Microwave heating device having transformer interposed between tuner and heating chamber

A microwave heating device includes a microwave generating portion outputting a microwave, a conductive heating chamber into which the microwave is led and having a short-circuited terminal end in a traveling direction of the microwave, and a tuner provided between the microwave generating portion and the heating chamber. The heating chamber has an opening for passing a member to be heated therethrough. The tuner re-reflects the microwave reflected at the terminal end of the heating chamber onto the heating chamber side. The microwave output end of the microwave generating portion and the tuner are connected by a square tubular waveguide made of a conductive material. The tuner and the terminal end of the heating chamber are connected by a square tubular waveguide, which is made of a conductive material except for the opening for passing the member to be heated therethrough.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 USC 119 of Japanese application no. 2011-238951, filed on Oct. 31, 2011, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a microwave heating device with high heating efficiency. The present invention also relates to an image fixing apparatus which uses such microwave heating device with high heating efficiency for fusing developing particles (toner).

2. Description of the Related Art

An image fixing apparatus fuses a toner material onto a sheet (object to be printed) to fix an image onto a sheet. A conventional image fixing apparatus applies heat or pressure onto the sheet by means of a fusing roller to fuse toner onto the sheet.

However, in the conventional configuration, the fusing roller wears with time. As a method for solving such a problem, a non-contact type method for fusing toner with a microwave has been developed in recent years (for example, see JP-A-2003-295692).

FIGS. 10A and 10Bare conceptual diagrams showing a configuration of a microwave device disclosed in JPA-2003-295692.

As shown inFIG. 10A, a microwave device100includes a magnetron110generating a microwave, an input coupling converter113which input couples the microwave generated from the magnetron110to a resonator chamber103, a water reservoir111, and a circulator112. Between the input coupling converter113and the resonator chamber103, a coupling aperture114with a diaphragm is provided. The resonator chamber103has a side surface109provided with a passing portion107for passing and guiding a sheet101therethrough. The resonator chamber103has on the downstream side a terminal end slider115made of metal. The terminal end slider115is horizontally movable relative to the resonator chamber103, and extends into the resonator chamber103.

FIG. 10Bis a schematic perspective view of the resonator chamber103portion. A microwave generated from the magnetron110is led into the resonator chamber103. For understanding,FIG. 10Bshows the microwave in a substantially sinusoidal wave form.

The resonator chamber103has the side surface109and a side surface109′ which are opposite to each other and are provided with the passing portion107and a passing portion107′, respectively. The sheet101passes through the passing portion107′, and is led into the resonator chamber103. Then, the sheet101passes through the passing portion107opposite to the passing portion107′, and is ejected therefrom. The moving direction of the sheet101is indicated by an arrow.

The passing portions107and107′ include therein a movable element104. The element104is a bar made of polytetrafluoroethylene (PTFE), and extends into the resonator chamber103.

In JP-A-2003-295692, the position of the element104can be longitudinally moved in the resonator chamber103. The position of the element104is moved to regulate the resonance conditions in the resonator chamber103. Therefore, the microwave absorption onto the sheet101can be enhanced.

In the technique of JP-A-2003-295692, the coupling aperture114with a diaphragm is provided between the input coupling converter113and the resonator chamber103. Thereby, a standing microwave is formed in the resonator chamber103. However, the diaphragm portion has an inclined side surface which causes microwave reflection, thereby lowering transmission efficiency. That is, to lead a high-energy microwave into the resonator chamber103, it is necessary to generate higher microwave energy from the magnetron. As a result, the energy consumption is increased.

In the microwave field, it has been known that the temperature of a microwave-exposed sheet is increased. However, in an application in which it is necessary to fuse toner onto a sheet in a very short time in, e.g., a printer and a copy machine, a method which enables temperature increase only for fusing toner in such a short time cannot be established at present. As a typical example of electronic equipment which performs heating with a microwave, e.g., a microwave oven has been known. However, even when a sheet put into an electronic oven is applied with a microwave for one to about several seconds, the temperature of the sheet cannot be increased by 100° C. or more.

In the technique of JP-A-2003-295692, it is difficult to fuse toner in a very short time. In addition, to shorten the fusing time by using the technique, it is necessary to generate very high microwave energy from the magnetron.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a microwave heating device which allows efficient microwave energy transmission to achieve both reduction in energy consumption and improvement in heating efficiency. In addition, an object of the present invention is to provide a non-contact type image fixing apparatus with high heating efficiency by using such a microwave heating device for fusing developing particles.

