Patent Publication Number: US-11395381-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 Literature 1). 
     Patent Literature 1 discloses an electromagnetic field distribution adjustment device that has a large number of metal pieces arranged in a matrix manner, and a large number of switches each connecting two metal pieces adjacent to each other among the large number of metal pieces. The electromagnetic field distribution adjustment device changes impedance near a metal piece, which is included in the large number of metal pieces, in response to operation of the switch. This makes it possible to move a position of a standing wave generated near the metal piece, so that uneven heating can be reduced. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: International Publication 2015/133081 
       
    
     SUMMARY OF THE INVENTION 
     Patent Literature 1, however, does not disclose clearly the way how to connect a metal piece and a switch. 
     To solve the above-mentioned conventional problem, the present disclosure provides a concrete configuration of an electromagnetic field distribution adjustment device. 
     The electromagnetic field distribution adjustment device in one aspect of the present disclosure includes: a plurality of metal pieces arranged to fill a predetermined two-dimensional region; and a switch provided between two metal pieces adjacent to each other among the plurality of metal pieces. 
     The switch is connected to the two metal pieces adjacent to each other through two conductor parts, the two conductor parts each being provided on a corresponding one of the two metal pieces adjacent to each other, the two conductor parts each being smaller than each of the two metal pieces adjacent to each other. 
     The present aspect can reduce uneven heating that occurs when a microwave heating device heats an object to be heated. 
    
    
     
       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 an exemplary embodiment of the present disclosure. 
         FIG. 2  is a longitudinal sectional view of the microwave heating device in accordance with the present exemplary embodiment. 
         FIG. 3  is a top view of the electromagnetic field distribution adjustment device in accordance with the present exemplary embodiment. 
         FIG. 4  is a perspective view of the electromagnetic field distribution adjustment device in accordance with the present exemplary embodiment. 
         FIG. 5A  is a view showing electric field distribution E 1  near the electromagnetic field distribution adjustment device when a switch is closed. 
         FIG. 5B  is a view showing electric field distribution E 2  near the electromagnetic field distribution adjustment device when the switch is opened. 
         FIG. 6  is a view exemplarily showing the switch included in the electromagnetic field distribution adjustment device in accordance with the present exemplary embodiment. 
         FIG. 7  is a plan view of an electromagnetic field distribution adjustment device in accordance with a modification of the present exemplary embodiment. 
         FIG. 8  is a perspective view of the electromagnetic field distribution adjustment device in accordance with the modification of the present exemplary embodiment. 
         FIG. 9  is a view showing frequency characteristics related to a reflection phase of a unit cell in accordance with the modification of the present exemplary embodiment. 
         FIG. 10A  is a view showing current vectors when current flows through a unit cell having a large metal piece. 
         FIG. 10B  is a view showing current vectors when current flows through a unit cell having a small metal piece. 
         FIG. 11  is a perspective view of a heating chamber used as a simulation model. 
         FIG. 12  is a view showing simulation results of electric field distributions generated in the heating chamber. 
         FIG. 13  is a perspective view of the heating chamber shown in  FIG. 11  in which an object to be heated is placed to analyze a temperature distribution. 
         FIG. 14  is a view showing temperature distributions on the object to be heated for three different configurations of the electromagnetic field distribution adjustment device. 
         FIG. 15  is a characteristic diagram showing a relationship between a diode impedance and a reflection phase of the unit cell. 
         FIG. 16  is a characteristic diagram showing a relationship between a diode impedance and a reflection rate of a microwave. 
         FIG. 17  is a view showing a diode connected to a microstrip line used for characteristic measurement. 
         FIG. 18A  is a block diagram showing an equivalent circuit of the diode when a forward bias is applied thereto. 
         FIG. 18B  is a block diagram showing an equivalent circuit of the diode when a reverse bias is applied thereto. 
         FIG. 19  is a view showing simulation results of electric field distributions generated on an object to be heated, when the equivalent circuit of the diode shown in  FIG. 18A  is used. 
         FIG. 20  is a view showing simulation results of electric field distributions generated on an object to be heated, when the equivalent circuit of the diode shown in  FIG. 18B  is used. 
