International audienceA mixed finite element formulation for analysing the propagation characteristics of lossy dissipative waveguides is presented. Maxwell's equations are expressed in terms of the transverse electric and magnetic field components. This formulation is discretized using the mixed finite element and the classical Lagrange's one. A comparison of these discretizations shows that the use of this mixed finite element eliminates the spurious modes. This method is applied to computing the propagation characteristics of a MIS microstrip line