To achieve the above object, a microwave heating device according to the present invention includes a microwave generating portion outputting a microwave, a conductive heating chamber into which the microwave is led and having a short-circuited terminal end in a traveling direction of the microwave, and a tuner provided between the microwave generating portion and the heating chamber. The heating chamber has an opening for passing a member to be heated therethrough in the heating chamber in a direction non-parallel to the traveling direction of the microwave. The tuner re-reflects the microwave reflected at the terminal end of the heating chamber onto the heating chamber side. The microwave output end of the microwave generating portion and the tuner are connected by a first square tubular waveguide made of a conductive material. The tuner and the terminal end of the heating chamber are connected by a second square tubular waveguide, the waveguide being made of a conductive material except for the opening for passing the member to be heated therethrough.

According to such a configuration, the microwave reflected at the terminal end of the heating chamber is re-reflected onto the heating chamber side by the tuner. Therefore, the microwave can be multi-reflected in the heating chamber. Accordingly, the electric field intensity of the standing microwave in the heating chamber can be higher without significantly increasing microwave energy generated from the microwave generating portion. Therefore, the temperature in the heating chamber can be abruptly increased in a short time.

In the above configuration, the tuner may be an E-H tuner.

With such a configuration, the microwave reflected at the terminal end of the heating chamber can be re-reflected onto the heating chamber side at a very high rate.

In addition to the above configuration, the microwave heating device may further include an electric field transformer which is a high dielectric having a higher dielectric constant than air, the transformer having a width more than (4N−3)λg′/8 and less than (4N−1)λg′/8 where λg′ is the wavelength of a standing microwave in the high dielectric and N (N>0) is a natural number, the transformer being interposed in a position including a node of the standing microwave between the tuner and the heating chamber.

In one configuration, the electric field transformer may have a width which is an odd multiple of λg′/4, and be provided such that a surface of the heating chamber on the terminal end side is in a position at the node of the standing microwave.

With such a configuration, the electric field intensity can be higher on the downstream side of the electric field transformer, that is, on the heating chamber side, than on the upstream side. Accordingly, the effect of abruptly increasing the temperature in the heating chamber in a short time can be enhanced.

The electric field transformer may be made of ultra high molecular weight (UHMW) polyethylene.

With such a configuration, the electric field transformer is excellent in processability, and can be relatively inexpensively available. The manufacturers' cost can be reduced.

An image fixing apparatus according to the present invention includes the microwave heating device having the above features, wherein a recording sheet with developing particles passes through the opening and is heated in the heating chamber, thereby fusing the developing particles onto the recording sheet.

With such a configuration, the image fixing apparatus can fuse the developing particles onto the recording sheet in a short time without having any mechanical fusing mechanisms.

According to the present invention, the microwave reflected at the terminal end of the heating chamber is re-reflected onto the heating chamber side by the tuner. Therefore, the microwave can be multi-reflected in the heating chamber. Accordingly, the electric field intensity of the standing microwave in the heating chamber can be higher without significantly increasing microwave energy generated from the microwave generating portion. Therefore, the temperature in the heating chamber can be abruptly increased in a short time.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1is a conceptual configuration diagram of a microwave heating device according to the present invention, and shows a state seen from one side. A microwave heating device1shown inFIG. 1includes a microwave generating portion3which is a magnetron, a heating chamber5for heating an object to be heated with a microwave, and a tuner7between the microwave generating portion3and the heating chamber5. In addition, in this embodiment, an isolator4is provided between the microwave generating portion3and the tuner7. The isolator4is a protective device which converts the electric power of the microwave reflected from the tuner7in the direction of the microwave generating portion3side into heat energy and stably operates the microwave generating portion3. However, in the device of the present invention, the isolator4is not always necessary.

In addition, as shown inFIG. 1, the downstream side of the heating chamber5is terminated by a conductor (5a). The terminal end5amay be made of the same metal material as the heating chamber5.

The microwave generating portion3and the tuner7, and the tuner7and the heating chamber5are connected by square tubular frames made of conductive materials (such as metals), thereby confining the generated microwave. However, the heating chamber5has a slit6(corresponding to an “opening”).

As in the conventional configuration shown inFIGS. 10A and 10B, in this embodiment, the heating chamber5is provided with the slit6for passing a sheet (corresponding to a “member to be heated”) therethrough. InFIG. 1, the sheet passes from the rear to the front in the direction of arrow d1. That is, the heating chamber5also has, in the rear side surface, a slit opposing the slit6. The sheet enters into the heating chamber5through the slit in the rear side surface, is heated in the heating chamber5, and is ejected from the slit6in the front side surface to the outside of the heating chamber5. Toner particles adhere onto the surface of the sheet. The adherent toner particles are heated in the heating chamber5, and are fused onto the sheet.

