Engineering Resources
Shielding
Shields perform many functions - some electrical, some mechanical. For example, in electrical and electronic circuits they may level out surge impedance along the length of the cable, screen a signal from external excitation, or confine a signal to an intended electrical path; in coaxial cables they may act as "return" wires; in telephone cables they may protect against shorts due to ground surges, provide a barrier against termites and rodents, or filter out low frequency interference from nearby power lines.
Shield Effectiveness - Stray magnetic and electrostatic fields can critically affect signal transmission in electrical and electronic circuits by inducing voltages that alter transmitted signals. Shield effectiveness is a measure of the success of a shield in reducing induced voltages and signal radiation. In selecting a particular shield construction, the design engineer must identify the potential fields in which the circuit will operate and specify a construction that reduces radiation and interference to specified limits. The optimum shielding system is usually the minimum cost construction that performs this function and meets applicable environmental and mechanical operating requirements. Quantitative effectiveness specifications that account for shield reflection and absorption characteristics in decibels are seldom used but can be specified when the exact nature of potential interference and shield materials specifications are known.
Shield Types - There are many shield varieties -round wire shields (braided, spiral, reverse spiral); tape shields (all metal, metal laminated with an insulating material); metal tubes; and conductive textiles and plastics. Each of these has a preferred use in some specific application.
