ORIGINS OF THE MICROWAVE OVEN Almost everyone has a microwave oven today, or has at least had food processed in such an oven. These ovens are found in homes, restaurants, industrial and medical facilities, etc. Where did they come from? Who developed them, and when? Actually, a Vice President and engineering manager at Raytheon Company in Waltham, Massachusetts, by the name of Percy Spencer, holds the first patent on a microwave oven. He conceived it in November 1945 while watching the manufacture and processing of a “magnetron” microwave tube. These tubes were the heart of all early radar systems, generating the high-power microwave energy in short pulses that were transmitted and bounced off targets to track and identify ships and aircraft in WW II. They were being produced in large quantities by Raytheon at that time. Other people had noticed the heating effect of microwaves. Radar technicians had found that they felt warm when standing in front of their radar antennas. Some German soldiers even were reported to warm their meals by placing them in front of the antennas. However, Percy Spencer was the first to assign development engineers, create a product, and pursue it for many years to make an oven practical for home use. How did I get involved in microwave oven development? After graduating from OHS in 1940 at age 16, earning a degree in Electrical Engineering from Union College in 1944, and spending 2 years in the U.S. Navy as a Radar/Sonar Specialist Officer (including training at Harvard and MIT and assignment to the Naval Research Lab), I joined Raytheon in October 1946 as a Magnetron Development Engineer. I worked there 43 years, mostly on high-power microwave tube designs for new radar systems. I also worked on marine and airport radars, missile guidance and weather radars. In the late 1960s and early 1970s I also contributed to the magnetron work for the microwave oven. At one time I had as many as 40 ovens assigned to “life tests”, running under various stress conditions and on-off cycles, to evaluate our design improvements and discover any weaknesses. (Not all 40 were running at once. Some were undergoing construction changes, circuit modifications, or magnetron tube revisions, but it was a very busy activity. Certain combinations of on-off cycles or power loads in the oven could cause short life – unacceptable in any use. The object was to achieve the equivalent of ten years of typical household usage.) As shown in Figures 1 and 2, the earliest microwave ovens were large and heavy, about the size of a common home refrigerator. They were based on tubes and circuit technology used in WW II radars and were really only suitable for industrial use, or in restaurants, military establishments, hospitals, railroad dining cars, etc. However Raytheon did some business in those areas for several years, while trying to optimize the ovens for the much broader home market. Fortunately, Raytheon Company acquired Amana Refrigeration Co. in 1963 with the purpose of using their expertise in manufacturing home appliances to convert the ovens for more practical home use. George Foerstner, President of Amana (see Figure 3), immediately made recommendations for major redesigns, which resulted in smaller size (“about the size of an air conditioner”; see Figure 4), air cooling instead of inconvenient water cooling, 120-volt operation (readily available in homes, versus the earlier 220-volt models), and lower cost (under $500). A new smaller and less expensive magnetron also appeared at this time from a Raytheon subsidiary, New Japan Radio.

Miniaturization of the magnetron over the years is illustrated in Figure 5. Raytheon's magnetron development technology, such as seen in Figure 6, provided important engineering background for these design advancements. Moreover, by now small solid-state rectifier diodes could replace the bulky vacuum-tube rectifiers that converted the high-voltage AC to DC to operate the magnetrons in the oven. All of these features resulted in the ovens becoming much more attractive for home use. As shown in Figure 3, the Raytheon-Amana RadaRange oven, which first appeared on the market in 1967, was a tremendous success, and home microwave oven sales surged dramatically. Raytheon licensed other U.S. companies, such as Litton, Tappan, GE, etc., to use similar design approaches and manufacture ovens. The search for still newer, better and cheaper designs continued. Soon companies began transferring manufacture overseas where labor rates were cheaper, and today nearly all microwave ovens are built in China, South Korea, Indonesia, etc. While a typical model of a 1946 oven might have cost as much as several thousand dollars, the 1967 RadaRange cost just under $500. I bought a small model of an Asian oven a few years ago for the Raytheon Archive display at a total cost of $27.00! U.S manufacturing costs cannot approach this. Meanwhile, the use of high-power microwave energy for various commercial applications has turned into a worldwide industry. Companies such as Hormel Foods, Firestone Tires, Frito-Lay, and myriads of food, medical and materials processing users have found the technology beneficial and profitable. As shown in Figure 7, these ovens are huge, however, with some magnetrons delivering as much as 100 kilowatts of power (100 times more powerful than a home microwave oven) and employing conveyor belts to speed production. The scope of this industry is reflected by international conferences, that attract attendees from across the world, including the US, Switzerland, Israel, Brazil, China, Japan, India, Russia, England, Germany, and France. At the recent Annual Conference of the International Microwave Power Institute (IMPI), held in Cambridge, MA, on August 10, 2006, I was honored to give its keynote speech on the subject “Invention of the Microwave Oven at Raytheon Company.” Raytheon eventually sold Amana in order to withdraw from some commercial markets and concentrate more on the defense industry, and today is a major defense contractor with nearly 80,000 employees worldwide. Amana is still an important appliance manufacturer, with products in the microwave power field. I am glad to have participated in the development phase of microwave ovens. It was an exciting and educating experience and left me with a feeling of having contributed to a product that benefits many people.

Robert Edwards (edwards96@juno.com, 781-861-0511) OHS Class of 1940 Lexington, MA Coauthor of the book Raytheon Company: The First Sixty Years, Arcadia, 2005

Figure 1. A display of early Raytheon microwave ovens, showing models from 1945 (left) to 1954 (right). Most went to commercial/industrial applications. [Photo #374045]

Figure 2. A technician is testing the high-voltage power supply of a partly assembled 1946 model. Note the huge size of the components and cabinet. [Photo #374044]

Figure 3. These three managers were largely responsible for Raytheon’s success in the microwave-oven field. At left is George Foerstner, Amana President; in the center is Percy Spencer, credited with inventing the oven (and holder of the first patent); and at right is Tom Phillips, Raytheon President and CEO. The three Raytheon Seniors are shown with an early Raytheon/Amana RR-1 Model oven, our first big success in the home oven field. [Photo #374109]

Figure 4. This stack of 5 later Raytheon/Amana ovens is about the same height as one of the early 1946 models. [Photo #374115]

Figure 5. Magnetrons for powering microwave ovens were improved by designs with lower weight, size and cost, as shown here, from the 1946 models (at right) progressing to the left. [Photo #374111]

Figure 6. Bob Edwards, the author, is seen in this photo working with a high-power magnetron for radar use. Raytheon’s experience with such designs provided a basis for microwave-oven tube development. [Photo #374147]

Figure 7. Industrial applications that use much higher power, larger ovens and conveyor belts, as this one for meat tempering, have multiplied greatly and are now common worldwide. [Photo #374117]