John
you are right.
it was shown by Kinn and Marcum (1947) that various sizes and shapes of heating cavities could be obtained by varying the wave guide termination. Similarly, the effect of heating and thawing foods with microwaves was studied by Morse and Rivercomb (1947) at 1,050 megacycles. They first described the "runaway" problem wherein the food thawed at one point and attracted a disproportionate share of energy.
In the early efforts, the Raytheon Company, the General Electric Company, and the Westinghouse Company were actively pursuing development programs in the United States. Raytheon selected 2,450 megacycles as a frequency for concentration while General Electric preferred to work with a frequency of about 915 megacycles. The U. S. Navy hoped to find a more appropriate method for preparing food in submarines and sponsored evaluations of ovens through the Bureau of Medicine and Surgery (Sussman 1947). They remained optimistic but respectful of the skills and special equipment involved.