Abstract The dispersion characteristics of the shielded planar transmission lines are analyzed by the transverse resonance method combining with variational technique. Two shorting planes are transversely inserted in the propagation direction so as to form a closed resonator. The dominant mode propagation constant of a transmission line is related to the distance be-tween shorting planes through the resonant condition. The characteristics equation for the distance is derived by using the variational formulation for a generalized resonator. The numerical results for the finlines are given. A comparision is made with available data in literatures.