Microwave oven having uneven bottom wall oven cavity
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a configuration of a microwave oven and, more particularly, to a configuration of an oven cavity of a microwave oven including an uneven bottom wall for purposes of assuring uniform heating. 
     2. Description of the Prior Art 
     Well known Microwave Ovens comprise, with reference to FIG. 1, an oven cavity 1, a turntable 2, a magnetron 3, a wave guide 4, a strut 5, two supporting rollers 6, and a driving roller 7. The turntable 2 is provided for carrying a food. The wave guide 4 functions to transfer microwave energy from the magnetron 3 into the oven cavity 1 while microwave heating operations are being carried out in the conventional microwave oven. The turntable 2 is positioned on the strut 5, two supporting rollers 6, and the driving roller 7. The strut 5 and two supporting rollers 6 are provided for only supporting the turntable 2 at the positions thereof while the driving roller 7 serves to rotate and support the turntable 2. 
     A bottom wall 1a of the oven cavity has a configuration, as best shown in FIG. 2, where a circular recess 1a--a lower than the plane of the bottom wall 1a is provided. The left half of FIG. 2 depicts a cross-sectional view of the bottom wall 1a while the right half illustrates a plane view of the same. The strut 5, the supporting rollers 6, and the driving roller 7 are all arranged within the area of the lower circular recess 1a--a, whereby the upper surface of the turntable 2 is at the same level as the plane of the remaining portion of the bottom wall 1a except for the circular recess 1a--a. 
     This configuration is to minimize a space reguirement within the oven cavity 1 irrespective of the presence of the turntable 2. However, because of the uniformity of the surface of the circular recess 1a--a, the distance between the bottom wall 1a and the food positioned on the turntable 2 is unvaried while the turntable 2 is driven for cooking purposes. The food is thus overheated in one region and underheated in other regions. Therefore, uniform heating operations can not be expected for the food positioned on the turntable 2. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     With the foregoing in mind, it is the primary object of the present invention to provide an improved microwave oven for performing uniform cooking operations for food arranged within the microwave oven. 
     It is another object of the present invention to provide an improved microwave oven which is resistant to water or the like in an oven cavity. 
     It is still another object of the present invention to provide an improved microwave oven comprising a turntable and a plurality of rollers related thereto, the rollers being compact and resistant to microwave energy. 
     Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     To achieve the above objects, pursuant to an embodiment of the present invention, the bottom wall of an oven cavity of a microwave oven is provided with an uneven configuration so as to make the distance between a food carried on the table means and the bottom wall dependent on the position of the bottom wall. The electric field intensity of microwave energy reflected from the bottom wall is modified according to changing distances. 
     The table means can be a rotatable turntable and a fixed table. Therefore, the food carried on the table means undergoes a uniform application of the microwave energy. 
     If the turntable is provided within the microwave oven, in the specific form of the present invention, a plurality of rollers are required to support and drive the turntable. The rollers are arranged in such a manner as to be provided at a rising portion of the bottom wall. When water or the like is spilled into the oven cavity of the microwave oven, the rising portion prevents the rollers from getting wet by water or the like. Also, no water or the like intrudes into an electric circuit from the area where the rollers are arranged. 
     The rollers are positioned in such a manner that the majority portion of the rollers is behind the bottom wall, whereby they are prevented from being unnecessarily exposed to microwave energy. 
     Conventionally, the rollers are made of synthetic resin. The rollers are prevented from being deformed or dissolved by the microwave energy even if the microwave energy is generated in the absence of the food in the oven cavity of the microwave oven. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention and wherein: 
     FIG. 1 is a side view of a conventional microwave oven; 
     FIG. 2 is a configuration of a bottom wall of an oven cavity of the microwave oven shown in FIG. 1, the left half being a cross-sectional view of the bottom wall and the right being a plane view of the same; 
     FIG. 3 is a plane view of a bottom wall of the present invention; and 
     FIG. 4 is a cross-sectional view taken on line I--I in FIG. 3. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is applicable to a microwave oven whether a rotatable turntable is provided. However, for convenience, the microwave oven with the turntable is described herein with reference to the drawings. The turntable may be substituted by a fixed table. 
