Ref: 11640061
Title: Cabling Basics:  Types of Shielding
Date: 2/27/90

Copyright 3Com Corporation, 1991.  All rights reserved.

When planning a data transmission system, it is vital to plan how to
preserve signal integrity from interference and noise, from inside or
outside the transmission line.

Sophisticated shields are particularly necessary for cables used to
interconnect data transmission systems, which are exposed to a wide range
of elecromagnetic and electrostatic interference.  When choosing shielded
cabling, consider the type of noise, its intensity and frequency, the
required length of cable, permissible loss tolerance, and cost.

COPPER BRAID SCREEN consists of copper wires woven into a braid to form
a metallic frame around the body of the cable, with a coverage from 80% to
95%.  A 100% coverage cannot be obtained with a braid.  Several factors
affect the braid's performance, including the number and diameter of wires,
number of groups, and stranding angle.  Braid shielding suits a wide range
of applications and offers low electric resistance and good mechanical
protection.  Its greater weight and volume and the difficulty in connection
sometimes inhibits its use.  Braid screens with coverage lower than 80% are
ineffective if not offered with other protections.

SPIRALLED SCREEN consists of copper wire binding, helically wound on
the cable.  It provides good shielding for audio applications and when
flexibility and endurance are the most important requirements.  It
should not be used for data transmission because the helical winding
produces an inductive effect at high frequencies.

COMMON COUPLED ALUMINUM/POLYESTER consists of an overlapped copper braid
used with aluminum foil.  It is commonly used for multicore and multipair
overall shielding, or when drain wire is outside and, in case of shielded
pairs (for low frequencies), when there is just one drain wire for all
the pairs.

SPECIAL COUPLED ALUMINUM/POLYESTER is commonly used for shielding of
pairs to reduce crosstalk and provide insulation among shields.

COUPLED ALUMINUM/POLYESTER/ALUMINUM is used with a copper braid for high
frequency coaxial or twinaxial cables.  Aluminum/polyester shields offer
100% coverage, occupy less space, and are lighter and less expensive than
copper shields, but they have a high electrical resistance, a minimal
mechanical resistance, and a short life when used in rough conditions and
extreme flexing (due to their short bend radius).

COMPOSED SHIELDS are combined forms of the types of shielding described
above, and are used to obtain the optimal shielding efficiency over a whole
spectrum of frequencies.  They provide good flexibility.  Usually aluminum
tape is combined with copper braid to provide the advantages of 100%
coverage with a lower electric resistance.  Other combinations are
aluminum/braid/aluminum or a double aluminum/braid, as in some cables for
Ethernet LANs.  The different shielding layers may or may not have
insulation between them using the aluminum/polyester coupling previously
described.

