Page 250 - belden

TE CH N I C A L INF O RM A T I O N 250

Shielding
®
Characteristics of Belden Shield Types

Foil Shields Spiral/Serve Shields • Duobond II (Foil/Braid) — Combines
Duobond with an outer braid, applied for
Foil shields consist of aluminum foil A spiral/serve shield consists of wire (usually greater protection against interference
laminated to a polyester or polypropylene copper) wrapped in a spiral around the inner and to increase
film. The film gives the shield mechanical cable core. the overall
strength and bonus insulation. Foil shields tensile strength.
Superior flexibility
provide 100% cable coverage, necessary • Duobond III (Tri-Shield) — Uses the
and flex life, ease
for electrostatic shield protection. Because Duobond II design (foil/braid) plus a
of termination and
of their small size, foil shields are commonly surrounding layer of Duofoil. The extra
up to 97% coverage are the advantages of
used to shield individual pairs of multi-pair foil layer improves shield reliability
spiral shields. They are best suited for audio and provides
data cables to reduce crosstalk. They have
applications. As a rule, spiral shields are not an additional
less weight, bulk and cost less than spiral
effective above the audio frequency range interference barrier.
or braid shields and are generally more
due to the coil effect produced by • Duobond Plus® — Features foil/braid/foil
effective than braid shields in RF ranges.
the inductance of served wire strands. construction with a shorting fold in the
Foil shields are more
outermost foil. This fold prevents a slot
flexible than braid “French Braid” Shields opening from being created in the shield,
but have a shorter thereby preventing
Belden’s patented “French Braid” shield is a
flex life than spiral signal egress
double spiral (double serve shield) with the
or braid. or ingress.
two spirals tied together by one weave. This
Drain wires are used with foil shields to construction provides improved flex life over • Duobond IV (Quad Shield) — Offers an
make termination easier and to ground standard spiral shields, improved flexibility extra layer of braid shield (foil/braid/foil/
braid) for improved
electrostatic discharges. The shortcomings over conventional braid shields, and lower strength and
in using the foil shield include higher DC levels of microphonic or triboelectric noise durability.
resistance and lower mechanical strength than either spiral
than braid or spiral shields. or conventional Other combination shields are available
braid shields. such as the foil/braid/foil/braid used on the
Braid Shields Ethernet cables, braid/braid or foil/spiral.
A braid shield consists of groups of Combination Shields
tinned or bare copper or aluminum Combination shields consist of more
strands, one set woven in a clockwise than one layer of shielding. They provide
direction and interwoven with another maximum shield efficiency across the
set in a frequency spectrum. The combination foil/
counterclockwise braid shield combines the advantages of
direction. 100% foil coverage, plus the strength and
low DC resistance of the braid.
Braid shields provide superior structural
integrity, while maintaining good flexibility Belden has also developed a number of
and flex life. These shields are ideal for shielding configurations for use with
minimizing low frequency interference broadband coaxial cables.
and have lower DC resistance than foil.
• Duobond® — Duobond is essentially the
Braid shields are effective at audio, as
same construction as Duofoil® (a laminated
well as RF ranges. tape of foil/film/foil), but with an extra
layer of adhesive bonding the foil shield to
Generally, the higher the braid coverage,
the dielectric core. This foil shield provides
the more effective the shield. However, the
100% coverage and insures maximum
trade-off between cost and braid coverage shield protection.
must be considered. Typical braid coverages
are between 80% and 95%. Coverage of
100% is unattainable with a braid shield.
Other features to consider when choosing
a braid shield are the weave angle, strand
diameter, number of carriers (strand groups)
and the number of ends (strands).
Braid shields are generally bulkier and
heavier than other shields and, in some
cases, harder to terminate because the
braid may be combed out and pigtailed.

Fo r m o r e i n fo r m at i o n, c o n t a c t Be ld e n Te ch n i ca l S u ppo r t : 1- 8 0 0 - B E L D E N -1 • www. b e l d e n . c o m