The unavoidable way of a better conception of cavities for industrial applications is electromagnetic field numerical computation with the aim of placing the fields E and H into the material, and furthermore to obtain some characteristics like the input impedance of the cavity or the dissipated power in the sample. From Maxwell's equations and using a Finite Element Method with 3D edge elements we get a linear system from which, after resolution, we can obtain several kinds of results: research of eigen modes of an empty or partially dielectric loaded cavity, that is, for a bounded domain, all the frequencies that could be excited, using only dimensionnal (L,l,h) and physical (εʹ,εʺ) characteristics, or response of a loaded cavity to an external excitation, that is, setting a TE10 mode in a waveguide feeding a cavity, obtain the distribution of field inside the oven and the dielectric. An original approach allows us, by projection of the excitation on eigen modes, to understand the behavior of the cavity and the interaction wave-product. Our computation is in very good agreement with analytical results we can obtain in some specific configurations (eigenvalues of an empty cavity, or transmission of only the TE10 mode in a correctly shaped waveguide).