Microwave heating apparatus with rotatable antenna and method thereof

Abstract

A microwave heating apparatus and a method for heating/browning a piece of food by means of microwaves are provided. The microwave heating apparatus comprises a cavity arranged to receive, in a substantially horizontal browning region, a piece of food to be browned. The microwave heating apparatus further comprises a microwave source for generating microwaves and a rotatable antenna arranged at the cavity bottom for supplying the generated microwaves. The antenna is configured to produce at least one radiating lobe pointing towards the browning region such that the intersection between the radiating lobe and the browning region forms a hot spot, thereby forming a ring-shaped heating pattern in the browning region under rotation of the antenna. The present invention is advantageous in that a microwave heating apparatus with an improved crisp function is provided.

Claims

1. A microwave heating apparatus comprising: a cavity arranged to receive a piece of food to be browned; a stationary browning plate having a thermally conductive layer on an upper side for directly receiving the piece of food to be browned, and a microwave absorbing layer on a lower side, the browning plate arranged in a substantially horizontal browning region configured outward of a center of the browning plate; a microwave source for generating microwaves below the browning plate; and a rotatable antenna arranged at a bottom of the cavity for distributing the generated microwaves, the antenna comprising a panel configured to distribute a first portion of the generated microwaves to a first opening configured to produce a first radiating lobe pointing towards the browning region such that the intersection between the first radiating lobe and the browning region forms a hot spot thereby forming a first ring-shaped heating pattern in the browning region as a result of rotation of the antenna, the panel configured to distribute a second portion of the generated microwaves to a second opening configured to produce a second radiating lobe pointing towards the browning region such that the intersection between the second radiating lobe and the browning region forms a hot spot thereby forming a second ring-shaped heating pattern in the browning region as a result of rotation of the antenna, wherein the first and second openings are configured such that the second ring-shaped heating pattern is closer to the center of the browning plate than the first ring-shaped heating pattern.

2. The microwave heating apparatus of claim 1, wherein the antenna is configured to produce the first radiating lobe such that the first ring-shaped heating pattern covers 10 to 50 percent of the browning region.

3. The microwave heating apparatus of claim 1, wherein the antenna is configured to produce the first radiating lobe pointing in a direction forming an angle comprised in the range of 0-90 degrees with the browning region.

4. The microwave heating apparatus of claim 1, wherein the antenna is configured such that the first radiating lobe points at the periphery of the browning region.

5. The microwave heating apparatus of claim 1, wherein the panel is a sector-shaped panel.

6. The microwave heating apparatus of claim 5, wherein the edge of the sector-shaped panel defining the opening at which microwaves exit the antenna is curved.

7. The microwave heating apparatus of claim 1, wherein a horizontal dimension of the browning plate is larger than a horizontal dimension of the rotatable antenna.

8. The microwave heating apparatus of claim 1, wherein the browning plate is configured to reduce coupling of microwaves from a compartment of the cavity defined by the bottom of the cavity and the browning plate to the rest of the cavity.

9. The microwave heating apparatus of claim 1, further comprising an additional feeding port for feeding microwaves in an upper part of the cavity.

10. The microwave heating apparatus of claim 1, further comprising holding means for holding a container in which the piece of food is located, the holding means being arranged such that the container is positioned in the browning region.

11. The microwave heating apparatus of claim 1, wherein the antenna is configured to produce the first radiating lobe pointing in a direction forming an angle comprised in the range of 30-60 degrees with the browning region.

12. A method of operating a microwave heating apparatus comprising a cavity arranged to receive, in a substantially horizontal browning region, a piece of food to be browned, the method comprising: providing a microwave source for generating microwaves; providing a stationary browning plate having a thermally conductive layer on an upper side for directly receiving the piece of food to be browned, and a microwave absorbing layer on a lower side, the browning plate arranged in a substantially horizontal browning region configured outward of a center of the browning plate; and arranging a rotatable antenna at a bottom of the cavity for supplying microwaves, the antenna comprising a panel configured to distribute a first portion of the generated microwaves to a first opening configured to produce a first radiating lobe pointing towards the browning region such that the intersection between the first radiating lobe and the browning region forms a hot spot thereby forming a first ring-shaped heating pattern in the browning region as a result of rotation of the antenna, the panel configured to distribute a second portion of the generated microwaves to a second opening configured to produce a second radiating lobe pointing towards the browning region such that the intersection between the second radiating lobe and the browning region forms a hot spot thereby forming a second ring-shaped heating pattern in the browning region as a result of rotation of the antenna, wherein the first and second openings are configured such that the second ring-shaped heating pattern is closer to the center of the browning plate than the first ring-shaped heating pattern.

