A transmission line is 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 $Y_0$ is the characteristic admittance of the line, then $I(x) / V(x) = Y_0$ for a wave moving rightward, or $I(x)/V(x) = -Y_0$ for a wave moving leftward.

Characteristic admittance is the mathematical inverse of the characteristic impedance. The general expression for the characteristic admittance of a transmission line is:

$Y_0=\sqrt{\frac{G+j\omega C}{R+j\omega L}}$

where

$R$ is the resistance per unit length,
$L$ is the inductance per unit length,
$G$ is the conductance of the dielectric per unit length,
$C$ is the capacitance per unit length,
$j$ is the imaginary unit, and
$\omega$ is the angular frequency.

The current and voltage phasors on the line are related by the characteristic admittance as:

$\frac{I^+}{V^+} = Y_0 = -\frac{I^-}{V^-}$

where the superscripts $+$ and $-$ represent forward- and backward-traveling waves, respectively.