     
    
    
     DESCRIPTION OF EMBODIMENT(S) 
     The electromagnetic field distribution adjustment device in a first aspect of the present disclosure includes a plurality of metal pieces arranged to fill a predetermined two-dimensional region, and a switch provided between two metal pieces adjacent to each other among the plurality of metal pieces. 
     The switch is connected to the two metal pieces adjacent to each other through two conductor parts, the two conductor parts each being provided on a corresponding one of the two metal pieces adjacent to each other, the two conductor parts each being smaller than each of the two metal pieces adjacent to each other. 
     According to the electromagnetic field distribution adjustment device in a second aspect of the present disclosure, in the first aspect, a distance between the two metal pieces is less than or equal to one half of wavelength of a microwave. 
     According to the electromagnetic field distribution adjustment device in a third aspect of the present disclosure, in the first aspect, the switch is a diode that is smaller than each of the two conductor parts and has a breakdown voltage characteristic. 
     According to the electromagnetic field distribution adjustment device in a fourth aspect of the present disclosure, in the third aspect, the diode has an impedance of 200Ω or less when a forward bias is applied thereto through electromagnetic waves, and has an impedance of 800Ω or more when a reverse bias is applied thereto through electromagnetic waves. 
     According to the electromagnetic field distribution adjustment device in a fifth aspect of the present disclosure, in the fourth aspect, an equivalent circuit of the diode is a series circuit constituted by a resistor with a resistance of approximately 3Ω and an inductor with an inductance of approximately 1.6 nH when a forward bias is applied to the diode through electromagnetic waves, and the equivalent circuit of the diode is a parallel circuit constituted by a resistor with a resistance of approximately 10 MΩ and a capacitor with a capacitance of approximately 0.22 pF when a reverse bias is applied to the diode through electromagnetic waves. 
     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 configured to generate microwaves; a wave guide tube configured to guide the microwaves to the heating chamber; and an electromagnetic field distribution adjustment device provided in a two-dimensional region located in at least a part of a wall face within the heating chamber. 
     The electromagnetic field distribution adjustment device has a plurality of metal pieces arranged to fill a predetermined two-dimensional region, and a switch provided between two metal pieces adjacent to each other among the plurality of metal pieces. The switch is connected to the two metal pieces adjacent to each other through two conductor parts each of which is provided on a corresponding one of the two metal pieces adjacent to each other and smaller than the two metal pieces adjacent to each other. 
     Hereinafter, exemplary embodiments of the present disclosure will be described with reference to the drawings. 
       FIG. 1  is a perspective view of microwave heating device  1  in accordance with an exemplary embodiment of the present disclosure.  FIG. 2  is a longitudinal sectional view of microwave heating device  1 . 
     In the present exemplary embodiment, microwave heating device  1  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  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 on 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 ). 
     Metal piece  11  is a square metal plate whose one side has a length less than one 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. 5A  shows electric field distribution E 1  near electromagnetic field distribution adjustment device  5 A when switch  12  is closed.  FIG. 5B  shows electric field distribution E 2  near electromagnetic field distribution adjustment device  5 A when switch  12  is opened. 
     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. 5A , 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 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. 5B , 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. 6  shows an example of switch  12  in accordance with the present exemplary embodiment. As shown in  FIG. 6 , 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. 
     Hereinafter, electromagnetic field distribution adjustment device  5 B in accordance with a modification of the present exemplary embodiment will be described. In electromagnetic field distribution adjustment device  5 B, a large number of metal pieces  11  are arranged on a dielectric substrate two-dimensionally and periodically. The back of the dielectric substrate is in contact with a wall face made of a conductive member within heating chamber  2 . In other words, electromagnetic field distribution adjustment device  5 B has no grounding conductor  14 . 
     In the following description, it is assumed that a plurality of unit cells are arranged two-dimensionally and periodically to constitute electromagnetic field distribution adjustment device  5 B, for convenience. Herein, the plurality of unit cells  21  each include metal piece  11  and a part of the dielectric substrate surrounding metal piece  11 . 
       FIG. 7  is a plan view of unit cell  21  constituting electromagnetic field distribution adjustment device  5 B in accordance with the modification of the present exemplary embodiment.  FIG. 8  is a perspective view of unit cell  21 . As shown in  FIGS. 7 and 8 , unit cell  21  includes metal piece  11 , dielectric  22 , and conductor parts  23 . 
