Novel analytical representations of the continuous-spectrum current in multilayer stripline structures

We consider a multilayer stripline structure excited by a delta-gap source and we study the continuous-spectrum current, which may be responsible of several spurious transmission effects such as interference with the quasi-TEM mode, undesired radiative effects, and crosstalk. In particular, a uniform asymptotic analysis of such continuous spectrum is presented, based on two alternative closed-form representations. Both representations are accurate also very close to the source and are valid inside a wide frequency range starting from DC. One formulation has a direct implementation, since it needs only knowledge of the spectral Green´s function of the background structure. The other formulation requires knowledge of the exact complex (leaky) poles of the stripline Green´s function and of their residues, but provides more physical insight since it gives quantitative information on the role of leaky waves in the continuous-spectrum representation in the entire considered frequency range. Numerical results which validate the accuracy and efficiency of the proposed approaches are provided at different frequencies for covered-microstrip structures with different strip widths.