A complex quantity that expresses the effect of a transmission line on a sinusoidal progressive wave. The propagation coefficient is defined for a uniform transmission line of infinite length supplied with a sinusoidal current of specified frequency at its sending end.
Under steady-state conditions, if the currents at two points along the line, separated by unit length, are I1 and I2, where I1 is nearer the sending end of the line, then
at the specified frequency; I1/I2 is the vector ratio of the currents. γ is a complex quantity and may be written
where j = √–1. The real part, α, is the attenuation constant and is measured in nepers per unit length of line. It measures the transmission losses in the line. The imaginary part, β, is the phase-change coefficient and is measured in radians per unit length of line. It is the phase difference between I1 and I2 introduced by the transmission line. Thus
If the displacement of the vibration is a maximum at a given point and equal to p1, then at the same instant the displacement, p2, at a distance x along the transmission line is given by
An infinite transmission line is not physically possible but conditions simulating those in an infinite line are realized when a transmission line of finite length is terminated by its characteristic impedance. Compare image transfer constant.