A magnetron includes a positive polar cylinder, a plurality of vanes, a filament, upper and lower shields, and upper and lower pole pieces. The vanes are disposed in the positive polar cylinder to constitute a positive polar section. The filament is disposed on an axis of the positive polar cylinder to define an activating space. The upper and lower shields cover a top and bottom of the filament, respectively. The upper and lower pole pieces are disposed to induce magnetic flux into the activating space. The upper shield preferably has a diameter ranging from 6.95 mm to 7.10 mm. Additionally, the lower shield preferably has a diameter ranging from 6.95 mm to 7.10 mm.