FIG. 2is a perspective view showing the configuration of the heating chamber5. The heating chamber5has a square tubular shape such that the periphery thereof is covered with a metal conductor with the slit6and a microwave inlet8being provided in predetermined surfaces thereof. That is, the heating chamber5is short-circuited by the conductor on the surface opposite to the microwave inlet8, located on the most downstream side seen from the microwave generating portion3. A constituent material of the heating chamber5includes a non-magnetic metal (having almost the same magnetic permeability as magnetic permeability of vacuum) such as aluminum, copper, silver or gold, an alloy having high electric conductivity, one or multi-layered plating having a thickness which is several times as large as a surface skin depth of the above metal or alloy, foil, surface-treated (including coating with a conductive material) metal, alloy such as brass, and resin.

The heating chamber5has the microwave inlet8in the side surface on the microwave generating portion3side. The microwave inlet8is an opening for leading a microwave into the heating chamber5. The microwave outputted from the microwave generating portion3is led from the microwave inlet8into the heating chamber5in the direction indicated by arrow d2. The microwave inlet8has a substantially rectangular shape such that a is a dimension perpendicular to advancing direction d1of a sheet10and b is a dimension parallel to d1.

In this embodiment, the microwave propagating in the heating chamber5is in the basic mode (H10mode or TE10mode).

The slit6preferably has a minimum size necessary for passing the sheet10to be heated therethrough. This is because when the slit6is excessively large, the introduced microwave leaks through the slit6, and the power of the microwave in the heating chamber5may be reduced.

FIG. 3is a conceptual diagram showing a waveguide electric field distribution when the heating chamber5is seen from the traveling direction of a microwave.FIG. 3conceptually shows the electric field intensity of a standing microwave W in the heating chamber5.

As shown inFIG. 3, the magnitude of the power of the standing microwave W is changed according to the position in the heating chamber5. The slit6is desirably provided in a position in which the power is maximum in the a direction.

FIG. 4is a conceptual diagram of the tuner7in this embodiment. The tuner7is a so-called E-H tuner and has two T-shaped branch type projecting portions on the surfaces parallel to the traveling direction d2of a microwave. That is, in the tuner7, among the side surfaces of a square tubular waveguide such that the periphery thereof is covered with a metal conductor, a side surface P1is parallel to the advancing direction d1of the sheet and has thereon a first T-shaped branch path11, and a side surface P2is perpendicular to d1and has thereon a second T-shaped branch path12. A constituent material of the tuner7includes a non-magnetic metal (having almost the same magnetic permeability as magnetic permeability of vacuum) such as aluminum, copper, silver or gold, an alloy having high electric conductivity, one or multi-layered plating having a thickness which is several times as large as a surface skin depth of the above metal or alloy, foil, surface-treated (including coating with a metal material) metal, alloy such as brass, and resin.

In this embodiment, the tuner7which is an E-H tuner is provided between the microwave generating portion3and the heating chamber5. The power of the standing microwave formed in the heating chamber5can thus be significantly high. More specifically, an incident microwave is reflected at the terminal end5aof the heating chamber5, and is then re-reflected onto the heating chamber5side by the E-H tuner7. These reflections are repeated a number of times, so that the electric field intensity of the standing microwave generated in the heating chamber5can be higher. Accordingly, time necessary for completely fusing toner can be shortened without significantly increasing the energy of the microwave outputted from the microwave generating portion3. The detailed results will be described later in Examples.

FIG. 5is a conceptual diagram of a microwave heating device according to a second embodiment. Hereinafter, for the d2direction, the terminal end5aside is called “downstream”, and the microwave generating portion3side is called “upstream”.

This embodiment is different from the first embodiment in that an electric field transformer15is further provided on the downstream side (the terminal end5aside) from the tuner7.

The electric field transformer15is made of a high dielectric constant material. In this embodiment, ultra high molecular weight (UHMW) polyethylene is used. However, a resin material such as polytetrafluoroethylene, quartz, and other high dielectric constant materials can be used. In addition, the electric field transformer15is preferably made of a hard-to-heat material where possible. From the viewpoint of the processability and the cost, UHMV polyethylene is preferably used.

