A magnetron including a cathode, an anode axially aligned with the cathode and including a plurality of radial vanes defining resonant cavities, an output coupler connected to a first set of the vanes, a second set of vanes not connected to the output coupler, and extensions formed on only the vanes of the second set, the extensions extending in the axial direction towards the output coupler in a direction parallel to the axis of the anode, the extensions not being connected to the output coupler, whereby a capacitance between the axial extensions and the cathode at least partly compensates for the capacitance between the output coupler and the cathode.