Patent Publication Number: US-11382187-B2

Title: Electromagnetic field distribution adjustment device and microwave heating device

Description:
TECHNICAL FIELD 
     The present disclosure relates to an electromagnetic field distribution adjustment device and a microwave heating device including the same. 
     BACKGROUND ART 
     For microwave heating devices such as a microwave oven, it is desired to heat an object to be heated, which is accommodated in a heating chamber, uniformly without heating it unevenly. To achieve the above-mentioned aim, various configurations have been considered (e.g., see Patent Literatures 1 to 3). 
     Patent Literature 1 discloses a turn-table that rotates an object to be heated, which is placed on the turn-table. Patent Literature 2 discloses a rotating antenna configured to supply microwaves to a heating chamber while rotating the microwaves. 
     Patent Literature 3 discloses an electromagnetic field distribution adjustment device that has a plurality of metal pieces arranged in a matrix manner, and a plurality of switches each connecting two metal pieces adjacent to each other among the plurality of metal pieces. The electromagnetic field distribution adjustment device is configured to change impedance near the plurality of metal pieces. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Examined Utility Model (Registration) Application Publication No. S58-005842 
     PTL 2: Japanese Unexamined Patent Publication No. S53-092939 
     PTL 3: International Publication 2015/133081 
     SUMMARY OF THE INVENTION 
     According to the invention described in Patent Literature 1, however, uneven heating occurs concentrically due to generation of standing waves in a heating chamber. According to the invention described in Patent Literature 2, heating intensity varies depending on a distance from an antenna, whereby uneven heating is occurred. 
     According to the invention described in Patent Literature 3, an electric power loss occurs when a switch is operated, thereby deteriorating heating efficiency. Moreover, in the invention described in Patent Literature 3, a large number of switches need to be wired physically and controlled simultaneously. Therefore, from the viewpoint of cost, it is not easy to apply the invention described in Patent Literature 3 to consumer appliances, such a microwave oven. 
     To solve the above-mentioned problem, the present disclosure aims to achieve a low-cost electromagnetic field distribution adjustment device that heats an object to be heated more uniformly while preventing a drop in heating efficiency. 
     An electromagnetic field distribution adjustment device in one aspect of the present disclosure includes: a plurality of metal pieces that are arranged to fill a predetermined two-dimensional region; a plurality of switches that connect the plurality of metal pieces with one another; and a serially connected row of metal pieces that includes a portion configured by connecting one metal piece among the plurality of metal pieces to at most two metal pieces adjacent to the one metal piece through at least two of the plurality of switches, the at most two metal pieces being included in the metal pieces. 
     According to the present aspect, there can be achieved a low-cost electromagnetic field distribution adjustment device that heats an object to be heated more uniformly while preventing a drop in heating efficiency. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a microwave heating device including an electromagnetic field distribution adjustment device in accordance with a first exemplary embodiment of the present disclosure. 
         FIG. 2  is a longitudinal sectional view of the microwave heating device including the electromagnetic field distribution adjustment device in accordance with the first exemplary embodiment. 
         FIG. 3  is a top view of the electromagnetic field distribution adjustment device in accordance with the first exemplary embodiment. 
         FIG. 4  is a perspective view of the electromagnetic field distribution adjustment device in accordance with the first exemplary embodiment. 
         FIG. 5  is a view showing electric field distribution E 1  near the electromagnetic field distribution adjustment device when a switch is closed. 
         FIG. 6  is a view showing electric field distribution E 2  near the electromagnetic field distribution adjustment device when the switch is opened. 
         FIG. 7  is a view exemplarily showing a switch included in the electromagnetic field distribution adjustment device in accordance with the first exemplary embodiment. 
         FIG. 8  is a top view of an electromagnetic field distribution adjustment device in accordance with a modification of the first exemplary embodiment. 
         FIG. 9  is a perspective view of a microwave heating device including an electromagnetic field distribution adjustment device in accordance with a second exemplary embodiment of the present disclosure. 
