Excitation of complex and backward mode on shielded lossless printed lines

In this paper, the surface current density excited by a delta-gap voltage s ource on shielded printed lines is computed and its modal decomposition ana lyzed. This analysis specifically aims to determine the practical excitatio n of complex and/or backward modes in this kind of structures. It has been found that complex modes in reciprocal structures are always excited in pai rs whose propagation constants and amplitudes of excitation have the same i maginary part and opposite sign real part. The combination of the pair of c omplex modes gives rise to an evanescent mode whose amplitude is modulated by a sinusoidal function, Backward modes have also been found to be part of the modal spectrum excited by the source and, thus, in a frequency swept, they appear after the attenuation constant of a pair of complex modes goes to zero.