Two propagation medium: substrate and air=> the mode of propagation is not TEM However for the sake of simplification and by making a slight approximation, it is considered that the mode of propagation is Quasi-TEM and that consequently the energy propagates in a homogeneous medium of effective relative permittivity er eff. The value of e r eff depends on the distribution of energy between the two propagation media of the microstrip line, that is to say the air and the substrate of relative dielectric permittivity e r and of height H. There are many Substrate categories, one of the best known is alumina.
so we have
characterisitc impedance Z0 (W) :
W / H ratio: