Graphene is in the focus of intense studies, as it represents an archetypal system for investigating low-dimensional quantum phenomena. Moreover, its topology results in a peculiar electronic structure with a massless dispersion near the Fermi level. Here we present a theoretical study on the Fabry-Perot interference patterns of graphene ribbons well contacted to external electrodes and capacitively coupled to a back gate. Our main focus comprises the effects of the orbital mixing between s and p bands and the role of defects.