Handbook for Sound Engineers

Transmission Techniques: Wire and Cable 419

tric of various cables. It will be immediately apparent

that some cables have a density (crush resistance)

double that of other designs. Soft foam can lead to

conductor migration over time which will change

timing, impedance, return loss, and bit errors over

distance.

Coaxial cable is used quite extensively with various

types of test equipment. When such cable is replaced,

the capacitance per foot, which is determined by the

dielectric constant of the insulator, must be taken into

consideration, particularly for oscilloscope probes.

14.10.2.1 CCTV Cable

CCTV (closed circuit television) cable has a 75: char-

acteristic impedance. CCTV is a baseband signal com-

prised of low-frequency vertical and horizontal sync

pulse information and high-frequency video informa-

tion. Since the signal is broadband, only cable with a

center conductor of solid copper should be used.

If the cable is constantly in motion as in pan and tilt

operation, a stranded center conductor should be used as

a solid conductor will work-harden and break. There are

also robotic coaxes designed to flex millions of times

before failure for intense flexing applications.

Shielding for CCTV cable should have a copper or

tinned-copper braid of at least 80% coverage, for

low-frequency noise rejection. If an aluminum foil

shield is used in conjunction with a braid, either tinned

copper or aluminum only may be used for the shield. A

bare copper braid will result in a galvanic reaction.

14.10.2.1.1 CCTV Distances

For common CCTV 75: cables, their rule-of-thumb

transmission distances are shown in Table 14-21. These

distances can be extended by the use of in-line booster

amplifiers.

14.10.2.2 CATV Broadband Cable

For higher-frequency applications, such as carrying
radio frequencies or television channels, only the skin of
the conductor is working (see Section 14.2.8, Skin

Effect). Television frequencies in the United States, for

instance, start with Channel 2 (54 MHz) which is defi-

nitely in the area of skin effect. So these cables can use

center conductors that have a layer of copper over a steel

wire, since only the copper layer will be working.

If one uses a copper-clad steel conductor for applica-

tions below 50 MHz, the conductor has a dc resistance

from four to seven times that of a solid copper

conductor. If a copper-clad cable is used on a baseband

video signal, for instance, the sync pulses may be atten-

uated too much. If such a cable is used to carry audio,

almost the entire audio signal will be running down the

steel wire.

CATV/broadband cable should have a foil shield for

good high-frequency noise rejection. CATV cable

should also have a braid shield to give the connector

something to grab onto, 40% to 60% aluminum braid

being the most common. Multiple layer shields are also

available such as tri-shielded (foil-braid-foil) and quad

shields (foil-braid-foil-braid). Assumptions that quad

shields give the best shield effectiveness are erroneous,

there being single foil/braid and tri-shield configura-

tions that are measurably superior. Refer to Section

14.8.6 on shield effectiveness.

Modern CATV/broadband cable will use a foamed

polyethylene or foamed FEP dielectric, and preferably

one with gas injected foam. This will reduce the losses

in the cable. The jacket material is determined by the

environment that the cable will be working in (see

Sections 14.4, 14.5, 14.6).

14.10.3 Coaxial Cable Installation Considerations

14.10.3.1 Indoor Installation

Indoor environments are the most common for coaxial

cable installations. A few tips on installing coaxial cable

are as follows:

1. First and foremost, follow all NEC requirements
   when installing coaxial cables.


2. Distribute the pulling tension evenly over the cable
   and do not exceed the minimum bend radius of ten
   times the diameter. Exceeding the maximum
   pulling tension or the minimum bend radius of a
   cable can cause permanent damage both mechani-
   cally and electrically to the cable.


3. When pulling cable through conduit, clean and
   deburr the conduit completely and use proper lubri-
   cants in long runs.

Table 14-21. Transmission Distances for CCTV Cable

RG-59 1000 ft

RG-6 1500 ft

RG-11 3000 ft