Definition

Quasi-TEM (quasi-transverse electromagnetic) describes signal propagation in microstrip and similar structures where the electromagnetic field exists in two different media (air and dielectric). Unlike true TEM mode in stripline where fields are purely transverse, microstrip fields have small longitudinal components because the wave travels at different speeds in air versus dielectric. This causes the effective dielectric constant and phase velocity to vary somewhat with frequency, creating dispersion at high frequencies. For most PCB applications below 10 GHz, the quasi-TEM approximation is accurate enough for impedance calculations.