Calculation of characteristic Impedance of parallel lines.
The following formula is used to calculate the impedance of two parallel lines.

Z_{o} = 276/sqrt(K) x Log( 2S/d )

where:-
S is the centre to centre Spacing of the conductors in mm,
d is the diameter of the conductors in mm and K is the relative permittivity or dielectric constant of the insulation.

For pvc insulated twin cable use 2.5 as the permittivity K. Use 1 for Air spaced.

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S (spacing between wires in mm)

(mm)

inch

d (diameter of conductor in mm)

(mm)

inch

K (dielectric constant)

Z_{o} (Impedance)

Ohms

Velocity factor = 1/Sqrt(K)

Once you hit Calculate and have a result, you can change any value and Calculate again.