Definition

Surface microstrip is a transmission line with a conducting strip on the outer PCB layer, separated from a ground plane by dielectric substrate, with air above the trace. The electromagnetic field exists partially in air (Dk=1) and partially in the dielectric, creating an effective dielectric constant lower than the substrate's rated Dk. This quasi-TEM propagation means the effective Dk for a typical FR-4 board (Dk 4.3) is around 3.3-3.5. Surface microstrip is the most common controlled impedance structure due to easy access for probing, testing, and component placement.