In this work, the problem of obtaining a closed form representation for the continuous-spectrum current excited by a finite source along open and covered microstrip lines is addressed inside the frequency range (the so-called spectral-gap region) in which the dominant leaky mode is not physical. In particular, the lowest asymptotic order of such continuous spectrum at large distances from the source is rigorously derived and the accuracy of such representation close to the source is further increased by properly taking into account the relevant spectral Green’s function singularities. Numerical results for different structures and frequencies inside the spectral gap region are provided which confirm the accuracy of the proposed formulation
Baccarelli, P., Galli, A. (2004). Analisi dello spettro continuo della corrente eccitata in microstrisce aperte e schermate nella regione di spectral gap del modo leaky dominante. In Atti della XV Riunione Nazionale di Elettromagnetismo (pp.21-24).