         FIG. 10A  is a block configuration diagram showing a concrete structure and an operation mode of the electromagnetic field distribution adjustment device in accordance with the second exemplary embodiment. 
         FIG. 10B  is a block configuration diagram showing the concrete structure and an operation mode of the electromagnetic field distribution adjustment device in accordance with the second exemplary embodiment. 
         FIG. 10C  is a block configuration diagram showing the concrete structure and an operation mode of the electromagnetic field distribution adjustment device in accordance with the second exemplary embodiment. 
         FIG. 11A  is a top view of an electromagnetic field distribution adjustment device in accordance with a modification of the second exemplary embodiment. 
         FIG. 11B  is a block configuration diagram showing a concrete structure of the electromagnetic field distribution adjustment device in accordance with the modification of the second exemplary embodiment. 
         FIG. 12  is a perspective view of a microwave heating device in accordance with a third exemplary embodiment of the present disclosure. 
         FIG. 13  is a perspective view of a microwave heating device in accordance with a fourth exemplary embodiment of the present disclosure. 
         FIG. 14  is a perspective view of a microwave heating device in accordance with a fifth exemplary embodiment of the present disclosure. 
         FIG. 15  is a perspective view of a microwave heating device in accordance with a first modification of the fifth exemplary embodiment. 
         FIG. 16  is a perspective view of a microwave heating device in accordance with a second modification of the fifth exemplary embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An electromagnetic field distribution adjustment device in a first aspect of the present disclosure includes: a plurality of metal pieces that are arranged to fill a predetermined two-dimensional region; a plurality of switches that connect the plurality of metal pieces with one another; and a serially connected row of metal pieces that includes a portion configured by connecting one metal piece among the plurality of metal pieces to at most two metal pieces adjacent to the one metal piece through at least two of the plurality of switches, the at most two metal pieces being included in the metal pieces. 
     According to the electromagnetic field distribution adjustment device in a second aspect of the present disclosure, in the first aspect, the plurality of metal pieces each have one side whose length is less than half of wavelength of a microwave. 
     In addition to the first aspect, the electromagnetic field distribution adjustment device in a third aspect of the present disclosure further includes a grounding conductor that is provided along the predetermined two-dimensional region, and a plurality of short-circuiting conductors that connect the plurality of metal pieces to the grounding conductor. 
     According to the electromagnetic field distribution adjustment device in a fourth aspect of the present disclosure, in the first aspect, the electromagnetic field distribution adjustment device has substantially infinite impedance near the plurality of metal pieces when the plurality of switches are opened, and has substantially zero impedance near the plurality of metal pieces when the plurality of switches are closed. 
     In addition to the first aspect, the electromagnetic field distribution adjustment device in a fifth aspect of the present disclosure further has a potential determination part that is configured to determine a potential of the serially connected row of metal pieces. 
     According to the electromagnetic field distribution adjustment device in accordance with a sixth aspect of the present disclosure, in the first aspect, the serially connected row of metal pieces is arranged in a part of the predetermined two-dimensional region. 
     A microwave heating device in a seventh aspect of the present disclosure includes: a heating chamber that accommodates an object to be heated; a microwave generator that is configured to generate microwaves; a wave guide tube that is configured to guide the microwaves to the heating chamber; and an electromagnetic field distribution adjustment device that is provided in a predetermined two-dimensional region within the heating chamber. 
     The electromagnetic field distribution adjustment device has a plurality of metal pieces and a plurality of switches. The plurality of metal pieces are arranged to fill the predetermined two-dimensional region. The plurality of switches connect the plurality of metal pieces with one another. A serially connected row of metal pieces is configured such that one metal piece among the plurality of metal pieces is connected to at most two metal pieces adjacent to the one metal piece through at least two of the plurality of switches. 
     According to the microwave heating device in an eighth aspect of the present disclosure, in the seventh aspect, the electromagnetic field distribution adjustment device is provided in at least one of wall faces within the heating chamber. 
     According to the microwave heating device in a ninth aspect of the present disclosure, in the eighth aspect, the electromagnetic field distribution adjustment device is partially provided in the at least one of wall faces. 
