Definition

A transmission line is a signal path where the propagation delay matters - typically when trace length exceeds 1/10 of the signal's rise time equivalent wavelength. At this point, the trace acts as a distributed impedance rather than a simple wire. Proper termination prevents reflections. PCB transmission lines include microstrip (trace over plane), stripline (trace between planes), and coplanar waveguide (trace with adjacent ground). Characteristic impedance depends on trace geometry and dielectric properties. Most digital signals above 100MHz and all RF signals require transmission line design.