A general dispersion relation is derived for a relativistic rectilinear electron beam of arbitrary momentum distributions interacting with a dielectric in a guide magnetic field,on the basis of Maxwell equations and the relativistic Vlasov equation.The instability occurs when the beam velocity exceeds the wave phase velocity in the medium.The linear wave dispersion relation,growth rate,spatial growth rate are studied analytically for delta and Lorentznian distributions of beam momentums in detail.The results are of importance for a new kind of high-power microwave generation or amplification devices based on anomalous Doppler effect.