Definition

Microstrip is a transmission line structure where a signal trace runs on an outer PCB layer with a ground or power plane as the reference below. Because part of the electromagnetic field travels through air (Dk=1) and part through the laminate, microstrip calculations use an effective dielectric constant lower than the laminate's rated Dk. For FR-4 with Dk of 4.3, effective Dk is typically around 3.3-3.5 depending on geometry. Microstrip is easy to probe and test but is more susceptible to crosstalk and EMI than stripline. Use stripline for sensitive signals that need better shielding.