Frequency Agile Sources for Microwave Ovens

It is shown that the overall energy conversion efficiency of a microwave oven can be raised from 40% to 65% with frequency-agile, broadband sources such as transistors and voltage tunable magnetrons. An even greater improvement in efficiency is shown to be possible for very light loads.Heating uniformity is improved considerably when the source is dynamically and sequentially tuned to discrete frequencies at constant power over a 4% bandwidth, The minimum bandwidth for good heating uniformity is 40 MHz at 2450 MHz. An automatic tuning circuit is described which samples reflected power and holds the source at the optimum frequency. A method by which temperature patterns can be programmed is suggested. Results on efficiency are given for a voltage tunable magnetron and compared with values obtained from a conventional magnetron source.