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The heating mechanism in a microwave oven results from the interaction of a high-frequency electromagnetic field and the food within it. In most ovens the field is generated by a magnetron which converts dc energy to high-frequency energy at approximately 2450 MHz. This energy is then propagated into the oven chamber which is, in reality, a multimode resonant cavity where the energy is reflected from the walls to create standing wave patterns. To properly design a microwave oven, one must consider not only the magnetron but also the dimensions and resonant properties of the oven cavity. The designer must strive for a uniform energy density within the cavity for uniform cooking of the food and must be sure the cavity presents the proper load to the magnetron. These objectives are discussed, and a procedure for obtaining them is outlined.