This paper reports on an enhanced implementation of the spectral-domain analysis (SDA) of boxed multistrip or multislot transmission lines embedded in a layered medium, including biaxial materials. Very high numerical efficiency is attained by a suitable basis and mixed SDA and spatial-domain technique to calculate the entries of the Galerkin matrix. Convergence properties of SDA are drastically improved, making it competitive with other analytical techniques [such as regular singular integral equation (RSIE)]. The method allows quick and accurate computation of current/field distributions.

This paper deals with the full-wave analysis of multiconductor microstrip lines used in electrooptic modulators (EOM), MMIC or high speed VLSI applications. An arbitrary number of coupled coplanar strips are embedded in a stratified medium involving iso/anisotropic dielectric and/or semiconductor layers. The numerical aspects of the computation of the propagation constants using the spectral domain analysis (SDA) are stressed. An efficient scheme is used to accurately compute attenuation and propagation constants and current distributions with reasonable CPU times. Convergence problems due to the existence of very thin layers adjacent to the metallized interface has been explicitly considered. An algorithm to compute the modal characteristic impedances is provided regardless of the number and nature of substrate layers. A reciprocity related definition of modal impedances is used in this paper. The use of this definition ensures the symmetry of the multiport scattering matrix associated to the structure.

The characteristic parameters of coplanar multistrip lines embedded in multilayered lossless/lossy substrates are commonly computed by using either quasi-TEM or fullwave models. Several methods are provided in the literature to deal with this type of structure. In this paper a comparative study of quasi-TEM and rigorous solutions is carried out in order to establish criteria for the validity of the quasi-TEM approach. Reliable quasi-TEM and full-wave numerical data have been properly generated by applying an enhanced spectral domain analysis. We conclude that the quasi-TEM model yields satisfactory results for many MIC and MMIC practical cases. However, significant errors arise when high conductivity substrates are involved in MMIC applications. A discussion about the computation of the dynamic modal characteristic impedance is also reported, showing how the insertion of the modal orthogonality can save computational effort in a lossy multiport system.

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