In a magnetron having a body 1 defining an anode 2 divided into resonant cavities 4 by vanes 3 and having a coaxial cathode 5 , r.f. energy produced when a magnetic field is applied parallel to the axis of the anode is launched along a waveguide 8 by an antenna 6 in an evacuated region of the magnetron closed by a dielectric window 19 . The latter has sector shaped conducting areas on its surface symmetrically arranged with respect to the antenna, the inductance of which balance the capacitance of the dielectric window, thereby reducing reflections at the window.
1. A magnetron comprising an antenna for launching r.f. energy along a waveguide, a dielectric window through which the r.f. energy is in use launched closing an evacuated region within the magnetron, and a conductive area on the window to reduce reflection of the r.f. energy by the window.
2. A magnetron as claimed in claim 1 , in which the window is a ceramic window.
3. A magnetron as claimed in claim 1 , in which the window is planar.
4. A magnetron as claimed in claim 1 , in which the periphery of the window is surrounded by a conductive region of the magnetron, and the conductive area is in contact with two spaced apart portions of the conductive region.
5. A magnetron as claimed in claim 4 , in which the axis of the antenna extends parallel to a line joining the spaced apart portions.
6. A magnetron as claimed in claim 4 , in which one boundary of the conductive region is the line joining the spaced apart portions.
7. A magnetron as claimed in claim 6 , in which another boundary of the conductive region is the periphery of the window.
8. A magnetron as claimed in claim 7 , in which the conductive region is sector-shaped.
9. A magnetron as claimed in claim 6 , in which there is a pair of such conductive areas symmetrically arranged with respect to the antenna.
10. A magnetron as claimed in claim 9 , in which there are pairs of such conductive areas on both sides of the window.
11. A magnetron as claimed in claim 1 , in which the window is disc shaped.
12. A magnetron as claimed in claim 1 , in which the window is rectangular.