13. A microwave heating apparatus comprising: a cavity arranged to receive, in a substantially horizontal browning region, a piece of food to be browned; a microwave source for generating microwaves; and a rotatable antenna arranged at a bottom of the cavity for supplying the generated microwaves, the antenna comprising a panel spaced from the bottom of the cavity to provide a path for a first portion of the generated microwaves to produce a first radiating lobe pointing towards the browning region such that the intersection between the radiating lobe and the browning region forms a hot spot thereby forming a first ring-shaped heating pattern in the browning region as a result of rotation of the antenna, wherein a top side of the panel comprises a top aperture from which a second portion of the generated microwaves exit the antenna to produce a second radiating lobe pointing towards the browning region such that the intersection between the second radiating lobe and the browning region forms a hot spot thereby forming a second ring-shaped heating pattern in the browning region as a result of rotation of the antenna, wherein the panel is configured such that the second ring-shaped heating pattern is closer to the center of the browning plate than the first ring-shaped heating pattern.

14. The microwave heating apparatus of claim 13, and further comprising a stationary browning plate having a thermally conductive layer on an upper side for directly receiving the piece of food to be browned, and a microwave absorbing layer on a lower side, the browning plate arranged in a substantially horizontal browning region configured outward of a center of the browning plate.

15. The microwave heating apparatus of claim 13, wherein the panel is a sector-shaped panel.

16. The microwave heating apparatus of claim 15, wherein the edge of the sector-shaped panel defining the opening at which microwaves exit the antenna is curved.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above, as well as additional features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, in which:

(2) FIG. 1 schematically shows a microwave heating apparatus according to an embodiment of the present invention;

(3) FIG. 2 shows a schematic view of a rotatable antenna and the electric field lines at the bottom of the cavity for a microwave heating apparatus according to another embodiment of the present invention; and

(4) FIG. 3 shows a schematic view of a microwave heating apparatus according to another embodiment of the present invention.

(5) All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the invention, wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) With reference to FIG. 1, there is shown a microwave heating apparatus in accordance with an embodiment of the present invention.

(7) FIG. 1 shows a microwave heating apparatus 100, e.g. a microwave oven, comprising a cavity 150, a rotatable antenna 120 and a microwave source 110. The cavity 150 is arranged to receive, in a substantially horizontal browning region 130, a piece of food to be browned. In FIG. 1, the browning region 130 is represented by a horizontal plane covering the whole section of the cavity 150. However, it will be appreciated that the browning region 130 may be defined to be slightly smaller than the whole area of the section of the cavity 150 in a horizontal plane. The microwave source 110 is adapted to generate microwaves which are supplied by means of, e.g., a transmission line (not shown in FIG. 1) to the rotatable antenna 120. The rotatable antenna 120 is arranged at the cavity bottom for supplying the generated microwaves under the browning region 130. The antenna 120 is configured to produce at least one radiating lobe pointing towards the browning region 130 such that the intersection between the radiating lobe and the browning region 130 forms a hot spot 131, thereby forming a ring-shaped heating pattern 132 in the browning region 130 under rotation of the antenna.

(8) Advantageously, the antenna may be configured to produce a radiating lobe such that the ring-shaped heating pattern 132 covers about 10 to 50 percent of the browning region 130 area (based on the size of the hot spot and as an effect of the rotation of the antenna only).

(9) In particular, the antenna may be configured to produce a radiating lobe pointing in a direction forming an angle comprised in the range of 0-90 degrees (and more preferably in the range of 30-60 degrees) with the browning region 130. Depending on the size of the rotatable antenna 120 relative to the size of the browning region 130 or depending on the location of the antenna opening through which microwaves are generated at the cavity bottom relative to the position of the browning region, the radiating lobe may be directed perpendicular to the browning region or inclined such that the radiating lobe points at the periphery of the browning region 130. As the electromagnetic field is concentrated at a specific point (or hot spot) 131 of the browning region 130, it is advantageous if the radiating lobe is not directed towards the center of the browning region 130 in order to avoid local overheating. Indeed, if the radiating lobe points too close to the center of the browning region, the coverage area of the heating pattern will be limited and the uniformity of the crisp function will be relatively poor. It is thus particularly advantageous if the radiating lobe is inclined and points at the periphery of the browning region 130 since a relatively large ring-shaped pattern 132 may then be created in the browning region 130 under rotation of the antenna 120.