The electric field transformer15has a width in the traveling direction d2of a microwave which is an odd multiple of λg′/4 (λg′/4, 3 λg′/4, . . . ) where λg′ is the wavelength of a standing microwave formed in the same dielectric as the electric field transformer15(hereinafter, called a “dielectric wavelength”). The electric field transformer15has a width which is an odd multiple of λg′/4, so that the interposition effect of the electric field transformer15can be the highest. However, the interposition effect of the electric field transformer15can be obtained by setting the width of the electric field transformer15to satisfy later-described relational equations.

When λ is the wavelength of a microwave generated from the microwave generating portion3, ∈′ is the dielectric constant of the electric field transformer15, λc is a cut-off wavelength, and λg′ is a dielectric wavelength, Equation 1 is established. From this relational equation, dielectric wavelength λg′ can be calculated.

As shown inFIG. 6, in this embodiment, the electric field transformer15is fixed. More specifically, the electric field transformer15is provided in a position20which is a node of a standing microwave formed in the heating chamber5. More specifically, the electric field transformer15is provided in the position20in which the surface of the electric field transformer15on the terminal end5aside (downstream side) is at the node.

The electric field transformer15has a higher dielectric constant than air, so that the wavelength of the standing microwave passing in the electric field transformer15becomes short. Accordingly, the electric field intensity of a standing microwave W′ on the downstream side (the terminal end5aside) from the electric field transformer15can be higher. In particular, when a width L of the electric field transformer15is set within the range of the following relational equation, the electric field intensity of standing microwave W′ can be significantly higher. In the following relational equation, N is a natural number.
(4N−3)λg′/8<L<(4N−1)λg′/8  (Relational equation)

These results will be apparent by later-described Examples.

As in the first embodiment and this embodiment, in the configuration generating the standing microwave in the heating chamber5, a high electric field intensity portion (antinode) and a low electric field intensity portion (node) are caused according to distance in the direction from the terminal end5atoward the microwave generating portion3. As shown inFIG. 6, in particular, by providing the electric field transformer15at the node of the standing microwave, the electric field intensity of standing microwave W′ on the downstream side from the electric field transformer15can be higher. The toner fusibility can thus be improved.

That is, the slit6is provided on the downstream side from the electric field transformer15to pass the sheet10therethrough, thereby performing heating treatment based on power-increased standing microwave W′. The toner fusing time can be further shortened.

By providing the electric field transformer15, the electric field intensity on the downstream side therefrom can be higher, which is also supported by the following theory.

(Description of the Theory)

As shown inFIG. 7A, the load end of the rectangular waveguide is terminated with an impedance Zr. When in consideration of the TE10mode, Eiis the amplitude of an incident electric field intensity at the load end and Eris the amplitude of a reflected electric field intensity at the load end, Eyand Hxat points on the Z axis of the waveguide are expressed by Equation 2. The a direction inFIG. 2corresponds to the X axis, the b direction therein corresponds to the Y axis, and the d2direction therein corresponds to the Z axis. Eycorresponds to the Y axis component of an electric field, and Hxcorresponds to the X axis component of a magnetic field.

In Equation 2, Z01is a characteristic impedance, and γ1is a propagation constant.

Here, as shown inFIG. 7B, a region I includes an atmosphere, and a region II is filled with the dielectric short-circuited at a terminal end c as an impedance ZR. When Ei1is the incident electric field intensity of the region I, Er1is the reflected electric field intensity of the region I, Ei2is the incident electric field intensity of the region II, and Er2is the reflected electric field intensity of the region II, Equation 3 is established by Equation 1 and under the boundary conditions at z=0.

Here, since inFIG. 7B, the surface of terminal end c is short-circuited, Equation 4 is established. The Z coordinate in the head position (on the microwave generating side) in the region II is 0, and the width of the region II in the Z axis direction is d.
Ex(z=d)=Ei2e−γ2d+Eγ2eγ2d=0  [Equation 4]

When Equation 4 is solved for Ei2, Equation 5 is established.

In Equation 5, when the loss is neglected to take the absolute values, Equation 6 is established.

In Equation 6, β1gis a complex component (phase constant) of a waveguide wavelength λ1gin the region I, and β2gis a complex component (phase constant) of a waveguide wavelength λ2gin the region II. In addition, K is a constant.

