A lossless transmission line is characterized by the surge impedance and surge velocity . When the transmission line is energized by a voltage source , a forward surge is created (the magnitude: , where is the input circuit impedance). The surge reaches the other end in the transit time , being the line length. Then, a backward surge is generated by the reflection factor , being the output impedance. This wave generates another forward surge by the factor at the initial terminal. The voltage and current in the transmission line are described by the superposition of those successive forward and backward components and .

This Demonstration shows the variation of voltage in space and time in a 3D graphic for a triangular-shaped input voltage, for a variety of conditions. You can vary the terminal impedances and input voltage duration. You can also display the corresponding current .

In the graphic, all the variables are shown in normalized bases: space and time by the line length and transit time , respectively. Impedances are shown by the surge impedance

Snapshot 1: voltage almost doubles at a large output impedance

Snapshot 2: a negative current surge is generated for