The
characteristic impedance or
surge impedance (usually written Z
0) of a uniform
transmission line is the ratio of the amplitudes of voltage and current of a single wave propagating along the line; that is, a wave travelling in one direction in the absence of
reflections in the other direction. Characteristic impedance is determined by the geometry and materials of the transmission line and, for a uniform line, is not dependent on its length. The
SI unit of characteristic
impedance is the
ohm.