     According to the microwave heating device in a tenth aspect of the present disclosure, in the seventh aspect, the electromagnetic field distribution adjustment device is detachably provided in any of wall faces within the heating chambers. 
     According to the microwave heating device in an eleventh aspect of the present disclosure, in the seventh aspect, the electromagnetic field distribution adjustment device is provided near an opening of the wave guide tube. 
     Hereinafter, exemplary embodiments of the present disclosure will be described with reference to the drawings. In the following drawings, the same reference numerals are assigned to the same components, and redundant description thereof is omitted. 
     First Exemplary Embodiment 
       FIG. 1  and  FIG. 2  are a perspective view and a longitudinal sectional view of microwave heating device  1 A in accordance with a first exemplary embodiment of the present disclosure, respectively. 
     In the present exemplary embodiment, microwave heating device  1 A is a microwave oven having heating chamber  2 . In  FIG. 1 , a front wall of heating chamber  2  is omitted such that the inside of heating chamber  2  can be seen. 
     As shown in  FIGS. 1 and 2 , in addition to heating chamber  2 , microwave heating device  1 A includes microwave generator  3 , wave guide tube  4 , and electromagnetic field distribution adjustment device  5 A. In the present disclosure, a back-and-forth direction, a horizontal direction, and a vertical direction of heating chamber  2  are defined as X-direction, Y-direction, and Z-direction, respectively. 
     In a front opening of heating chamber  2 , a door (not shown) is provided, and object  6  to be heated is accommodated in an inner space of heating chamber  2 . 
     Microwave generator  3  is constituted by a magnetron or the like, and generates a microwave. Wave guide tube  4  guides the microwave from microwave generator  3  to heating chamber  2 . In the present exemplary embodiment, an opening of wave guide tube  4  is provided in a side wall of heating chamber  2 . 
     Electromagnetic field distribution adjustment device  5 A is provided in a predetermined two-dimensional region within heating chamber  2 . Electromagnetic field distribution adjustment device  5 A changes impedance on its face opposite to the inner space of heating chamber  2 . Thus, electromagnetic field distribution adjustment device  5 A changes an electromagnetic field distribution, i.e., a standing wave distribution in the vicinity thereof. As a result, the heating distribution of object  6  to be heated can be changed, so that uniform heating of object  6  to be heated can be achieved. 
     If object  6  to be heated is placed near electromagnetic field distribution adjustment device  5 A, uniform heating effect will be obtained easily. In the present exemplary embodiment, the predetermined two-dimensional region corresponds to an entire bottom face of heating chamber  2 . In this case, object  6  to be heated is placed on electromagnetic field distribution adjustment device  5 A. 
       FIG. 3  and  FIG. 4  are a top view and a perspective view of electromagnetic field distribution adjustment device  5 A, respectively. As shown in  FIGS. 3 and 4 , electromagnetic field distribution adjustment device  5 A includes a plurality of metal pieces  11 , a plurality of switches  12 , a plurality of short-circuiting conductors  13 , and grounding conductor  14 . 
     Grounding conductor  14  is provided along the bottom face of heating chamber  2 . Grounding conductor  14 , which corresponds to a bottom face of electromagnetic field distribution adjustment device  5 A, is an electrically grounded surface having a reference potential. 
     Switch  12  is provided between two metal pieces  11  adjacent to each other in a column direction (X-direction shown in  FIGS. 3 and 4 ). 
     Electromagnetic field distribution adjustment device  5 A has eight serially connected rows  15  of metal pieces arranged in a row direction (Y-direction shown in  FIGS. 3 and 4 ). The plurality of metal pieces  11  are connected in series with one another through switches  12  provided therebetween to constitute serially connected row  15  of metal pieces. 
     In other words, serially connected row  15  of metal pieces includes a portion configured such that one metal piece  11  among the plurality of metal pieces  11  is connected to at most two metal pieces  11  adjacent to the one metal piece  11  through at least two of the plurality of switches  12 . 
