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# 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

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.