     Dielectric  22  is a part of the dielectric substrate surrounding metal piece  11 . Dielectric  22  has a square shape whose one side has a length of 45 mm. Metal piece  11  has a square shape whose one side has a length of 36 mm, and is placed on a surface center of dielectric  22 . 
     Conductor part  23  is a metallic member extending outwardly from a center portion of each side of metal piece  11 . The metallic member is provided integrally with metal piece  11  and has a rectangle shape with a width of 5 mm. 
     Switch  12  is formed in a gap with a length of 1.8 mm. The gap is interposed between two conductor parts  23  provided between two adjacent metal pieces  11  so as to face each other. Two diodes  24  are parallelly connected in reverse directions from each other to constitute switch  12  (see  FIG. 6 ). Diode  24  is a Zener diode, for example. 
     The width of conductor part  23  is made smaller than the width of metal piece  11  such that unit cell  21  is not prevented from functioning as electromagnetic field distribution adjustment device  5 B. 
     As mentioned above, in the present modification, switch  12  is connected to two metal pieces  11  adjacent to each other through two conductor parts  23  each of which is provide on a corresponding one of the two metal pieces  11  adjacent to each other, and smaller than metal piece  11  adjacent to each other. 
       FIG. 9  is a view showing frequency characteristics related to a reflection phase of unit cell  21 . In  FIG. 9 , characteristic curve group  25  is a bundle of characteristic curves when a forward bias is applied to diode  24  and then diode  24  is turned on. Characteristic curve group  26  is a bundle of characteristic curves when a reverse bias is applied to diode  24  and then diode  24  is turned off. 
     When unit cell  21  is irradiated with the microwave at an incident angle θ of 0 degrees, the characteristic curve is indicated by a dashed line. Further, when unit cell  21  is irradiated with the microwave at an incident angle θ of 30 degrees, the characteristic curve is indicated by a dotted line. Furthermore, when unit cell  21  is irradiated with the microwave at an incident angle θ of 60 degrees, the characteristic curve is indicated by a solid line. Herein, the incident angle θ of 0 degrees means that the microwave enters perpendicular to metal piece  11 , and the incident angle θ of 90 degrees means that the microwave enters in parallel to metal piece  11 . 
     As shown in  FIG. 9 , for the microwave with a frequency of 2.45 GHz, which is used for a microwave oven, when diode  24  is turned on, unit cell  21  has a reflection phase of 180 degrees. In this case, unit cell  21  functions as an electric wall. 
     When diode  24  is turned off, the reflection phase changes into 0 degrees. In this case, unit cell  21  turns into a resonance state, so that unit cell  21  functions as a magnetic wall. In this way, the reflection phase can be reversed depending on a direction of the bias applied to diode  24 . 
     It is thought that the phenomenon is caused by changing the impedance of unit cell  21  through operation of diode  24 . This applies to all cases, i.e., at incident angles of 0 degrees, 30 degrees, and 60 degrees. In other words, electromagnetic field distribution adjustment device  5 B in accordance with the present exemplary embodiment can reverse the reflection phase in response to irradiation of the microwave, not depending on an incident angle of the microwave. 
     Hereinafter, the influence of distance L between two metal pieces  11  adjacent to each other on characteristics of unit cell  21  will be described with reference to  FIGS. 10A through 14 . 
       FIG. 10A  shows current vectors when current flows through unit cell  21  having large metal piece  11  and short conductor part  23 .  FIG. 10B  shows current vectors when current flows through unit cell  21  having small metal piece  11  and long conductor part  23 . These results have been obtained through simulation. 
     As shown in  FIGS. 10A and 10B , current components flowing along edges of metal piece  11  and conductor part  23  are more than current components flowing through the remaining portions. 
     In  FIG. 10A , path  7 A indicated by an arrow line is a current path flowing along left-hand side edges of metal piece  11  and conductor part  23  downwardly. In  FIG. 10B , path  7 B indicated by an arrow line is a current path flowing along left-hand side edges of metal piece  11  and conductor part  23  downwardly. 
