CHAPTER
16 Coaxial Cable and Antennas
U
nlike most other signal and power wiring, coaxial cable (or coax) is not merely a
conduit between devices; it represents an inherent part of most marine antennas,
and needs to be addressed as a critical component.Coaxial Cable Basics
We need to focus on coaxial cable selection and installation requirements since coax is the
connection between antennas and their transceivers; there are quite a few commonly used
variations of coax cable available; and signal losses through it can diminish the performance
of a transceiver, or even render it almost useless. After voltage and amperage problems, loss
through this vital link is perhaps the biggest factor in poor equipment performance.
Coaxial cable is different from the wire and cable used strictly to conduct electricity or
large amounts of electrical current. And although coax is sometimes used to carry electri-
cal power, its more common use, and the one we’ll focus on here, is to carry radio, video,
and data signals.
Coaxial cable consists of four layers (see illustrations page 166). In the center is a con-
ductor composed of either solid copper wire or stranded copper (commonly used in
marine applications). The core conductor is surrounded by a dielectric material or insula-
tor. Surrounding the insulator is the second conductor, a sheath or shield, which can be
either braided copper or a foil. The final layer is the outer jacket (insulation), which pro-
tects the cable from the environment and provides some flame retardation.
The signal is carried by the center conductor, while the shield protects the signal from
RFI and serves as a ground. The design is intended to keep the signal in place, so that it
will not “couple” with adjacent wiring routed in the same cable run, and noise from adja-
cent cables will not interfere with the signal.
Boat-system installers and troubleshooters have three specifications to be concerned
with: attenuation, impedance, and velocity of propagation.
Attenuation
Coaxial cable will always have some attenuation, which is the amount of signal loss per
given length of the cable. This value is measured in decibels (dB). The idea is to keep the
loss within parameters described later in this chapter.Copyright © 2007 by Edwin R. Sherman. Click here for terms of use.