin, it is harder to work with, especially when applying a connector. It provides 100% coverage for the conductors it surrounds.
As copper has higher conductivity than aluminum and the braid has more bulk for conducting noise, the braid is more effective as a shield. If the environment is extremely noisy, a cable may use multiple layers of shielding with both the braided and foil designs. Here is figure of braided shield, foiled shield and braided/foiled shield, from which you can have a better understanding of inner structure of these three types of shields.
When you plan to buy shielded 100m cat6 cable on store, you may have to make a choice between different construction of shielding. And here are are some commonly used shielding constructions:
U/FTP is the typical individual shielding using aluminum foil. This kind of construction has one shield for each twisted pair or quad above the conductor and insulation. Individual shield especially protects neighboring pairs from crosstalk.
F/UTP, S/UTP, and SF/UTP are overall shielding with different shield materials. Overall shield refers to the entire coverage around the whole cable. This type of shielding helps prevent EMI from entering or exiting the cable.
F/FTP, S/FTP, and SF/FTP are individual and overall shield. This type of construction has both layers of shielding. And its immunity to EMI disturbance is greatly improved.
Meanings of the abbreviated letters:
U = unshielded
F = foiled shielding
S = braided shielding
TP = twisted pair
Power cables are constructed with shielding to be electromagnetic compatible (EMC) to minimize noise generation, which affects many other systems like radio and data communication. Bulk Ethernet cables are shielded to prevent the effects on the data transmitted from EMI. To further prevent cross talk and coupling, some bulk Ethernet cables are also paired and individually shielded. In some applications, such as those needing servo cables, double or even triple, shielding is required around individual conductors, around twisted pairs, and around the entire cable. By the way, if you use shielded cat6 cable for your network, the cat6 patch panel you choose must be shielded, too.
Adopting twisted pair cable shielding is an effective method to prevent EMI from interfering signal transmission. For the purpose of providing a reliable connection between electronic devices, choosing a proper shielded twisted pair cable is essential. FS.COM provides various types of shielded cables, such as FTP cat5e cable 305m, SFTP 100m cat6 cable, SFTP 1000 feet cat6 cable.