Characteristic Impedance
The characteristic impedance of a transmission line is an important parameter at higher frequencies of operation. For a parallel 2-wire transmission line,
where is half the distance between the wire centres, is the wire radius and, are respectively the permeability and permittivity of the surrounding medium. A commonly used approximation that is valid when the wire separation is much larger than the wire radius and in the absence of magnetic materials is
where is the relative permittivity of the surrounding medium.
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