     Metal piece  11  is a square metal plate whose one side has a length less than half of wavelength of the microwave. The plurality of metal pieces  11  are arranged on a plane, which is in parallel to grounding conductor  14 , in a matrix manner such that the plurality of metal pieces  11  are opposite to grounding conductor  14 . 
     Short-circuiting conductor  13  connects metal piece  11  to grounding conductor  14 . A combination of metal piece  11  and short-circuiting conductor  13  is referred to as a unit cell with a mushroom structure. 
     Dimensions such as length of one side of metal piece  11  and height of short-circuiting conductor  13  are designed such that, when switch  12  is opened, electromagnetic field distribution adjustment device  5 A functions as a magnetic wall, with respect to the microwave. 
       FIG. 5  shows electric field distribution E 1  near electromagnetic field distribution adjustment device  5 A when switch  12  is closed.  FIG. 6  shows electric field distribution E 2  near electromagnetic field distribution adjustment device  5 A when switch  12  is opened. 
     In serially connected row  15  of metal pieces, a plane including switch  12  and metal piece  11  functions as a conductor plate when switch  12  is closed. In this case, electromagnetic field distribution adjustment device  5 A constitutes a short-circuit plane that has substantially zero impedance near the plurality of metal pieces  11 . 
     As shown in  FIG. 5 , if electromagnetic waves are reflected on the short-circuit plane, a standing wave whose node lies on the short-circuit plane, i.e., surfaces of the plurality of metal pieces  11  will be formed. 
     Electromagnetic field distribution adjustment device  5 A functions as an electric wall that has substantially zero impedance near the plurality of metal pieces  11 . 
     When switch  12  is opened, electromagnetic field distribution adjustment device  5 A constitutes a meta-material in which a large number of unit cells are arranged two-dimensionally and periodically. In this case, electromagnetic field distribution adjustment device  5 A functions as a magnetic wall that has substantially infinite impedance near the plurality of metal pieces  11 . Herein, the expression of “arranged two-dimensionally and periodically” means that a plurality of objects with the same structure are arranged at constant intervals in a longitudinal direction and a transverse direction. 
     Even if switch  12  is opened, two metal pieces  11  adjacent to each other included in serially connected row  15  of metal pieces are conducted through two short-circuiting conductors  13  and grounding conductor  14 . Therefore, direct current can flow between these metal pieces. The microwave, however, can hardly propagate between these metal pieces because metal piece  11  and short-circuiting conductor  13  have the above-mentioned dimensions. 
     Accordingly, electromagnetic field distribution adjustment device  5 A constitutes an open plane that has substantially infinite impedance near the plurality of metal pieces  11 . As shown in  FIG. 6 , if electromagnetic waves are reflected on the open plane, a standing wave whose antinode lies on the open plane, i.e., surfaces of the plurality of metal pieces  11  will be formed. 
     In this way, by changing the impedance, electromagnetic field distribution adjustment device  5 A can interchange positions of a node and an antinode of the standing wave generated by reflecting on electromagnetic field distribution adjustment device  5 A. 
       FIG. 7  shows an example of switch  12  in accordance with the present exemplary embodiment. As shown in  FIG. 7 , two Zener diodes are parallelly connected in reverse directions from each other to constitute switch  12 . 
     In the case where switch  12  is an element that has a breakdown voltage characteristic such as that of a Zener diode, if electromagnetic waves reach near switch  12 , a potential difference larger than a predetermined threshold (breakdown voltage) will occurs between two metal pieces  11  connected to both ends of switch  12 . At this time, switch  12  is changed from an open state to a closed state automatically. 
     Therefore, at a portion having a strong electromagnetic field in electromagnetic field distribution adjustment device  5 A, the impedance changes into substantially zero automatically, so that a node of the standing wave occurs at the portion. Thus, the electromagnetic field at the portion is weakened automatically, thereby making it possible to prevent occurrence of uneven heating. Switch  12  may be a PIN diode or the like, for example. 
