Editing Characteristic impedance (section)

{{Short description|Property of an electrical circuit}}
{{About|impedance in electrical circuits|impedance of electromagnetic waves|Wave impedance|characteristic acoustic impedance|Acoustic impedance}}
[[File:TransmissionLineDefinitions.svg|thumb|upright=1.4|A [[transmission line]] drawn as two black wires. At a distance ''x'' into the line, there is current [[phasor]] ''I''(''x'') traveling through each wire, and there is a [[voltage difference]] phasor ''V''(''x'') between the wires (bottom voltage minus top voltage). If <math>Z_0</math> is the '''characteristic impedance''' of the line, then <math>V(x) / I(x) = Z_0</math> for a wave moving rightward, or <math>V(x) / I(x) = -Z_0</math> for a wave moving leftward.]]
[[File:Transmission line schematic.svg|thumb|upright=1.35|Schematic representation of a [[electrical circuit|circuit]] where a source is coupled to a [[electrical load|load]] with a [[transmission line]] having characteristic impedance <math>Z_0</math>]]

The '''characteristic impedance''' or '''surge impedance''' (usually written Z<sub>0</sub>) of a uniform [[transmission line]] is the ratio of the amplitudes of [[voltage]] and [[Electric current|current]] of a wave travelling in one direction along the line in the absence of [[Reflections of signals on conducting lines|reflections]] in the other direction.  Equivalently, 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 (unit)|ohm]].

The characteristic impedance of a lossless transmission line is purely [[Real number|real]], with no [[Electrical reactance|reactive]] component (see [[Characteristic impedance#Lossless line|below]]). Energy supplied by a source at one end of such a line is transmitted through the line without being [[Dissipation|dissipated]] in the line itself. A transmission line of finite length (lossless or lossy) that is terminated at one end with an [[Electrical impedance|impedance]] equal to the characteristic impedance appears to the source like an infinitely long transmission line and produces no reflections.