Definition

Transverse electromagnetic (TEM) mode is the ideal propagation mode for transmission lines where both electric and magnetic fields are entirely perpendicular to the direction of signal propagation - no field components exist in the direction of travel. Stripline supports true TEM mode because the signal is completely surrounded by homogeneous dielectric. TEM propagation has no cutoff frequency and no dispersion (phase velocity is constant with frequency). Microstrip only approximates TEM (quasi-TEM) because part of the field travels through air with a different dielectric constant than the substrate.