     As mentioned above, according to the present exemplary embodiment, the impedance of electromagnetic field distribution adjustment device  5 A is set to be substantially zero or substantially infinite, thereby making it possible to interchange positions of a node and an antinode of the standing wave generated near electromagnetic field distribution adjustment device  5 A, selectively. Thus, uneven heating can be reduced. 
       FIG. 8  is a top view of electromagnetic field distribution adjustment device  5 B in accordance with a modification of the present exemplary embodiment. As shown in  FIG. 8 , electromagnetic field distribution adjustment device  5 B has four serially connected rows  15  of metal pieces. Each of four serially connected rows  15  of metal pieces has twelve metal pieces  11  that are connected in series with one another through eleven switches  12  to form a U-shape. On the other hand, sixteen metal pieces  11 , which are provided near the center of electromagnetic field distribution adjustment device  5 B, are not connected with one another through switches  12 . 
     Typically, in processing where frozen food is thawed, it is hard to heat a center portion of object  6  to be heated, whereas the remaining portion thereof is comparatively easy to heat. According to the modification, since a center portion of electromagnetic field distribution adjustment device  5 B, which is close to the center portion of object  6  to be heated, is caused to function as a magnetic wall constantly, an electromagnetic field can be formed so as to heat the center of object  6  to be heated intensively. 
     The shape of metal piece  11  is not limited to a square. Any shape other than a square may be employed as long as the plurality of metal pieces  11  are arranged to fill the predetermined two-dimensional region. 
     Grounding conductor  14  will not be limited to a plate-like shape as shown in  FIG. 3 , if metal piece  11  can be grounded. As an example, mesh-like grounding conductor  14  of which each opening does not pass electromagnetic waves may be employed. 
     Short-circuiting conductor  13  will not be limited to a column-like shape as shown in  FIG. 4 , if metal piece  11  can be grounded. 
     Metal piece  11  may be a conductive pattern provided on a dielectric substrate. In this case, metal piece  11  is supported by the dielectric substrate rather than short-circuiting conductor  13 . 
     Second Exemplary Embodiment 
       FIG. 9  is a perspective view of microwave heating device  1 B in accordance with a second exemplary embodiment of the present disclosure. In  FIG. 9 , a front wall of heating chamber  2  is omitted such that the inside of heating chamber  2  can be seen. 
     As shown in  FIG. 9 , microwave heating device  1 B includes electromagnetic field distribution adjustment device  5 C whose impedance varies depending on a control signal from the outside. In the present exemplary embodiment, switch  12  is an element having a breakdown voltage characteristic such as that of a Zener diode, for example (see  FIG. 7 ). 
     In addition to the configuration of microwave heating device  1 A, microwave heating device  1 B further includes controller  21  and temperature sensor  22 . Controller  21  selects an operation mode of electromagnetic field distribution adjustment device  5 C. Temperature sensor  22  detects temperature inside heating chamber  2 . 
       FIGS. 10A through 10C  are block configuration diagrams each showing a concrete structure and an operation mode of electromagnetic field distribution adjustment device  5 C. As shown in  FIGS. 10A through 10C , electromagnetic field distribution adjustment device  5 C has potential determination part  19  that includes selection switches  17  and  18  each being connected to serially connected row  15  of metal pieces. Selection switch  17  and selection switch  18  correspond to a first selection switch and a second selection switch, respectively. 
     Selection switch  17  selects whether metal piece  11  provided at one end of serially connected row  15  of metal pieces is connected to either direct-current voltage source  16  or ground, or neither of them. Selection switch  18  selects whether metal piece  11  provided at the other end of serially connected row  15  of metal pieces is connected to ground or not. 
     Hereinafter, metal piece  11  provided at one end of serially connected row  15  of metal pieces is referred to as one end of serially connected row  15  of metal pieces, and metal piece  11  provided at the other end of serially connected row  15  of metal pieces is referred to as the other end of serially connected row  15  of metal pieces. 
     Controller  21  controls selection switch  17  and selection switch  18 , as follows, to select an operation mode of electromagnetic field distribution adjustment device  5 C. In the present exemplary embodiment, a diode is used as switch  12 , for example. 
