A Short History and Review of Shielded Cables
August 4th, 2009 by calmont
Effective shielding prevents interference from entering or emanating from a cable
In the late 1800s, as the use of electricity was first being used for communications, interface reared its ugly head. The imperative for some form of mitigation became apparent. The first attempts involved using custom cable for wire spacing. Bare wires entered buildings and were forced closer to each other, twisting the pair was the preferred technique for reducing interference says military wire specialist. Look at early pictures of New York City; the sky is dark with thousands of wires for telegraph and telephone wires. Then radio came along, and radio broadcasts could be heard coming from the telephone wires. The need for shields became a problem begging for a solution. Wrapping the cables similar to military cable with grounding wires (a simple served shied), worked or fixed installations; but when the cable was subjected to flexing, the wrapped wires separated and interference returned. Enter the braider. Braiders were invented in the mid – 1800’s to manufacture shoe laces and other textile items. Tinsel wire, a flattened wire wrapped around a cotton or flax yarn, was used for the epaulettes on military uniforms. Early telephone engineers discovered that this was great, very flexible conductor for telephone earpiece cords. The tinsel wire could also be braided over the conductors in the earpiece cords to reduce radio interference. When the custom extrusion cable was not subjected to high flexing conditions of a telephone cord, tinsel wire could be replaced by simple strands of wire.
These days, the purpose of a shield is to prevent interference from entering or emanating from a cable. The three types of interference radio are: radio frequency interference (RFI), electromagnetic interference (EMI) and electromagnetic pulse (EMP). RFI was the earliest type of interference that engineers had to deal with, Early radio signals easily found their way into devices. Coaxial cables and shielded twisted pairs using copper eliminated most of this type of interference. Today, EMI has become more problematic because of high power transmission lines; higher magnetic fields, such as MRI machines in hospitals; and other high power applications. Copper may not offer much resistance to higher magnetic fields; therefore,the use of magnetic materials such as high permeable irons may be required. EMP is produced by the detonation of nuclear devices. When the hydrogen bombs were tested in the 1950s at Bimini Atoll in the South Pacific, circuit breakers at power stations in Hawaii were tripped by the EMP
wave from the detonation. Critical military and civilian circuits must be protected from
an EMP condition. These shields require the use of both high and low permeable materials
to reduce the effect of an EMP.
