This guide provides an overview of Raytheon’s pioneering work in microwave technology, focusing on the evolution and design principles of early microwave ovens, particularly the 21X radar technology and its applications.
Historical Development
Raytheon’s development of the “Radarange” remains a significant program in microwave oven technology. Figure 101 illustrates the progression of models up to 1954. In 1945, they developed a lightweight device for aircraft use, capable of defrosting and heating eight-ounce meals from 10 to 170F (-12 to 77C) in one minute. This model featured a drawer-like cavity with a pull handle.
The 1946 model was a sandwich heater, where a removable box served as the heating cavity. When positioned, a sandwich could be heated in seconds. This anticipated the fixed-load, short-duty cycle principle seen in today’s microwave-equipped vending machines.
First Cooking Model
The 1947 model, known as the 1132 or “white range,” had an output of approximately 1.6 kilowatts, utilizing a water-cooled, permanent magnet magnetron. Compared to later versions, it was a bulky and heavy piece of electronic hardware. Microwaves entered through a U-shaped transmission line waveguide and an “iris” window of Pyrex glass on the rear wall. The range introduced a “stirrer” or mode agitator consisting of two chromium-plated back-to-back hollow hemispheres that rotated to improve heat distribution. The magnetron was mounted with the antenna positioned vertically in the waveguide under the oven. It operated on direct-current power from a power supply designed to protect the magnetron. The high power output and small oven size often resulted in uneven heating.
Range with Adjustable Power
The 1150 model, dating back to 1951, utilized two air-cooled QK-312 magnetrons in a large oven. The range, which measured 21 X 29 X 26 inches in height, width, and depth, had two magnetrons directly inserted into the roof. The oven opening measured 14 X 29 inches.
This model was known for its unusual power output, theoretically exceeding two kilowatts, but cooking was recommended within a setting of 200 or 300 milliamperes of magnetron current, making the useful power between 1.1 and 1.7 kilowatts. The operator, likely a chef, could adjust the magnetron operation using a variac on the control panel, allowing for flexible control over the cooking speed.
The energy supplied to the oven depended on the magnetron current, which was controlled by varying the voltage applied to the magnetron. This flexibility provided a beginner with a slow microwave heating process and allowed skilled users to operate closer to the “red” portion of the variac dial for faster cooking.
The 1150 range also featured a drainage hole at the low point in a sloping oven floor. This was designed to address the issue of melted fat cooling the bottom of the roast and interfering with cooking results. The stainless steel oven floor allowed fat to drain into a receptacle, and the surface was easily wiped clean due to the cool oven temperatures.
Commercial Models
In 1954, the 1161 model was designed for commercial use in restaurants, hospitals, hotels, and institutions. This model generated microwaves with two QK-390 air-cooled magnetrons, providing about 1.5 kilowatts for cooking and heating. The oven measured 20 X 22 X 12 inches in width, depth, and height, with magnetrons directly inserted into the top.
The uniformity of heating in this oven was significantly improved through optimized dimensions, an improved stirring device with fan-type blades, and the use of two magnetrons. A switch provided options for high, medium, and low power input settings. The skills needed for cooking with this range limited its use to carefully designed food service systems or simple, standard-quantity heating operations. The oven size contributed to performance, even with lighter loads. A smaller oven would not heat small, randomly placed loads as reliably. The timer was initially set to ten minutes but later changed to 21 minutes.
A companion oven with a single magnetron was also available in 1954. Called the 1170, it provided a smaller oven measuring 14 X 18 X 10 inches. However, its performance was overshadowed by the 1161 due to longer heating times for standard loads.
Domestic Ranges
Further development led to the introduction of domestic microwave ovens in 1955 and 1956. Raytheon became an original equipment manufacturer for domestic range companies, including Hotpoint, Westinghouse, Tappan, Kelvinator, and Whirlpool. They furnished power supplies and magnetrons, while each company designed its own domestic range around these components. Tappan continuously developed the 1180 models. One significant change in an 1180 model was the introduction of microwaves through a transparent oven bottom, which allowed for the addition of a broiler element for browning.
Magnetron Improvements
Magnetron improvements were crucial due to the high cost of the power supply and magnetron. In 1960, a new magnetron became available, meeting many requirements and increasing the power by 20 percent. Produced by Litton Industries and called the “Microtron,” the L-3510 included a cw magnetron, high voltage transformer, filament isolation transformer, and an electromagnet and filter assembly.
Two new magnetrons were developed by Raytheon: the QK707 and the QK904. The QK707, with an output of 0.8 kilowatt, replaced the QK390 in microwave ovens. The QK904, developed in 1962, operated on unregulated alternating current as well as direct current, and had a higher output, bringing single generator ovens up to 1.25 kilowatts.
Design
A microwave oven consists of eight major components:
- The power supply
- The generator or power tube
- The transmission section
- Coupling devices
- The distributor
- The cavity or oven
- Energy sealing or trapping structures
- Operating and safety controls
In addition, auxiliary heat may be incorporated, and there are subsidiary requirements for cooling and exhausting the oven.
Waves, Fields, and Food
The disappearance of energy from the waves in the cavity depends on the amount of energy and absorbing material present. For resonant cavities, Q, the quality factor, is defined as:
Q = Wo (energy stored in the circuit) / average power loss
Where Wo = l/VLC, L is the inductance, and C is the capacitance. In a high Q cavity, there is considerable reactive power and a high SWR in the oven, which is efficient for heating.
The rigorous definition of cavity dimensions in terms of wavelengths is:
2
AO= V (mlax)2 + (n/ay)2 + (plaz)2
Where AO is the free space wavelength, , y, are rectangular coordinates, and , n, are integers.
Broadening of resonant peaks occurs as energy is carried by the modes and distributed to the food, expanding resonance points in the frequency space. A mode stirrer helps distribute the incoming energy in various directions, irradiating the food from many directions.
If the cavity is filled with food, reflections are diminished, depending on the frequency and penetration. The stirrer should be located between the generator and the load.
Other Tuning Procedures
Hall (1952) outlined six objectives in the design of an effective microwave oven:
- Uniformity of distribution of microwaves
- Uniformly integrated microwave heating pattern
- Maximum use of input power
- Uniformity of heating obtained from periodic change in field distribution
- Continuous changes in the mode of the waves in the cavity
- Uniform heating of food masses
He recommended that the oven dimensions differ, for example, by a quarter wavelength.
Hall also described a type of mode shifter with a provision for polarizing the energy using two grids of rods on different levels.
Conclusion
Raytheon’s early work on microwave technology laid the foundation for modern microwave ovens. Their focus on magnetron design, cavity optimization, and energy distribution techniques shaped the industry and continue to influence microwave oven technology today. Understanding these foundational principles provides valuable insights into the workings and evolution of this ubiquitous appliance.
Bibliography
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