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MULTI MODE RESONATOR
APPLICATIONS
     

The common microwave oven has a cooking cavity known technically as a multimode applicator, “multimode” meaning a metal box of fixed dimensions that can support a number of resonant modes at a given frequency. The current magnetron has a typical oscillating frequency of around 2455MHz with capabilities of variation within a bandwidth of ±5MHz for a typical microwave oven. However, these mode patterns are purely theoretical and represent an empty cavity condition. When food is placed within the cavity, and dependent on its shape and container type, the mode pattern changes. Then, when the food is heated, its electrical (relative permittivity and dielectric constant) properties change, causing the mode patterns to change even further.

To minimise these effects most ovens have either a turntable or mode stirrer. The mode stirrer as the name implies, rotates axially about the microwave launch plane, providing a continuously variable impedance change to the incoming microwave energy. This impedance change causes the magnetron to oscillate at different frequencies within its narrow bandwidth, thus satisfying the conditional mode change.

The turntable on the other hand has two functions: Firstly, because of its physical distance from the microwave launch plane, the rotating food becomes the mode stirring mechanism but, in this case, has less impact on the magnetron’s frequency variation. Secondly, and to compensate for this, the food travels through constantly changing microwave energy fields, improving the heat distribution.

In both methods there is a direct coupling relationship between the microwave energy source and the food load to be heated.

All ovens have different sized cavities, meaning they require their own dedicated microwave coupling system. So the challenge was to develop a standard system that could be fitted to any size of oven, with the minimum of changes required. With this in mind, a new microwave coupling system was designed that would produce a constant impedance source, irrespective of food load, and allow microwave energy transfer to be made in the most effective and unrestricted manner.

  The system is known as the “Multi-Entry-Mode-Resonator” or for short “Mode Resonator”. It gives unparalleled vertical and lateral heating distribution, regardless of shelf position, and is unaffected by the use of metal baking utensils or multiple metal shelves. It follows the standard equation for resonance:
  • Freq. in the Frequency of resonance;
  • C is the velocity of light;
  • W, D and H represent the dimensions of the cavity;
  • L, M and N are the corresponding numbers of integers of hot spots/modes.
   
In the diagram shown to the right, many more frequencies are excited with the mode resonator system giving rise toa much more even ernergy uniformity.

Reducing the sensitivity of microwave oven heating and cooking performance to load volume, load placement and load properties has been a longstanding challenge for microwave and microwave-convection oven designers. To begin with, conventional design problems and solution methods were reviewed to provide greater insight into the challenges and optimum operation of a microwave oven. A new design strategy was introduced, using a special load isolating and energy modulating device called a transducer-exciter. It contains a microwave launch box and iris, a phase amplitude and frequency modulator, and a mode coupling matchplate designed to provide spatially distributed energy coupling to the oven.
The idea being to establish cooking in the perceived ‘far-field’. So, if it were technically feasible to make the mode coupling system present the food at being a long way away from its transmission point, then foods could be cooked more evenly, with one item not impacting or having an effect on another. This implies that different types of foods could be cooked at the same time and on different shelves simultaneously. After intensive research this goal was achieved.

The Laws of Physics teaches us that electric and magnetic energy propagate at the speed of light and as such transmit electro-magnetic energy at high velocity, so, when received in the far-field where the food is perceived to be, there is very limited loss in the transmitted cooking power. By studying the electro-magnetic spectrum, an innovative way was found to increase the range of the magnetron’s operating frequencies, thus creating more even-heating modes within the cavity. A way was also found to reduce the microwave’s electric (E) field and increase the magnetic (H) field intensities for cooking, thus preventing any electrical sparking between adjacent metal parts and cooking utensils.
Applied to consumer and catering foodservice convection ovens, or equipment such as combination-steamers and pizza conveyor ovens, this new design (Patent No. US 6,909,077 B2) gives astounding performance improvements. Large and small metal cooking containers and utensils can be used in the oven with minimal or no performance penalty on energy uniformity and cooking results. Cooking times are greatly reduced without sacrificing food quality; thus presenting a fast and efficient system that has a major impact on energy conservation.

Furthermore, it was found that with some redevelopment to accommodate a circular cavity, and another patent registration that is pending, it was possible to incorporate the mode coupling resonator system into tumble dryer appliance applications.

The idea of applying microwave energy to the drying process in laundry appliances is not new. In fact there has been much research into the concept over the past couple of decades and in recent years many articles have been written on the topic, but even oday, no such appliance of this kind is being marketed or sold, primarily because of the many technical difficulties that remain in place that have caused some damage to the drying clothes load. Clearly, industry could not tolerate any burning or arcing problems associated with this type of appliance technology.

Of primary concern is the arcing caused by the interaction of the generated microwave field with metallic objects. In order to be commercially viable it must be possible to dry clothes of all material types in a speedy and efficient manner. That is, even when pockets are inadvertently filled with items such as nails, safety pins, keys and coins,

butane filled lighters, or pen and pencils, and other metal objects that could easily be trapped or left within the folds of clothing.

After long-term, exhaustive testing and evaluation of tumble dryer development samples using the revised system, it is reported that the results have exceeded expectations and no such instances of damage have occurred.

The future for this major breakthrough in microwave heating technology is not only limited to food products. In addition to the clothing washer/dryer applications coupled with International concerns over energy conservation, the “multi-mode-coupling-technology” now allows microwaves to be effectively used for many other commercial and industrial applications.
 
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