Transmission Techniques: Wire and Cable 39714.1 IntroductionIt was not long ago that wire was the only method to
inexpensively and reliably transmit sound or pictures
from one place to another. Today we not only have wire,
but we also have fiber optics, and wireless radio fre-
quency (RF) transmission from Blu-tooth to wireless
routers, cell phones, and microwave and satellite deliv-
ery. RF transmission is discussed briefly in Chapter
16.10, Wireless Microphones. This chapter will discuss
the various forms of wire and cable used in audio and
video.
Wire is a single conductive element. Wire can be
insulated or uninsulated. Cable, on the other hand, is
two or more conductive elements. While they theoreti-
cally could be uninsulated, the chance of them touching
each other and creating a short circuit requires that they
are usually both insulated. A cable can be multiple insu-
lated wires, called a multiconductor cable, or wires that
are twisted together, called a twisted pair cable, or
cables with one wire in the center, surrounded by insula-
tion and then a covering of metal used as another signal
path, called coaxial cable.14.2 ConductorsWire and cable are used to connect one circuit or compo-
nent to another. They can be internal, connecting one cir-
cuit to another inside a box, or externally connecting one
box to another.14.2.1 Resistance and Wire SizeWire is made of metal, or other conductive compounds.
All wire has resistance which dissipates power through
heat. While this is not apparent on cables with small sig-
nals, such as audio or video signals, it is very apparent
where high power or high current travels down a cable,
such as a power cord. Resistance is related to the size of
the wire. The smaller the wire, the greater the resistance.14.2.2 Calculating Wire Resistance
The resistance for a given length of wire is determined
by:
(14-1)where,
R is the resistance of the length of wire in ohms,
K is the resistance of the material in ohms per circular
mil foot,
L is the length of the wire in feet,
d is the diameter of the wire in mils.The resistance, in ohms per circular mil foot (:/cir
mil ft), of many of the materials used for conductors is
given in Table 14-1. The resistance shown is at 20°C
(68°F), commonly called room temperature.When determining the resistance of a twisted pair,
remember that the length of wire in a pair is twice the
length of a single wire. Resistance in other construc-R KL
d 2=-------Table 14-1. Resistance of Metals and Alloys
Material Symbol Resistance
:/cir mil ft)
Silver Ag 9.71
Copper Cu 10.37
Gold Au 14.55
Chromium Cr 15.87
Aluminum Al 16.06
Tungsten W 33.22
Molybdenum Mo 34.27
High-brass Cu-Zn 50.00
Phosphor-bronze Sn-P-Cu 57.38
Nickel, pure Ni 60.00
Iron Fe 60.14
Platinum Pt 63.80
Palladium Pd 65.90
Tin Sn 69.50
Tantalum Ta 79.90
Manganese-nickel Ni-Mn 85.00
Steel C-Fe 103.00
Lead Pb 134.00
Nickel-silver Cu-Zn-Ni 171.00
Alumel Ni-Al-Mn-Si 203.00
Arsenic As 214.00
Monel Ni-Cu-Fe-Mn 256.00
Manganin Cu-Mn-Ni 268.00
Constantan Cu-Ni 270.00
Titanium Ti 292.00
Chromel Ni-Cr 427.00
Steel, manganese Mn-C-Fe 427.00
Steel, stainless C-Cr-Ni-Fe 549.00
Chromax Cr-Ni-Fe 610.00
Nichrome V Ni-Cr 650.00
Tophet A Ni-Cr 659.00
Nichrome Ni-Fe-Cr 675.00
Kovar A Ni-Co-Mn-Fe 1732.00