     When metal piece  11  and conductor part  23  have a square shape or a rectangular shape, peripheral length of an area obtained by combining metal piece  11  and conductor part  23  is constant, not depending on sizes of metal piece  11  and conductor part  23 . Therefore, the length of path  7 A is equal to the length of path  7 B. 
     In other words, as long as metal piece  11  and conductor part  23  have the above-mentioned shapes, these shapes may scarcely affect the resonance frequency. 
     However, when electromagnetic field distribution adjustment device  5 B is actually placed in a microwave oven, it has been founded that heating performance changes depending on the shape of unit cell  21 . Hereinafter, this will be described. 
       FIG. 11  is a view showing heating chamber  20  used as a simulation model. In  FIG. 11 , a front wall of heating chamber  20  is omitted such that the inside of heating chamber  20  can be seen. As shown in  FIG. 11 , heating chamber  20  of the present simulation has wave guide tube  27  provided on an upper surface of heating chamber  20 , and electromagnetic field distribution adjustment device  5 B provided over the entire lower surface of heating chamber  20 , which faces wave guide tube  27 . 
       FIG. 12  shows simulation results of electric field distributions generated on virtual planes  2 A and  2 B within heating chamber  20  in the cases of “short-circuiting between patches” and “opening between patches.” 
     In the present simulation, the following three configurations of electromagnetic field distribution adjustment device  5 B are employed. Virtual plane  2 A virtually divides heating chamber  2  into a front half portion and a rear half portion, and virtual plane  2 B virtually divides heating chamber  20  into a left half portion and a right half portion (see  FIG. 11 ). 
     As shown in  FIG. 12 , in the three configurations, metal piece  11  has the same size. A first configuration is set to have a distance L of 18 mm. A second configuration and a third configuration are set to have a distance L of 40 mm and a distance L of 80 mm, respectively. The length of conductor part  23  is determined based on distance L. In  FIG. 12 , shades of an image, which are displayed as the simulation result, indicate an electric field distribution. In other words, the electric field at a lighter color portion is stronger than the electric field at a deeper color portion. 
     “Short-circuiting between patches” means that conductor part  23  is provided between metal pieces  11 , and “opening between patches” means that conductor part  23  is not provided between metal pieces  11 . 
     When distance L is 18 mm, a great difference occurs in electric field distributions between the case of “short-circuiting between patches” and the case of “opening between patches.” In other words, the operation of switch  12  seriously changes the electric field distributions, so that a heating pattern of an object to be heated is changed significantly. 
     When distance L is 80 mm, similar electric field distributions are generated between the case of “short-circuiting between patches” and the case of “opening between patches.” In other words, the operation of switch  12  does not change the electric field distributions so much, so that a heating pattern of an object to be heated is not changed so much. 
     The results at a distance L of 40 mm are similar to the results at a distance L of 18 mm, rather than the results at a distance L of 80 mm. 
     As mentioned above, desirable effects are obtained at a distance L of 18 mm, and certain effects are obtained at a distance L of 40 mm. At a distance L of 80 mm, however, no desirable effects are obtained. In short, the smaller distance L is, the better it is. 
     This phenomenon is thought to depend on the wavelength of a microwave to be used. In other words, in the case where the microwave has a frequency of 2.45 GHz, one half of wavelength of the microwave is approximately 60 mm. If distance L is less than or equal to 60 mm, a desirable result will be obtained. If not, the microwave passing through the gap will be increased. This may deteriorate the performance of electromagnetic field distribution adjustment device  5 B. However, the evaluation of only one unit cell is not enough to find this. 
     For instance, in the simulation shown in  FIGS. 10A and 10B , the result shows that the size of metal piece  11  has no influence thereon when only one unit cell is evaluated. 
     However, in the case where a plurality of unit cells are arranged two-dimensionally, if the size of metal piece  11  is small, distance L will be large. When distance L is larger than one half of the wavelength, the effect of reducing uneven heating is deteriorated. Accordingly, to obtain better effect of reducing uneven heating, distance L is desirably less than or equal to one half of wavelength of the microwave. 
       FIG. 13  is a perspective view of heating chamber  20  shown in  FIG. 11 . In heating chamber  20 , object  6  to be heated (agar) is placed to analyze a temperature distribution.  FIG. 14  shows simulation results of temperature distributions generated on object  6  to be heated, which is placed in heating chamber  20 , in the cases of “short-circuiting between patches” and “opening between patches.” In the simulation, electromagnetic field distribution adjustment devices  5 B whose distances L each are set to be a corresponding one of 18 mm, 40 mm, and 80 mm are employed. 
