The CharacteristicImpedance of a Transmission line is determined by taking the limit of the Impedance given by the unit length capacitance and unit length inductance of a transmission line as unit length tends towards zero. The CharacteristicImpedance is irrespective of the SeriesResistance? of the cable and is only valid when the cable length is more than 4-8 times longer than the electrical length of the smallest electrical feature (usually the rising edge). This is because a reflection on a short cable will return before the edge is complete, avoiding the (maximum) effect of the TransmissionLine's CharacteristicImpedance.