1. Field of the Invention
The present invention relates to the field of microwave ovens for heating of a load arranged in a cavity and in particular to microwave ovens adapted to switch between mode fields. More specifically, the microwave oven comprises a cavity designed to support at least two predefined modes and a frequency-controllable microwave source connected to the cavity. The microwave oven further comprises a control unit for switching the operating frequency of the microwave source in order to obtain uniform heating.
2. Description of the Related Art
The art of microwave heating involves feeding of microwave radiation in a cavity. When heating a load in the form of food by means of a microwave oven, there are a number of aspects which have to be considered. Most of these aspects are well-known to those skilled in the art and include, for instance, the desire to obtain uniform heating of the food at the same time as a maximum amount of available microwave power is absorbed in the food to achieve a satisfactory degree of efficiency.
As known to a person skilled in the art, uneven heating using microwave radiation may be due to the presence of hot and cold spots in the mode field. Traditional solutions to eliminate, or reduce, the effect of hot and cold spots are the use of a turntable to rotate the load in the cavity of the microwave oven during heating or the use of a so-called “mode stirrer” to continuously alter the mode patterns within the cavity. Drawbacks of such techniques are that they are not fully satisfying in terms of heating uniformity and that they involve rotating or moving parts.
Alternatively, in U.S. Pat. No. 5,632,921, a microwave oven with a quadratic arrangement between a first and a second feed aperture and a phase-shift of ninety degrees between the microwaves input from a first waveguide feed connected to the first feed aperture and a second waveguide feed connected to the second feed aperture is disclosed to produce a rotating microwave pattern in the cavity, thereby producing a more even heating. However, a drawback is that such a microwave oven requires a rather advanced structure for feeding the microwaves to the cavity of the microwave oven and a non-standard design of the cavity.
Thus, there is a need for providing new methods and devices that would overcome these problems.