Transmission line theory

Abstract

The propagation of waves along a transmission line is discussed in this chapter. Expressions for the voltage and current at a point on a uniform line are derived. Both the voltage and the current along the line can be written as a superposition of two waves, one travelling in the forward direction and the other in the reverse or backward direction. Solutions of the equations are found for an infinite line and for a finite line with the receiving end either short-circuited or open-circuited or terminated by an impedance. Primary and secondary line constants, propagation constant, characteristic impedance, phase and group velocities, reflection coefficient, standing-wave ratio, attenuation and phase constants and a correctly terminated or matched line are introduced. The behaviour of lines at high frequencies is discussed where approximations can be made because the series resistance and shunt leakage conductance are small in comparison with the series inductive reactance and the shunt capacitive reactance respectively. The properties of quarter-wavelength and half-wavelength lines are considered. The special case of a distortionless line is briefly mentioned.