Handbook for Sound Engineers

426 Chapter 14

14.14.3 French BraidTM

The French Braid shield by Belden is an ultraflexible
double spiral shield consisting of two spirals of bare or
tinned copper conductors tied together with one weave.
The shield provides the long flex life of spiral shields
and greater flexibility than braided shields. It also has
about 50% less microphonic and triboelectric noise.
Because the two layers are woven along one axis, they
cannot open up as dual spiral/serve constructions can. So
French Braid shields are effective up to high frequen-
cies, and are used up to the Gigahertz range of
frequencies.

14.14.4 Braid

Braid shields provide superior structural integrity while
maintaining good flexibility and flex life. These shields
are ideal for minimizing low-frequency interference and
have lower dc resistance than foil. Braid shields are
effective at low frequencies, as well as RF ranges. Gen-
erally, the higher the braid coverage, the more effective
the shield. The maximum coverage of a single braid
shield is approximately 95%. The coverage of a dual
braid shield can be as much as 98%. One hundred per-
cent coverage with a braid is not physically possible.

14.14.5 Foil

Foil shields can be made of bare metal, such as a bare
copper shield layer, but more common is an alumi-
num-polyester foil. Foil shields can offer 100% cover-
age. Some cables feature a loose polyester-foil layer.
Other designs can bond the foil to either the core of the
cable or to the inside of the jacket of the cable. Each of
these presents challenges and opportunities.
The foil layer can either face out, or it can be
reversed and face in. Since foil shields are too thin to be
used as a connection point, a bare wire runs on the foil
side of the shield. If the foil faces out, the drain wire
must also be on the outside of the foil. If the foil layer
faces in, then the drain wire must also be inside the foil,
adjacent to the pair.
Unbonded foil can be easily removed after cutting or
stripping. Many broadcasters prefer unbonded foil
layers in coaxial cable to help prevent thin slices of foil
that can short out BNC connectors. If the foil is bonded
to the core, the stripping process must be much more
accurate to prevent creating a thin slice of core-and-foil.

However, with F connectors, which are pushed onto
the end of the coax, unbonded foil can bunch up and

prevent correct seating of these connectors. This

explains why virtually all coaxes for broadband/CATV

applications have the foil bonded to the core—so F

connectors easily slip on.

In shielded paired cables, such as analog or digital

audio paired cables, the foil shield wraps around the

pair. Once the jacket has been stripped off, the next step

is to remove the foil shield. These cables are also avail-

able where the foil is bonded (glued) to the inside of the

jacket. When the jacket is removed, the foil is also

removed, dramatically speeding up the process.

A shorting fold technique is often used to maintain

metal-to-metal contact for improved high-frequency

performance. Without the shorting fold, a slot is created

through which signals can leak. A isolation fold also

helps prevent the shield of one pair contacting the shield

of an adjacent pair in a multipair construction. Such

contact significantly increases crosstalk between these

pairs.

An improvement on the traditional shorting fold

used by Belden employs the Z-Fold™, designed for use

in multipair applications to reduce crosstalk, Fig. 14-14.

The Z-Fold combines an isolation fold and a shorting

fold. The shorting fold provides metal-to-metal contact

while the isolation fold keeps shields from shorting to

one another in multipair, individually shielded cables.

Since the wavelength of high frequencies can even-

tually work through the holes in a braid, foil shields are

most effective at those high frequencies. Essentially, foil

shields represent a skin shield at high frequencies,

where skin effect predominates.

14.14.6 Combination Shields

Combination shields consist of more than one layer of

shielding. They provide maximum shield efficiency

Figure 14-14. Z-Fold foil type shielded wire improves high

frequency performance. Courtesy Belden.

Drain wire

Insulating film

Aluminum

Insulated

conductor

Isolation fold

Shorting fold