From Equation 6, when β2gd is an odd multiple of π/2, the electric field intensity of the region II is equal to the incident electric field intensity, and when β2gd is an even multiple of π/2, the electric field intensity of the region II is 1/K of the incident electric field intensity. When the boundary surface between the regions having different dielectric constants is at the antinode of the electric field, the electric field intensities of the regions on both sides of the boundary surface are equal, When the boundary surface between the regions having different dielectric constants is at the node of the electric field, the electric field intensities of the regions on both sides of the boundary surface are inversely proportional to the ratio between phase constants βgof the regions.

Therefore, as shown inFIG. 7C, the waveguide is filled with the dielectric having a thickness of λ2g/4 on the downstream side from a reference surface a (region II), and a short-circuited surface c is then placed at the distance of λ1g/4 on the downstream side of the region II from b (region III). Equation 7 is thus established. EI, EII, and EIIIindicate electric field intensities in the regions I, II, and III, respectively.

In consideration of the condition |EI|=|EII|, Equation 8 is established.
|EIII|−K|EI[Equation 8]

From Equation 8, the electric field intensity of the region III is K times the electric field intensity of the region I. That is, by interposing the dielectric having a thickness of λ2g/4, that is, the electric field transformer15, the electric field intensity on the upstream side therefrom is amplified to be propagated to the downstream side.

When the region I includes an atmosphere and the region II includes the dielectric having a dielectric constant ∈r, the constant K is defined by Equation 9.

<1> In the embodiments, the microwave is used for fusing toner onto the sheet. However, the present invention can be used for other typical applications in which abrupt heating is required in a short time (e.g., calcination and sintering of ceramics, chemical reaction requiring high temperature, and manufacturing of a wiring (conductive) pattern with toner as metal particles).

<2> In the second embodiment, the width of the electric field transformer15is preferably an odd multiple of λg′/4. However, the width of the electric field transformer15should satisfy at least the relational equations, and is desirably close to an odd multiple of λg′/4 where possible. When the width of the electric field transformer15is an even multiple of λg′/4, impedance conversion is not performed. Therefore, the effect of increasing the electric field intensity on the later stage (terminal end5a) side cannot be exhibited.

Most preferably, the surface of the electric field transformer15on the terminal end5aside is in the position at the node of the standing microwave, but should be in at least a non-antinode position.

<3> In the embodiments, the heating chamber5has the slit6as the opening. However, the opening is not limited to have the slit shape. For example, the opening may be circular, square, and polygonal. In particular, when the member to be heated is in a sheet form, such as paper and a cloth, the opening preferably has the slit shape. When the member to be heated is in a linear form such as a thread, the opening is preferably circular, square, and polygonal.

EXAMPLES

First Example

Hereinafter, the experimental results of Examples and Comparative Example by assuming the configurations of the embodiments are shown. In Examples and Comparative Example, the following devices are commonly used.

The microwave generating portion3: A product manufactured by MICRO DEVICE CO. LTD (at present, MICRO ELECTRO CO. LTD) is used. As the generating conditions, an output energy is 400 W, and an output frequency is 2.45 GHz.

The isolator4: A product manufactured by MICRO DEVICE CO. LTD (at present, MICRO ELECTRO CO. LTD) is used.

The heating chamber5: An aluminum waveguide provided with the slit6

The sheet10: A commercially available PPC (Plain Paper Copier) sheet called neutralized paper is used.

As the tuner7, an E-H tuner (a product manufactured by MICRO DEVICE CO. LTD (at present, MICRO ELECTRO CO. LTD) is used. The heating chamber5has dimensions of a=109.2 mm and b=54.6 mm. The electric field transformer15is not provided. When the E-H tuner is used in Examples and Comparative Example, the same E-H tuner is used.

As the tuner7, the E-H tuner is used. The heating chamber5has dimensions of a=109.2 mm and b=54.6 mm. As the electric field transformer15, UHMW polyethylene (dielectric constant ∈r=2.3) is used. More specifically, in the heating chamber5, UHMW polyethylene having a width of 25 mm is interposed from the position at a distance of 500 mm from the terminal end5atoward the upstream side.

This example has the same conditions as Example 1 except that the heating chamber5has dimensions of a=70 mm and b=54.6 mm. However, the size of the E-H tuner is different from the size of the heating chamber5. Therefore, the tuner7and the heating chamber5are connected by a taper-shaped waveguide.

This example has the same conditions as Example 2 except that the heating chamber5has dimensions of a=70 mm and b=54.6 mm. However, from the same reason as Example 3, the tuner7and the heating chamber5are connected by a taper-shaped waveguide.

This example has the same conditions as Example 1 except that as the tuner7, an iris (a product manufactured by MICRO DEVICE CO. LTD (at present, MICRO ELECTRO CO. LTD) is used.

