Characteristic impedance

In radio communications, characteristic impedance (Z₀) 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.

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 $Z_{0}={\sqrt {\mu \over \epsilon }}$ where μ is the magnetic permeability and ε is the electric permittivity of the medium. This definition is used in Maxwell's equations. A fundamental physical constant, the characteristic impedance of free space, can be calculated from this equation, and turns out to be equal to 120π (about 377) ohms.

Adapted from Federal Standard 1037C.