The function could be defined thusly,

This will use the units of Ampere*Second/Meter*Volt for epsilon.Code:Needs["PhysicalConstants`"]; Z[W_,d_]:=If[W/d<=1, (60 Pi/Sqrt[VacuumPermittivity]) Log[(8d/W)+(W/4d)], (120 Pi)/(Sqrt[VacuumPermittivity] (W/d+1.393+0.667 Log[W/d+1.444]))];