418 Chapter 14
14.10 Coaxial Cable
Coaxial cable is a design in which one conductor is
accurately centered inside another with both conductors
carrying the desired signal currents (source to load and
return), as shown in Fig. 14-12. Coaxial cable is so
called because if you draw a line through the center of a
cross-sectional view, you will dissect all parts of the
cable. All parts are on the same axis, or coaxial.
14.10.1 History of Coaxial Cable
It has been argued that the first submarine telegraph
cable (1858) was coaxial, Fig. 14-13. While this did
have multiple layers, the outer layer was not part of the
signal-carrying portion. It was a protective layer.
Modern coaxial cable was invented on May 23, 1929
by Lloyd Espenscheid and Herman Affel of Bell Labo-
ratories. Often called coax, it is often used for the trans-
mission of high-frequency signals. At high frequencies,
above 100 kHz, coax has a dramatically better perfor-
mance than twisted pairs. However, coax lacks the
ability to reject noise that twisted pairs can do when
configured as balanced lines. Coaxial cable was first
installed in 1931 to carry multiple telephone signals
between cities.14.10.2 Coaxial Cable ConstructionThe insulation between the center conductor and the
shield of a coaxial cable affects the impedance and the
durability of the cable. The best insulation to use
between the center conductor and the shield would be a
vacuum. The second best insulation would be dry air, the
third, nitrogen. The latter two are familiar insulators in
hard-line transmission line commonly used to feed
high-power antenna in broadcasting.
A vacuum is not used, even though it has the lowest
dielectric constant of “1,” because there would be no
conduction of heat from the center conductor to the
outer conductor and such a transmission line would
soon fail. Air and nitrogen are commonly used under
pressure in such transmission lines. Air is occasionally
used in smaller, flexible cables.
Polyethylene (PE) was common as the core material
in coaxial cables during WW II. Shortly after the war,
polyethylene was declassified and most early cable
designs featured this plastic. Today most
high-frequency coaxial cables have a chemically
formed foam insulation or a nitrogen gas injected foam.
The ideal foam is high-density hard cell foam, which
approaches the density of solid plastic but has a high
percentage of nitrogen gas. Current state-of-the-art
polyethylene foam velocity is 86% (dielectric constant:
1.35) although most digital video cables are 82–84%
velocity of propagation. High-density foam of this
velocity resists conductor migration when the cable is
bent, keeping impedance variations to a minimum. This
high velocity improves the high-frequency response of
the cable.
A problem with soft foam is it easily deforms, which
changes the distance between the center conductor and
the shield, changing the cable impedance. This can be
caused by bending the cable too sharply, or running
over it, or pulling it too hard, or any other possibility. To
reduce this problem, a hard cell foam is used. Some
cable that is rated as having a very high velocity of
propagation might use very soft foam. A simple test can
be performed where the user squeezes the foam dielec-Figure 14-12. Construction of a coaxial cable.
Figure 14-13. First submarine cable.