TABLE OF CONTENTS In this appendix, the basic design parameters of coupled microstrip lines are outlined. We assume two uniform coupled microstrip lines of the same width in one plane, as illustrated in Figure 2.23. Asymmetric coupled lines in an inhomogeneous medium are analyzed in [1]. First, we will analyze the quasi-static frequency region, and subsequently dispersion.
It was discussed in Section 2.8 that two fundamental modes with zero cut-off frequency, the even and odd modes, can propagate along the coupled lines. As illustrated in Figure 2,25, the electromagnetic field distributions show strong differences. With growing slot width s, the coupling of the two lines becomes weaker, and the distribution of the electromagnetic field approaches that of two isolated single microstrip lines. When the distance s is continuously reduced, the even mode degenerates to a microstrip mode with a doubled microstrip width. A similar limit consideration is not possible for the odd mode. Its existence depends on the different potential of the coupled lines. Therefore, a limit value s ? 0 for the odd mode parameters, such as effective dielectric constant and characteristic impedance, cannot be found. This circumstance hampers the derivation of design formulas from numerical results based on full-wave analysis.
Figure J.1 shows the principal dependence of the quasi-static effective dielectric constant of the even mode, 
, and odd mode, 
, respectively, as a function of the normalized coupling slot s/ h [2], From the plot, it becomes apparent that
| (J.1) |
