Transmission lines are used to carry electromagnetic energy from one point to another . It means it transfer from one point to another. Generally it consists of two conductors. It is used to connect a source to a load. The source may be a transmitter or an electric generator and the load may be an antenna. Transmission lines are used for operational frequencies equal to or less than about 3GHz. Above 3GHz they will be replaced by waveguides.
Transmission lines are commonly used in power systems for power transmission. These lines can be used as circuit elements like capacitors, inductors at ultra high frequencies(300-3000MHz)
Types of Transmission Lines
(a) Two wire transmission line:- This transmission line consists of a pair of parallel conducting wires separated by a uniform distance .These are used in power systems or telephones lines.
(b) Coaxial transmission line:- This consists of an inner and a coaxial outer conducting sheath separated by a dielectric medium . They are used as TV cables, telephones cables and power cables.
(c) Parallel plate transmission line or planar line:- It has two parallel conducting plates separated by a dielectric slab of uniform thickness
(d) Optical fibres transmission line:- It consists of core and cladding . Information passes through the core in the form of totally internal reflected TEM waves.