     In  FIG. 10A , the one end of serially connected row  15  of metal pieces is connected to neither direct-current voltage source  16  nor ground through selection switch  17 . The other end of serially connected row  15  of metal pieces is not connected to ground through selection switch  18 . In this case, serially connected row  15  of metal pieces is set in a self-controlled wall mode in which a state of switch  12  changes autonomously depending on an electric field generated on metal piece  11 , so that the electric field is made uniform. 
     In  FIG. 10B , the one end of serially connected row  15  of metal pieces is connected to direct-current voltage source  16  through selection switch  17 . The other end of serially connected row  15  of metal pieces is connected to ground through selection switch  18 . In this way, since metal piece  11  is forced to be short-circuited, serially connected row  15  of metal pieces is set in a successive conductor-plate mode in which one successive conductor plate is formed. 
     In  FIG. 10C , the one end of serially connected row  15  of metal pieces is connected to ground through selection switch  17 . The other end of serially connected row  15  of metal pieces is connected to ground through selection switch  18 . In this way, since switch  12  is forced to be opened, serially connected row  15  of metal pieces is set in a magnetic wall mode in which serially connected row  15  of metal pieces functions as a magnetic wall. 
       FIG. 11A  is a top view of electromagnetic field distribution adjustment device  5 C in accordance with a modification of the present exemplary embodiment. As shown in  FIG. 11A , the plurality of metal pieces  11  arranged to fill the predetermined two-dimensional region are not connected with one another through switches  12 , except for sixteen metal pieces  11  placed in a center portion of electromagnetic field distribution adjustment device  5 C. 
       FIG. 11B  is a block configuration diagram showing a concrete structure of electromagnetic field distribution adjustment device  5 C, especially, serially connected row  15  of metal pieces. As shown in  FIG. 11B , unlike serially connected rows  15  of metal pieces shown in  FIGS. 10A through 10C , serially connected row  15  of metal pieces in accordance with the present modification is configured such that sixteen metal pieces, which are arranged in a square-like shape, are connected in series with one another through fifteen switches  12  to form a W-shape. 
     Hereinafter, thawing operation of the present exemplary embodiment will be described. The thawing operation is performed based on temperature detected by temperature sensor  22 , with respect to object  6  to be heated, i.e., frozen food. 
     As heating progresses, object  6  to be heated changes from a low-dielectric state in which an entire surface area thereof is frozen to a high-dielectric state in which almost all surface area thereof is thawed, via a state in which the surface area thereof is partially thawed. 
     At the beginning of heating, controller  21  sets electromagnetic field distribution adjustment device  5 C in the magnetic wall mode (see  FIG. 10C ), because object  6  to be heated, which is in a frozen state, should be heated as strongly as possible. 
     As heating progresses, when the temperature of object  6  to be heated approaches zero degrees, at least a part of the surface area of object  6  to be heated is thawed, thereby increasing a dielectric constant of the area. For this reason, if electromagnetic field distribution adjustment device  5 C remains in the magnetic wall mode, heat will concentrate at the area. 
     Therefore, to achieve uniform thawing, controller  21  sets electromagnetic field distribution adjustment device  5 C in the successive conductor-plate mode (see  FIG. 10B ). 
     When the temperature of object  6  to be heated exceeds zero degrees, an entire surface area of object  6  to be heated is thawed. Under this condition, controller  21  sets electromagnetic field distribution adjustment device  5 C in the self-controlled wall mode (see  FIG. 10A ). 
     In this way, the operation modes of electromagnetic field distribution adjustment device  5 C are changed based on the temperature of object  6  to be heated, thereby making it possible to heat frozen food appropriately. 
     In the first and second exemplary embodiments mentioned above, the electromagnetic field distribution adjustment device is provided over the entire bottom face of heating chamber  2 . The present disclosure, however, is not limited to the above-mentioned exemplary embodiments. 
