We propose a coherent jet model for radio loud AGNs. This model is based on the assumption that the jet is composed of two flows: an external MHD plasma, surrounding an internal highly relativistic plasma of electron-positrons. The leptons present in the spine, responsible of most of the emission, are supposed to be continuously heated via diffuse acceleration along the jet. The model is thus naturally inhomogeneous and includes most of the expected electromagnetic processes in radio loud sources: synchrotron, self-synchrotron Compton and external Compton emission (with Klein-Nishina cross section) on external sources (accretion disk, dusty torus and broad line region), photon-photon absorption and pair creation. A key point of the model is that the jet bulk Lorentz factor is not a free parameter but is the result of the Compton rocket effect and is thus imposed by external sources, themselves constrained by observation.The particles energy distribution is computed coherently along the jet applying energy conservation between heating and cooling and flux conservation, and the emerging spectrum is computed numerically. This model is applied to the quasar 3C273 and successfully reproduces its broad band emission over 15 orders of magnitude.