Handbook for Sound Engineers

Transmission Techniques: Wire and Cable 427

across the frequency spectrum. The combination
foil-braid shield combines the advantages of 100% foil
coverage and the strength and low dc resistance of a
braid. Other combination shields available include vari-
ous foil-braid-foil, braid-braid, and foil-spiral designs.

14.14.6.1 Foil + Serve

Because of the inductive effects of serve/spiral shields,
which relegate them to low-frequency applications, this
combination is rarely seen.

14.14.6.2 Foil + Braid

This is the most common combination shield. With a
high-coverage braid (95%) this can be extremely effec-
tive over a wide range of frequencies, from 1 kHz to
many GHz. This style is commonly seen on many
cables, including precision video cable.

14.14.6.3 Foil + Braid + Foil

Foil-braid-foil is often called a tri-shield. It is most com-
monly seen in cable television (CATV) broadband coax-
ial applications. The dual layers of foil are especially
effective at high frequencies. However, the coverage of
the braid shield in between is the key to shield effective-
ness. If it is a reasonably high coverage (>80%) this
style of braid will have excellent shield effectiveness.

One other advantage of tri-shield coax cable is the
ability to use standard dimension F connectors since the
shield is essentially the same thickness as the common
foil + braid shield of less expensive cables.

14.14.6.4 Foil + Braid + Foil + Braid

Foil-braid-foil-braid is often called quad-shield or just
quad (not to be confused with starquad microphone
cable or old POTS quad hookup cable). Like trishield
above, this is most common in cable television (CATV)
broadband coaxial applications. Many believe this to be
the ultimate in shield effectives. However, this is often
untrue.

If the two braids in this construction are high
coverage braids (>80%) then, yes, this would be an
exceptional cable. But most quad-shield cable uses two
braids that are 40% and 60% coverage, respectively.
With that construction, the tri-shield with an 80%+
braid is measurably superior. Further, quad-shield

coaxial cables are considerably bigger in diameter and

therefore require special connectors.

Table 14-27 shows the shield effectiveness of

different shield constructions at various frequencies.

Note that all the braids measured are aluminum braids

except for the last cable mentioned. That last cable is a

digital precision video (such as Belden 1694A) and is

many times the cost of any of the other cables listed.

14.15 Shield Current Induced Noise

There is significant evidence that constructions that fea-

ture bonded foil with an internal drain wire may affect

the performance of the pairs, especially at high frequen-

cies. Since an ideal balanced line is one where the two

wires are electrically identical, having a drain wire in

proximity would certainly seem to affect the symmetry

of the pair. This would be especially critical where

strong RF fields are around audio cables.

Despite this evidence, there are very few cables

made with appropriate symmetry. This may be based on

lack of end-user demand, as manufacturers would be

glad to redesign their cables should the demand arise.

The drain wire could be easily substituted with a

symmetrical low-coverage braid, for instance.

14.16 Grounds of Shields

With any combination shield, the braid portion is the part

that is making the connection. Even if we are shielding

against high-frequency noise, in which case the foil is

doing the actual work, the noise gets to ground by way of

the braid which is much lower in resistance than the foil.

Where the foil uses a drain wire, it is that drain wire

that is the shield connection. Therefore, that drain wire

must be bare so it can make contact with the foil. If the

foil is floating, not glued or bonded to the core of the

cable, then another plastic layer is used to carry the foil.

Table 14-27. Shield Effectiveness of Different Shield

Constructions

Shield Type

(Aluminum Braid)

5

MHz

10

MHz

50

MHz

100

MHz

500

MHz

60% braid, bonded foil 20 15 11 20 50

60% braid, tri-shield 3 2 0.8 2 12

60%/40% quad shield 2 0.8 0.2 0.3 10

77% braid, tri-shield 1 0.6 0.1 0.2 2

95% copper braid, foil 1 0.5 0.08 0.09 1