Advanced Marine Electrics and Electronics Troubleshooting A Manual for Boatowners and Marine Technicians

Impedance

Each type of coaxial cable has an inherent

impedance, or opposition to current (it’s the

high-frequency equivalent of resistance). More

impedance means more attenuation. Interest-

ingly, this impedance is not related to the

length of the cable but to the wire gauge of the

conductors and the thickness and effectiveness

of the insulation between them.

Coaxial cable specifications typically assign

a “characteristic” impedance, which is calcu-

lated from the ratio of the inner and outer

166 marine electronics installation and troubleshooting

diameters and the dielectric constant. Assum-

ing the dielectric properties of the material

inside the cable do not vary too much over the

operating range of the cable, this impedance is

frequency independent.

The most common impedance values asso-

ciated with coaxial cable today are 50 ohm and

75 ohm. Usually, the 50-ohm variety is used to

carry radio signals and data (Ethernet), and the

75-ohm variety is used for audio, video, and

some telecommunications applications. The

attenuation associated with each of these values

is related to the frequency of the signals each

carries. So attenuation and frequency will help

determine the best cable choice and rated

impedance for a given application.

Equipment should be matched to the

cable’s impedance. When radio frequency

signals are transmitted through coaxial

cabling, the cable impedance is significant in

determining the load of the transmission

source and the overall efficiency of the trans-

mission. Another way of saying this is that

the source impedance must be equal to the

load impedance (often the antenna) to

achieve maximum power transfer and mini-

mal signal loss. We need to have matched

impedance values for the transmission

source, the cable, and the receiving device at

the other end of the cable. Therefore, make

sure the correct impedance cable is used for

the application at hand, as recommended by

the equipment manufacturer.

Velocity of Propagation

As we covered in Chapter 3, VOP is a percent-

age of the speed of light that describes the speed

of current through a cable. Different conduc-

tors have different VOP values due to their

inherent resistances to electrical current flow.

However, as coaxial cable ages, its imped-

ance changes, and consequently, so does its

VOP value (increased impedance results in lower

VOP). This indicates a general degradation of

copper wire (solid or stranded)

insulation

copper mesh

outer insulation

Coaxial cable construction.

Three sizes of coaxial cable with the inner insulation

stripped back to show the center conductor, and the outer

insulation stripped back to show the (somewhat crumpled)

braided shield.