The invention relates to a very high power microwave generator that employs the virtual cathode phenomenon. The inventive microwave generator (60) comprises an emission device (62) that can produce electrons in an output microwave circuit (64, 66), the quantity of electrons emitted being sufficient to cause a regular variation in the density of electrons in the output microwave circuit. Said circuit transforms the kinetic energy of the electrons into microwave energy according to a resonance mode. The electron emission device, which emits said electrons in several regions of the microwave circuit, comprises field maximas (Exa1, Exa2, Exa3) of the resonance mode. The invention is suitable for use in relation to high power microwave emitters and jammers.