Transmission Mediums : Coaxial Cable (Coax)

Coaxial cable is a transmission line that is constructed from a center conductor that is completely surrounded by shield conductor. The center conductor and the shield conductor are separated by an insulation material. The shield can be inter-woven strands of wire or metal foil. Because the center conductor is complete surrounded by the shield, in an ideal coaxial cable, all the transmitted energy is contained within the cable.

Figure below shows a cross sectional view of a coaxial cable. This diagram shows a center conductor that is surrounded by an insulator (dielectric). The insulator is surrounded by the shield. This diagram shows that during transmission, electric fields extend perpendicular from the center conductor to the shield and magnetic fields form a circular pattern around the center conductor.


Cross Sectional View of Coax Cable

Coaxial cable is best known as the medium for cable television. It was primarily chosen because of its durability, wide frequency bandwidth capacity (often up to 1 GHz bandwidth), and less rigid length restrictions. Coax (as it is normally called) is often used in local area networks (LAN) to transport high-speed data signals with relatively high security (low signal leakage).

Twinax is a derivative of coax and is constructed as noted above with the exception that twinax uses two center conductors. Each center conductor is individually insulated, but, as with coax, each references the single shield for ground.

The first local area networks (LAN’s) were almost exclusively centered around coax as the network medium of choice. It became the standard for the early LAN’s. There are three common types of coax that are, or have been, in use extensively with many computer systems: thicknet, thinnet, and twinax.

Thicknet is often associated with the first Ethernet LAN’s and with high-speed bus cables used between mainframe computers and their peripherals. It is bulky and difficult to install but provides high speed and capacity where it is most critical. With respect to LAN’s, use of thicknet provides extra protection from electrical interference that may be encountered (e.g., such as on factory floors near assembly equipment).

For most LAN’s installed in the early 2000’s, UTP or STP is used. Coax is used in cable television systems and for interconnection trunks at telephone company switching centers. This provides for relatively high data transmission capacity (e.g., for DS3 45 Mbps transmission). Figure below lists the most frequently encountered types of copper and coax line and their approximate information transmission capabilities.


Copper and Coax Cable Information Capacity