721,967. Magnetrons. RAYTHEON MANUFACTURING CO. May 20, 1952. No. 12773/52. Class 39 (1). In a multi-cavity magnetron tuned by axially movable plungers, one for each cavity, strapping is provided at the ends of the plungers extending into the cavities. The tuning structure is dimensioned to resonate in the # mode at a frequency different from the operating frequency of the magnetron; the strapping prevents resonance of the tuning structure in modes other than the # mode. The anode segments 12 are provided with notches 35 to allow the strapped plungers 32 to enter the cavities. The strapping may consist of a pair of concentric rings 33, each connected to alternate plungers, the rings lying in notches in the plungers, or below the plungers, but connected to them by small extensions on the lower ends of the plungers. In another arrangement, all the plungers may be connected together by a single strap set in notches in the lower outer corners of the plungers. The tuning plungers 32 extend from a metal ring 31 secured to the lower end of a movable pole-piece 23. The pole-piece 23 contains a recess 24 for accommodating the upper end of the cathode cylinder 18. The anode structure is provided with conventional strapping 13 at its lower end.