GB1417156A - Microwave heating apparatus - Google Patents

Abstract

1417156 Microwave oven RAYTHEON CO 7 Nov 1972 [30 Nov 1971] 51364/72 Heading H5H A microwave oven having a non-contact making energy seal between the peripheral metal wall portions of the oven opening and the door is characterized by a solid dielectric in the gap between the door and the opening and an elongated peripherally extending choke groove, one of the peripheral wall portions being provided with a plurality of peripherally spaced circuit parameter variations which inhibit energy propagation in a peripheral direction, the distance between adjacent maxima and minima of the circuit parameter variations being less than the wavelength at the excitation frequency of the oven. In the drop-down door type of oven shown the door 52 comprises a perforated panel 62 secured to a frame 64 supporting an outer window 61, an inner window member 63 also being provided. A choke arrangement 76 around the door has tapered wall portions which mate with correspondingly tapered wall portions 15 of the access opening. An upstanding peripheral wall 78 of door panel 62 defines with wall portion 15 an elongated peripheral energy path 80. Frame 64 is stepped to provide a surface 84 forming part of the choke as well as a lateral surface 86 overlapping the casing walls 72. A second peripheral energy path 88 is defined between frame 64 and wall 78 for energy escaping through gap 82. Dielectric material 90, e.g. polystyrene or polypropylene, may fill the parallel paths 80, 88. Entrance and exit to path 88 is provided by the gap 92 between the end of wall 78 and the opposing wall 84. Elongated energy absorbing bodies 94 and 96, e.g. of carbon or ferrite loaded rubber or plastics, are provided externally of the choke seal to absorb any escaping radiation. To prevent peripheral energy propagation in the Y- direction in the choke seal a plurality of capacitive slots 106 are provided in at least one of the choke wall sections, e.g. 78 to leave a plurality of conductive fingers 78a. The slots 78 may be evenly spaced as shown, randomly spaced, if differing lengths, may be obliquely located and may be filled with dielectric material. The choke arrangement may be formed wholly in the door, wholly in the surround to the access opening or as shown, partly in the door and partly in the surround. The door may be of the vertically hinged or slidably withdrawable types. In alternative embodiments the conductive bars of the elongated propagation line type seal of Specification 1,257,217 are transversely slotted to impede peripheral propagation. The oven is provided with a mode stirrer and the conventional safety interlock switches. The circuit variations may be provided by transversely extending peripherally spaced conductive ribs of one of the wall portions.