     For temperature distributions generated on the agar in  FIG. 14 , when distance L is 18 mm, a great difference occurs in temperature distributions between the case of “short-circuiting between patches” and the case of “opening between patches.” In other words, this configuration enhances the effect of reducing uneven heating. 
     When distance L is 80 mm, little difference occurs in temperature distributions between the case of “short-circuiting between patches” and the case of “opening between patches.” In other words, this configuration deteriorates the effect of reducing uneven heating. 
     The result at a distance L 40 mm, if anything, is similar to the result at a distance L of 18 mm. However, there is a big difference therebetween, in fact. 
     For temperature in a center portion of the agar in  FIG. 14 , when distance L is 18 mm, the temperature in the case of “short-circuiting between patches” is high, and the temperature in the case of “opening between patches” is low. When distance L is 40 mm, however, the center temperature in both the cases is low. 
     As mentioned above, when distance L is 18 mm, the best heating characteristic is obtained among the three above-mentioned configurations. This phenomenon is thought to depend on the wavelength of a microwave to be used. 
     When the microwave has a frequency of 2.45 GHz, one fourth of wavelength of the microwave is approximately 30 mm. If distance L is less than or equal to 30 mm, a desirable result will be obtained. If not, the microwave passing through the gap will be increased. This may deteriorate the performance of electromagnetic field distribution adjustment device  5 B. However, the evaluation of only the electric field distribution shown in  FIG. 12  is not enough to find this. 
     For instance, in the simulation shown in  FIG. 12 , the result shows that, if distance L is smaller than one half of wavelength of the microwave, better effects will be obtained. To obtain the maximum effect of reducing uneven heating, however, distance L is desirably less than or equal to one fourth of wavelength of the microwave. 
     Hereinafter, specifications required for diode  24  used in unit cell  21  shown in  FIGS. 7 and 8  will be described with reference to  FIGS. 15 through 20 . 
       FIG. 15  is a characteristic diagram showing a relationship between an impedance of diode  24  and a reflection phase of unit cell  21 . 
     As shown in  FIG. 15 , to achieve the state where unit cell  21  has a large reflection phase, i.e., a reflection phase of 140 degrees or more, diode  24  is required to have an impedance of 200Ω or less. In other words, when a forward bias is applied to diode  24  through the microwave supplied into heating chamber  20  and then switch  12  turns into a short-circuiting state, diode  24  is required to have an impedance of 200Ω or less. 
     To achieve the state where unit cell  21  has a small reflection phase, i.e., a reflection phase of 40 degrees or less, diode  24  is required to have an impedance of 800Ω or more. In other words, when a reverse bias is applied to diode  24  through the microwave supplied into heating chamber  20  and then switch  12  turns into an open state, diode  24  is required to have an impedance of 800Ω or more. 
     Referring to  FIG. 15 , diode  24  to be adopted is required to have an impedance of 200Ω or less when a forward bias is applied thereto through the microwave, and required to have an impedance of 800Ω or more when a reverse bias is applied thereto through the microwave. 
       FIG. 16  is a characteristic diagram showing a relationship between an impedance of diode  24  and a rate of reflection to incidence of the microwave in unit cell  21 . Not-reflected microwaves become a loss. Therefore, diode  24  is desirably selected to reflect as much microwave as possible. 
     In the present exemplary embodiment, the selection criteria of diode  24  is to reflect more than or equal to one half of the incident microwave, i.e., have a reflection rate of more than −3 dB. 
     With reference to  FIG. 16 , it is desired that diode  24 , which is to be adopted, has an impedance of 50Ω or less when a forward bias is applied thereto through the microwave, and has an impedance of 3 kΩ or more when a reverse bias is applied thereto through the microwave. 
       FIG. 17  shows the state where diode  24 , which satisfies the above-mentioned condition, is connected to a microstrip line with a width of 1.6 mm. The microstrip line is used for characteristic measurement. As shown in FIG.  17 , diode  24  has a package with a length of 1.8 mm, which is quite small compared with conductor part  23  with a width of 5 mm (see  FIG. 8 ). For this reason, the characteristics of unit cell  21  are not affected by diode  24 , adversely. 
