Coaxial cables are used as transmission lines and are constructed to provide protection against outside signal interference.
Coaxial cables are used primarily for CATV and other wide band and video application. Coaxial cable is called “coaxial” because it includes one physical channel that carries the signal surrounded (after a layer of insulation) by another concentric physical channel, both running along the same axis. The outer channel serves as a ground. Many of these cables or pairs of coaxial tubes can be placed in a single outer sheathing and, with repeaters, can carry information for a great distance.
Coaxial cabling is the primary type of cabling used by the cable television industry and is also widely used for computer networks such as Ethernet. Although more expensive than standard telephone wire, it is much less susceptible to interference and can carry much more data.
Most common coaxial cable impedances in use in various applications are 50 ohms and 75 ohms.
50 ohms cable is used in radio transmitter antenna connections, many measurement devices and in data communications (Ethernet).
75 ohms coaxial cable is used to carry video signals, TV antenna signals and digital audio signals.
There are also other impedances in use in some special applications (for example 93 ohms).
It is possible to build cables at other impedances, but those mentioned earlier are the standard ones that are easy to get. Different impedances have different characteristics. For maximum power handling, somewhere between 30 and 44 Ohms is the optimum. Impedance somewhere around 77 Ohms gives the lowest loss in a dielectric filled line. 93 Ohms cable gives low capacitance per foot.