# Characteristic impedance

In radio communications, characteristic impedance (acoustic impedance or sound impedance) [itex]Z_0 \ [itex] 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

[itex]Z_0 = \sqrt{\mu \over \epsilon} = {1 \over {c \ \epsilon}} [itex]

where

[itex]Z_0 \ [itex]is the characteristic impedance
[itex]\mu \ [itex]is the magnetic permeability of the medium
[itex]\epsilon \ [itex]is the electric permittivity of the medium
[itex]c = {1 \over \sqrt{ \mu \epsilon}} \ [itex] is the speed of propagation in the medium

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

[itex]Z_0=\sqrt{\mu_0 \over \epsilon_0} = {1 \over {c \ \epsilon_0}} = 376.730313461... \ \Omega[itex]
here [itex]c = {1 \over \sqrt{ \mu_0 \epsilon_0}} \ [itex] is the speed of propagation in vacuum.

## Source

• Art and Cultures
• Countries of the World (http://www.academickids.com/encyclopedia/index.php/Countries)
• Space and Astronomy