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Communication – Transmission Media
Suggested answers
Activity 1
Speaker wire dissected
In its simplest form, speaker wire consists of two cold drawn copper wires embedded by extrusion into a straight, flat, parallel pair cable. This is simple to manufacture, easy for the user to connect (terminate) and to join and is quite suitable for many applications of short distance communication. By substituting a multistrand wire for the single wire for each side of the cable the manufacturer is able to produce a flexible cable with similar characteristics as those described. This is the more common form of untwisted pairs in use today.
Figure 1: Untwisted pair, stripped to expose the two multistrand wires.
Activity 2
Telephone cable
Figure 2: With the outer shield removed the telephone cable consists of two twisted pairs where each wire is individually insulated. Most twisted pair cables show a greater degree of twist that that used in telephone cabling in order to increase the protection against noise. To operate a telephone a single pair is required.
Activity 3
Coaxial cable
Figure 1: By removing the outer shield from coaxial cable we are able to clearly see the braided outer conductor.
Figure 2: Untwisting the braid exposes a foil covering an inner, heavily insulated central core.
Coaxial cable construction
The inner core, which is often stranded, is the conductor and will be made from silver plated copper, oxygen free high-conductivity (OFHC) copper or silver.
The dielectric, made from PVC, polyethylene, foamed (gas injected) polyethylene, polypropylene, nylon and teflon, separates and insulates the centre conductor from the ground shielding.
Aside from creating a negative ground, the ground shield, made from thin OHFC copper, helps prevent signal leakage (resulting in signal loss) and noise from entering into the conductor.
Friction reducers include materials such as thin paper or Mylar. They are found sandwiched within some coaxial cable to help reduce friction that occurs in bending, especially during installation. By reducing friction between these layers, it minimises the potential of damage to the internal conductor and/or ground braid layers.
For coaxial cables, there are three basic shielding techniques, including aluminium foil shielding, braid shielding and combination shielding. Jackets or sheaths, made from PVC, polyurethane, polyethylene or nylon, over coaxial cables act as a protective covering.
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