Transmission Line,Representation,Types and Applications

Transmission Line 
A transmission line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic waves or acoustic waves, as well as electric power transmission. Components of transmission lines include wires, coaxial cables, dielectric slabs, optical fibers, electric power lines, and waveguides.

An electric transmission line can be generally represented by aseries combination of resistance,inductance and shunt combination of conductance and capacitace as shown below.The transmission line model represents the transmission line as an infinite series of two-port elementary components, each representing an infinitesimally short segment of the transmission line: 

Transmission lines are basically circuits with distributed constants i.e R,L,C and G are distributed along the whole length of line.each small length at any section of the line will have its own values and concentration of all such parameters for complete length of line in to a single one is not possible.In the above figure

• The distributed resistance R of the conductors is represented by a series resistor (expressed in ohms per unit length).
• The distributed inductance L (due to the magnetic field around the wires, self-inductance, etc.) is represented by a series inductor (henries per unit length).
• The capacitance C between the two conductors is represented by a shunt capacitor C (farads per unit length).
• The conductance G of the dielectric material separating the two conductors is represented by a conductance G shunted between the signal wire and the return wire (siemens per unit length).

Among the parameters R,L,G and C , R and G are least important in the sense that they do not affect much the total equivalent impedance of the line and hence the transmission capacity.They are very much important when transmission efficiency and economy are to be evaluated as they completely determine the real transmission line losses.