Comparative Example 1

This example has the same conditions as Example 1 except that the tuner is not provided.

Under the respective conditions, the sheet10with toner put on a predetermined region thereof is set into the slit6of the heating chamber5to measure time required for fusing the toner. Then, the measured time is multiplied by the ratio between the area of the predetermined region and the area of an A4 sheet to calculate time for toner fusion onto the A4 sheet. Table 1 shows the results.

When the tuner is not provided, it is difficult to fuse the toner onto the A4 sheet even after the elapse of 120 seconds. On the contrary, in Examples 1 to 5 in which the tuner7is provided, the toner is fused in time significantly shorter than 120 seconds. Accordingly, by providing the tuner7, the power of the standing microwave formed in the heating chamber5can be significantly increased.

Second Example

FIG. 8is a graph showing electric field intensity in the heating chamber5in Second example. The horizontal axis shows positions in the microwave traveling direction (z axis direction) in the heating chamber5, and the vertical axis shows electric field intensity. Referring toFIG. 8, the electric field intensity is greatly increased on the downstream side from the electric field transformer15. InFIG. 8andFIGS. 9A to 9F, the electric field intensity on the vertical axis has relative values (dimensionless values) when a predetermined value is a reference.

FIGS. 9A to 9Fare graphs showing electric field intensity in the heating chamber5when the width of the electric field transformer15is changed in Example 2. In this example, the dielectric having the same width is interposed directly ahead of a short-circuited plate. This is performed for making the experimental conditions identical, and does not affect the effect of Examples. In addition, depending on the graphs, the magnitude of the electric field intensity in a position at the wave trough of the standing microwave is slightly varied, which is within the calculation error range.

FIG. 9Gis a graph showing change in the ratio between the magnitudes of electric field intensities on the upstream side and the downstream side of the electric field transformer15when the width of the electric field transformer15is changed.FIG. 9His a table thereof.

InFIG. 9A, since the electric field transformer15is not interposed, as a matter of course, the electric field intensity is not changed at the front and back of the electric field transformer15(electric field intensity=4.2).

InFIG. 9B, the width of the electric field transformer15is 6 mm this corresponds to 0.06 λg′). On the upstream side of the electric field transformer15, the electric field intensity=4.2. On the downstream side of the electric field transformer15, the electric field intensity=5.3. The electric field intensity is 1.26 times higher at the back than at the front of the electric field transformer15.

InFIG. 9C, the width of the electric field transformer15is 13 mm (this corresponds to 0.13 μg′). On the upstream side of the electric field transformer15, the electric field intensity=3.8. On the downstream side of the electric field transformer15, the electric field intensity=6.8. The electric field intensity is 1.79 times higher at the back than at the front of the electric field transformer15.

InFIG. 9D, the width of the electric field transformer15is 25 mm (this corresponds to 0.25 λg′). On the upstream side of the electric field transformer15, the electric field intensity=3.4. On the downstream side of the electric field transformer15, the electric field intensity=6.2. The electric field intensity is 1.82 times higher at the back than at the front of the electric field transformer15.

InFIG. 9E, the width of the electric field transformer15is 37 mm (this corresponds to 0.37 λg′). On the upstream side of the electric field transformer15, the electric field intensity=3.5. On the downstream side of the electric field transformer15, the electric field intensity=6.0. The electric field intensity is 1.7 times higher at the back than at the front of the electric field transformer15.

InFIG. 9F, the width of the electric field transformer15is 44 mm (this corresponds to 0.44 λg′). On the upstream side of the electric field transformer15, the electric field intensity=4.2. On the downstream side of the electric field transformer15, the electric field intensity=4.5. The electric field intensity is 1.1 times higher at the back than at the front of the electric field transformer15.

Although not shown on the graphs, when the width of the electric field transformer15is 50 mm (this corresponds to 0.50 λg′), the upstream end point and the downstream end point of the electric field transformer15are both in the position at the wave trough of the standing microwave. Therefore, the electric field intensity is not changed on the downstream side and the upstream side of the electric field transformer15.

According to the above results, a width L of the electric field transformer15is set to satisfy (4N−3)λg′/8<L<(4N−1)λg′/8 by using the relational equations, that is, natural number N, so that the electric field intensity of the standing microwave on the downstream side of the electric field transformer15can be higher. Accordingly, the electric field intensity in the heating chamber5can be higher to greatly shorten time necessary for toner fusion.