The Ultimate Characteristic impedance - American History Information Guide and Reference

In radio communications, characteristic impedance (acoustic impedance or sound impedance) $Z_0 \$ of a uniform transmission line is the impedance of a circuit that, when connected to the output terminals of a line of arbitrary length, causes the line to appear infinitely long. The SI unit of characteristic impedance is the ohm.

A uniform line terminated in its characteristic impedance will have no standing waves, no reflections from the end, and a constant ratio of voltage to current at a given frequency at every point on the line.

If the line is not uniform, the iterative impedance must be used.

The characteristic impedance of a linear, homogeneous, isotropic, dielectric propagation medium free of electric charge is given by the relation

$Z_0 = \sqrt{\mu \over \epsilon} = {1 \over {c \ \epsilon}}$

where

$Z_0 \$ is the characteristic impedance

$\mu \$ is the magnetic permeability of the medium

$\epsilon \$ is the electric permittivity of the medium

$c = {1 \over \sqrt{ \mu \epsilon}} \$ is the speed of propagation in the medium

When the medium is free space, the magnetic permeability $\mu_0 \$ and electric permittivity $\epsilon_0 \$ of free space are used and this defines the universal physical constant, the characteristic impedance of free space :