Analytical investigation on the continuous spectrum in the spectral gap of the dominant leaky mode for open and covered microstrip lines

In this work, a rigorous analysis is carried out in order to characterize the behaviour of the continuous-spectrum current excited by a delta-gap source along open and covered microstrip lines at large distances from the source. The proposed analysis is valid and particularly efficient in the neighbourhood of the spectral gap of the dominant leaky mode. In particular, a closed-form representation for the continuous-spectrum current is obtained at the lowest asymptotic order; the accuracy of such representation close to the source can be further increased by properly taking into account the role of the singularities of the involved spectral integrands, which directly influence the behaviour of such continuous spectrum. Numerical results for different structures and frequencies are provided, which confirm the accuracy and the efficiency of the proposed approach.