Consider a wave travelling through a transmission line with a step in impedance from Z A to Z B . When the wave transitions through the impedance step, a portion of the wave Γ Γ will be reflected back to the source. Because the voltage on a transmission line is always the sum of the forward and reflected waves at that point, if the incident wave amplitude is 1, and the reflected wave is Γ Γ , then the amplitude of the forward wave must be sum of the two waves or ( 1 + Γ Γ ) .