The inhomogeneous dielectric configuration of the microstrip line supports only Quasi-TEM hybrid modes. The mode-coupling efficiency between the longitudinal section electric (LSE) and longitudinal section magnetic (LSM) modes increases with frequency. Microstrip circuits can be analyzed assuming TEM modes but only below a frequency fstat. Theoretical formulas predict that undesirably strong coupling between the Quasi-TEM and TM0modes, and the onset of the lowest-order surface wave TE0mode occur at frequencies fTEMand fc,TEo'respectively. In this paper, the full-wave FDTD method shows the existence of the longitudinal field components, and verifies to a certain extent the validity of the operating frequency range as predicted by the theoretical formulas.