     FIG. 3 shows a bottom wall of an oven cavity of the present invention of the microwave oven with the turntable. FIG. 4 illustrates the same in a cross-sectional view taken on line I--I in FIG. 3. 
     Like elements corresponding to those illustrated in FIGS. 1 and 2 are indicated by like numerals. In FIG. 3 the turntable 2 is represented by a chain line circle. 
     The bottom wall 1a includes a trefoil recess 1a-b, a declining portion 1a-d, and a rising portion 1a-c. The supporting rollers 6 and the driving roller 7 are arranged at the rising portion 1a-c. 
     The driving roller 7 has a gear at the pheripery thereof which is engaged with a grooved cam 2a which is formed along a circle of the turntable 2. The turntable 2 supported on the supporting rollers 6 and the driving roller 7 are rotated in unison with the rotation of the driving roller 7 by engagement of the gear of the roller 7 with the grooved cam 2a of the turntable 2. The driving roller 7 is activated by a motor (not shown). 
     With reference to FIG. 4, a predetermined area B of the turntable 2 travels on the trefoil recess 1a-b and the rising portion 1a-c as the turntable 2 is driven. This means that a distance A between the area B and the bottom wall 1a are varied in accordance with the rotation of the turntable 2. The distance A is A MAX  of the largest value while the area B is positioned on the trefoil recess 1a-b. The distance A is A MIN  of the smallest value while the area B&#39; referred to the area B is passing on the rising portion 1a-c. Similarly, the distance A is a value between A MAX  and A MIN  while the area B is traveling on the declined portion 1a-d. 
     Electric field intensity of the microwave energy reflected from the bottom wall 1a is modified according to the variations in the distance A. The microwave energy depicted by M1 which is reflected from the trefoil recees 1a-b travels along the distance A MAX . On the other hand, the microwave energy referred to M2 which is reflected from the rising potion 1a-c passes through the distance A MIN . Therefore, the food carried on the turntable 2 undergoes uniform microwave application. 
     Even if the food is positioned at the center of the turntable 2, the food receives different intensities of an electric field of the microwave energy reflected from the different depths of the bottom wall 1a to have a uniform microwave application. That is, the food undergoes the uniform microwave application irrespective of its location on the turntable 2. 
     Although the lowest portion of the bottom wall 1a has a trefoil configuration, any to desired patterns can be used for the lowest portion inasmuch as the distance between the bottom wall 1a and the food carried on the turntable 2 is varied in the direction of the rotation on the turntable 2. 
     That is because the food passes above the uneven bottom wall 1a while the turntable 2 is rotated. 
     The variation in the distance between the bottom wall 1a and the food results in variations in the electric field intensity by the microwave energy propagation. More particularly, the variation of the electric field intensity by the microwave energy is derived from the difference between the distances A MAX  and A MIN . It is preferred that the difference be approximately λ/4 where λ is the wavelength of the microwave energy. For the microwave frequency of 2.450 Hz, λ is approximately equal to 4.8 inches (122.4 mm), and λ/4 then equals to 1.2 inches e.g. 30.6 mm. In a specific form of the present invention in practice, the difference is approximately 1 cm. When the difference is 15.0 mm, the strength of the microwave energy is approximately 70.7%. 
     Since the supporting rollers 6 and the driving roller 7 are all arranged at the rising portion 1a-c, the rising portion prevents the rollers 6 and 7 from being wet by in water of the like which is spilled within the microwave oven. No water or the like is drawn into the electric circuit portion from the area where the rollers 6 and 7 are disposed. 
     It is further desirable that the major majority portion of the rollers 6 and 7 are positioned behind wall 1a, especially, the rising portion 1a-c, whereby they are prevented from being unnecessarily exposed to the microwave energy. In case the rollers 6 and 7 are made of synthetic resin, for example, plastic, they are prevented from being damaged or dissolved by the microwave energy while the microwave energy is generated in the absence of the food in the oven cavity. 
     Compact rollers are available for the rollers 6 and 7 because the distance A MIN  is selected to be small. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be included within the scope of the following claims.