We apply directly a perturbative expansion to thehoppingmodel of Feinberg. The first three harmonics of the space‐charge field are presented, considering the contributions from the higher‐order terms at large modulation depth. The dependence of the harmonics on spatial frequency, modulation depth and applied field strength are discussed. An analytical solution of the two‐wave coupling equations is given in this case. The mixing gain &ggr; is also discussed. ©1996 American Institute of Physics.