Characteristic - impedance of a cable:

The characteristic impedance also known as surge impedance (usually written Z0) 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. It can be defined as the input impedance of a transmission line when its length is infinite.

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. The general expression for the characteristic impedance of a transmission line is:

Where,

R = Resistance per unit length

L = Inductance per unit length

C = Capacitance per unit length

G = Conductance of dielectric per unit length

j = Imaginary unit

w = Angular frequency