Further development is reported of the modeling of the two-layer dielectric microstrip line structure by computing the scattering parameter S/sub 21/ derived from the model and comparing the computed value with the measured value over the frequency range from 90 MHz to 18 GHz. The sensitivity of the phase of S/sub 21/, and the magnitude of the characteristic impedance to various parameters of the equivalent circuit is also discussed. Examples are given of the measurement and modeling of the SiO/sub 2/ on silicon system to 18 GHz and the modeling of the GaAs on silicon system to 100 GHz. >