An anode structure for a magnetron includes T-shape anode vanes 3 having a radially extensive component 3a and a circumferentially extensive portion 3b, the cylindrical faces 3c of the circumferential portions 3b facing a cathode in the complete magnetron. The use of T-shape vanes increases inductance and hence permits low frequency radiation to be generated without increasing the dimensions of the magnetron compared to those of a conventional magnetron. Also, capacitance is increased to give a further reduction in frequency by using more than two anode straps, and preferably four anode straps 5 to 8, at each end of the anode structure. Preferably, the anode structure is incorporated in a magnetron in which a high magnetic field of the order of 500 Gauss for a magnetron operating at 100 MHz is used. The anode shell 2 itself may form part of the magnetic return path.