     Third Exemplary Embodiment 
     It is not necessary to provide the electromagnetic field distribution adjustment device in the entire bottom face of heating chamber  2 . 
       FIG. 12  is a perspective view of microwave heating device  1 C in accordance with a third exemplary embodiment. In  FIG. 12 , a front wall of heating chamber  2  is omitted such that the inside of heating chamber  2  can be seen. 
     As shown in  FIG. 12 , microwave heating device  1 C includes electromagnetic field distribution adjustment device  5 D. Electromagnetic field distribution adjustment device  5 D is provided in a part of the bottom face of heating chamber  2 , rather than the entire bottom face thereof. 
     Electromagnetic field distribution adjustment device  5 D may be detachably provided in any of wall faces of heating chambers  2 . Thus, electromagnetic field distribution adjustment device  5 D can be moved to a desired wall face within heating chamber  2  such that the standing wave distribution is changed more variously. 
     Fourth Exemplary Embodiment 
     The electromagnetic field distribution adjustment device may be provided over a plurality of two-dimensional regions within heating chamber  2 . 
       FIG. 13  is a perspective view of microwave heating device  1 D in accordance with a fourth exemplary embodiment. In  FIG. 13 , a front wall of heating chamber  2  is omitted such that the inside of heating chamber  2  can be seen. 
     As shown in  FIG. 13 , microwave heating device  1 D includes two electromagnetic field distribution adjustment devices  5 A provided in a bottom face and a side wall of heating chamber  2 . According to the present exemplary embodiment, the standing wave distribution can be changed more variously. 
     Fifth Exemplary Embodiment 
     The electromagnetic field distribution adjustment device may be provided in other wall faces, such as a side wall and a ceiling of heating chamber  2 , rather than the bottom face of heating chamber  2 . 
       FIG. 14  is a perspective view of microwave heating device  1 E in accordance with a fifth exemplary embodiment. As shown in  FIG. 14 , microwave heating device  1 E includes electromagnetic field distribution adjustment device  5 E provided near an opening of wave guide tube  4 . In the present exemplary embodiment, the opening of wave guide tube  4  is provided in a side wall of heating chamber  2 . 
       FIG. 15  is a perspective view of microwave heating device  1 F in accordance with a first modification of the present exemplary embodiment. As shown in  FIG. 15 , microwave heating device  1 F includes electromagnetic field distribution adjustment device  5 F provided near an opening of wave guide tube  4 . In the present modification, the opening of wave guide tube  4  is provided in a bottom face of heating chamber  2 . 
       FIG. 16  is a perspective view of microwave heating device  1 G in accordance with a second modification of the present exemplary embodiment. As shown in  FIG. 16 , microwave heating device  1 G includes electromagnetic field distribution adjustment device  5 G provided near an opening of wave guide tube  4 . In the present modification, the opening of wave guide tube  4  is provided in a ceiling of heating chamber  2 . 
     According to the present exemplary embodiments including these modifications, the electromagnetic field distribution adjustment device is provided near the opening of wave guide tube  4 , thereby making it possible to change the electromagnetic field distribution near the electromagnetic field distribution adjustment device satisfactorily. 
     INDUSTRIAL APPLICABILITY 
     The electromagnetic field distribution adjustment device in accordance with the present disclosure is applicable for not only a microwave oven but also other heating devices using dielectric heating, such as a garbage disposal. 
     REFERENCE MARKS IN THE DRAWINGS 
     
         
         
           
               1 A,  1 B,  1 C,  1 D,  1 E,  1 F, and  1 G microwave heating device 
               2  heating chamber 
               3  microwave generator 
               4  wave guide tube 
               5 A,  5 B,  5 C,  5 D,  5 E,  5 F, and  5 G electromagnetic field distribution adjustment device 
               6  object to be heated 
               11  metal piece 
               12  switch 
               13  short-circuiting conductor 
               14  grounding conductor 
               15  serially connected row of metal pieces 
               16  direct-current voltage source 
               17  and  18  selection switch 
               19  potential determination part 
               21  controller 
               22  temperature sensor