TABLE OF CONTENTS Planar transmission-lines are formed by metallized plane dielectric plates [14 17]. Planar transmission-lines play an important role as the basic line structures in hybrid and monolithic integrated circuits.
The microstrip line is depicted in Figure 7.17. In the microstrip line the electromagnetic field is not confined to the substrate, but spreads over the free-space. The microstrip line is an inhomogeneous waveguide since the transverse dielectric region is nonhomogeneous. Due to this circumstance the fundamental mode is not a TEM mode, but a quasi-TEM mode. The quasi-TEM mode approaches a TEM mode if the frequency goes to zero.
The analysis of microstrip lines only can be performed using numerical methods. Accurate methods of modeling (integral equation method, spectral domain method, partial wave synthesis, finite difference method, transmission-line matrix method) in some cases may require a high numerical effort. The easiest approximation is the quasistatic approximation. In the quasistatic approximation the transmission-line wave is approximated by a TEM wave and the transmission-line properties are calculated from the electrostatic capacitance. This approximation can be used if the transmission-line width as well as the thickness of the substrate are very small in comparison with the wavelength. In the following we give simple formulae for the quasistatic approximation.
Let C a be the capacitance per unit of length of the microstrip line for the case in which the dielectric is replaced by free-space and let C be the...
