# Characteristic impedance

In

radio communications,

**characteristic 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.

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

where μ is the magnetic permeability and ε is the electric permittivity of the

medium. Where the magnetic permeability and electric permittivity of the

vacuum are used, this equation defines a fundamental physical constant, the

**characteristic impedance of free space** and turns out to be equal to 120π (about 377) ohms.

See also:

Adapted from

Federal Standard 1037C.