       FIG. 18A  shows an equivalent circuit of diode  24  when a forward bias is applied thereto through the microwave, and  FIG. 18B  shows an equivalent circuit of diode  24  when a reverse bias is applied thereto through the microwave. 
     As shown in  FIG. 18A , the equivalent circuit of diode  24  when forwardly biased is a series circuit constituted by a resister with a resistance of approximately 3Ω and a inductor with an inductance of approximately 1.6 nH. As shown in  FIG. 18B , the equivalent circuit of diode  24  when reversely biased is a parallel circuit constituted by a resistor with a resistance of approximately 10 MΩ and a capacitor with a capacitance of approximately 0.22 pF. 
       FIG. 19  shows simulation results of temperature distributions generated on object  6  to be heated (agar) when frequencies of the microwave and inductance values are changed. In the simulation, the diode having the equivalent circuit shown in  FIG. 18A  is employed. 
       FIG. 20  shows simulation results of temperature distributions generated on object  6  to be heated when frequencies of the microwave and capacitance values are changed. In the simulation, the diode having the equivalent circuit shown in  FIG. 18B  is employed. 
     In  FIGS. 19 and 20 , shades of an image, which are displayed as the simulation result, indicate a temperature distribution. In other words, the temperature at a lighter color portion is higher than the temperature at a deeper color portion. 
     As shown in  FIG. 19 , when frequencies of the microwave are changed, different electric field patterns are generated on object  6  to be heated. Even when inductance values are changed, however, almost the same electric field patterns are generated on object  6  to be heated. In other words, the electric field generated on object  6  to be heated is not affected by a variation in inductance. 
     As shown in  FIG. 20 , when the frequencies of microwave are changed, different electric field patterns are generated on object  6  to be heated. Even when capacitance values are changed, however, almost the same electric field patterns are generated on object  6  to be heated. In other words, the electric field generated on object  6  to be heated is not affected by a variation in capacitance. 
     From the above results, to achieve electromagnetic field distribution adjustment device  5 B with stable characteristics, the following conditions are required, i.e., switch  12  is constituted by diode  24  whose equivalent circuit is the series circuit shown in  FIG. 18A  when forwardly biased, and is the parallel circuit shown in  FIG. 18B  when reversely biased, for example. 
     According to the present exemplary embodiment, the electromagnetic field distribution is changed automatically at a portion having a strong electromagnetic field in electromagnetic field distribution adjustment device  5 B. As a result, the heating distribution on object  6  to be heated is changed, thereby heating object  6  to be heated more uniformly. 
     According to the present exemplary embodiment, in unit cell  21  shown in  FIGS. 10A and 10B , conductor part  23  and switch  12  are disposed on all sides of metal piece  11 . Conductor part  23  and switch  12 , however, are not necessary to provide on all sides of metal piece  11 . Unit cell  21  may not have conductor part  23  and switch  12 , if necessary. 
     In other words, electromagnetic field distribution adjustment device  5 B may have unit cell  21  whose conductor part  23  and switch  12  are not provided on at least one side of metal piece  11 , and unit cell  21  whose conductor part  23  and switch  12  are not provided on all sides of metal piece  11 . 
     According to the present exemplary embodiment, electromagnetic field distribution adjustment device  5 B is provided in the entire bottom face of the heating chamber. Electromagnetic field distribution adjustment device  5 B, however, may not be provided in the entire bottom face of the heating chamber, if necessary. 
     As long as the sizes of the unit cell and metal piece  11  are determined based on the size of the diode used as switch  12 , switch  12  may be connected to metal piece  11  directly, not through conductor part  23 . 
     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  microwave heating device 
               2  and  20  heating chamber 
               2 A and  2 B virtual plane 
               3  microwave generator 
               5 A and  5 B electromagnetic field distribution adjustment device 
               6  object to be heated 
               7 A and  7 B path 
               11  metal piece 
               12  switch 
               13  short-circuiting conductor 
               14  grounding conductor 
               21  unit cell 
               22  dielectric 
               23  conductor part 
               24  diode 
               25  and  26  characteristic curve group