Advantages and Drawbacks of Shielded Cables
There are several types of shields used. The more common shields used in cable assemblies are foil sheaths and metal braids. In specialty applications, shields can be made of conducting polymers. In this article, advantages and drawbacks about shield cables are outlined as follows.
Both foil and braid shields have their advantages. Foils provide 100% coverage while braids only provide 70%-95% coverage. A foil is thinner than a metal braid therefore it rejects less noise but it does provide protection over a wider range of frequencies. A braid shield provides better overall defense against EMI because it is denser than foil. For these reasons a combination of a foil and a braid shield are commonly used for the best protection. Each type of shield supports the other, overcoming any deficiencies of the other type while compensating with each own strengths. A combination of shields provides improved shielding effectiveness superior to either type alone.
One drawback of a foil shield is that it is thin, making it harder to terminate to a connector. The termination of a foil shield is made easier through the use of a drain wire, which terminates and grounds the foil shield. One drawback of a braid shield is that it does not provide 100% coverage as the foil shield does. Depending on the tightness of the mesh, metal braids typically provide between 70% and 95% coverage. Copper has a much higher conductivity than aluminum as well as having more bulk, therefore copper is more efficient for conducting noise and/or EMI.