Coaxial cable components

A coaxial cable consists of the following layers (moving outward from the center) as shown in Figure 3.12.

Figure 3.12

Coaxial cable components

Carrier wire

A carrier wire or signal wire is in the center of the cable. This wire is usually made of copper and is normally solid but may be stranded. There are restrictions regarding the wire composition for certain network configurations. The diameter of the signal wire is one factor in determining the attenuation of the signal over distance. The number of strands in a multi-strand conductor also affects the attenuation.

Insulation

An insulation layer consists of a dielectric around the carrier wire. The dielectric is often air, with a helical spacer made of polyethylene or Teflon. For short distance cables foamed or solid dielectric is used.

Foil shield

A thin foil shield around the dielectric. This shield usually consists of aluminum bonded to both sides of a mylar tape. Not all coaxial cables have foil shielding. Some have two foil shield layers, interspersed with copper braid shield layers.

Braid shield

A braid, or mesh, conductor, made of copper or aluminum, surrounds the insulation and foil shield. This conductor is normally connected to ground to create an electrostatic shield for the carrier wire. Together with any foil shield, the earthed braid protects the carrier wire from electromagnetic interference (EMI) and radio frequency interference (RFI). You should carefully note that the braid and foil shields provide good protection against electrostatic interference when earthed correctly, but little protection against magnetic interference.

Sheath

The outer cover of the cable provides physical protection for the cable. This is usually made of polyethylene (PE) for outdoor applications and polyvinyl chloride (PVC) for internal cabling.

The shield layers surrounding the carrier wire also help prevent signal loss due to radiation from the carrier wire. The signal and shield wires are concentric, or coaxial and hence the name.

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