TABLE OF CONTENTS The wave propagation of the quasi-TEM mode along the microstrip transmission line is mainly determined by the layered air-dielectric material system. As is known, lossless TEM transmission lines with homogeneous dielectric filling exhibit no dispersion effects. In this case, the phase velocity of the fundamental mode versus frequency is constant.
Regarding the microstrip transmission line with inhomogeneous dielectric filling in the transversal plane with respect to the direction of propagation, the electromagnetic field is partly located in air and in the dielectric material. At very low frequencies, near dc, the electromagnetic field can be assumed to be purely transversal. In this case, the phase velocity of the wave is related to the velocity of light ( c 0) divided by the square root of the quasi-static effective dielectric constant [ ? eff( f=0)], which is a function of the relative dielectric constant ( ? r) and thickness of the substrate material ( h), and the dimensions of the upper strip of the microstrip line ( w, t). Electrostatic field analysis can be applied to derive relations for the quasi-static effective dielectric constant [1, 2].
The power flow along the microstrip line occurs both in the air and dielectric regions. With increasing frequency, the portion of power in the dielectric is growing continuously. In the limited case of infinite frequency, the electromagnetic field can be supposed to be totally concentrated in the dielectric material. In this case, the phase velocity is calculated as
