Differential impedance is defined as the impedance between the two lines when the line pair is driven differentially. The symmetric stripline is reliable method for creating a transmission line with a PCB. Here is an online Stripline PCB Differential Impedance Calculator to calculate the differential impedance of stripline using PCB (Printed Circuit Board) based on the characteristic values of impedance, space between traces and height of trace.
An electrical transmission line with impedance of 30 Ohm, space between traces as 15 cm and height of trace as 12 cm.
d=2 x z x (1 - 0.347 x exp((-2.9 x (s / h))))
= 2 x 30 x (1 - 0.347 x exp((-2.9 x (15 / 12))))
= 59.4452 Ohm