(10) Although various shapes of rotatable antenna are envisaged, the rotatable antenna 120 may comprise a sector-shaped panel 121 arranged at a distance from the cavity bottom for providing at least one opening 124 through which the generated microwaves are supplied. In particular, the distance between the cavity bottom and the sector-shaped panel 121 of the rotatable antenna 120 together with the sector geometry defines the level of the microwave power supplied from the antenna 120 via the opening 124 to the cavity 150. Thus, the distance between the cavity bottom and the sector-shaped panel 121 and the sector geometry itself are parameters that can be used for designing the rotatable antenna 120 and improving the crisp function of the microwave heating apparatus. In particular, the rotatable antenna 120 may be equipped with at least one (substantially horizontal) lateral wing 122 connected to the sector-shaped panel 121 via a (substantially vertical) side wall 123 for providing the opening 124. The height of the side wall 123 may then determine the level of microwave power supplied by the rotatable antenna 120.

(11) Further, the edge of the sector-shaped panel 121 that together with the cavity bottom defines the opening 124 through which the microwaves are supplied, may be curved. In particular, the radius of curvature defines the direction at which the radiating lobe exits the opening. Referring to some of the above concerns, the curve of the edge may then be designed such that the radiating lobe points at the periphery (or any other advantageous locations) in the browning region 130.

(12) With reference to FIG. 2, there is shown a microwave heating apparatus according to another embodiment of the present invention.

(13) In particular, FIG. 2 shows a schematic view of a rotatable antenna 220 and the electric field lines at the bottom of the cavity 150.

(14) The microwave heating apparatus in which the rotatable antenna 220 is mounted may be equivalent to the microwave heating apparatus 100 described with reference to FIG. 1, i.e. comprising a cavity and a rotatable antenna arranged at the bottom of the cavity.

(15) In FIG. 2, the arrows represent the direction of propagation of the microwaves. In this specific example, the microwaves come from the right-hand side and propagate in a transmission line 160, which is provided for transmitting microwaves generated by a microwave source (not shown in FIG. 2) to the rotatable antenna 120. The transmission line 160 may be a standard one such as a waveguide, a coaxial cable or a strip line. The microwaves are transmitted from the transmission line 160 to the rotatable antenna 220 via an opening in the cavity bottom in which the rotation axis of the rotatable antenna is arranged. The microwaves are then transmitted from the rotatable antenna via an opening 124.

(16) Further, in FIG. 2, the lines represent the electric field lines, i.e. the electric field vector of the electromagnetic field corresponding to the microwaves emitted from the rotatable antenna 220.

(17) Depending on the design of the rotatable antenna 220 and its boundary conditions, a radiating opening 124 in the antenna may result in one or several radiating lobes, e.g. horizontal and/or inclined lobes. A horizontal radiating lobe may contribute to the excitation of the cavity volume modes whereas the upwardly inclined radiating lobe is conveniently used for forming the hot spot in the browning region 130. If there were only one inclined radiating lobe emitted from the antenna and coupling to the cavity volume mode was desired, the directivity could be adjusted such that the horizontal radiation intensity would not be null or negligible.

(18) Alternatively, it may be considered that the radiating lobe emitted from the opening 124 of the antenna comprises a horizontal part, i.e. a component directed along a horizontal direction I1, and another part directed along an inclined direction I2, as illustrated in FIG. 2. In this case, the horizontal part of the radiating lobe contributes to the excitation of the cavity volume modes while the inclined part is intended to energize a crisp function in the browning region 130.

(19) Advantageously, the rotatable antenna may be configured to produce at least two radiating lobes directed towards the browning region 130 at two different locations of the browning region 130. As a result, two hot spots are created in the browning region 130, which in turn provide two ring-shaped heating patterns at two different places in the browning region, thereby further improving the uniformity of the crisp function.

(20) The rotatable antenna 220 shown in FIG. 2 is generally identical to the rotatable antenna 120 shown in FIG. 1, i.e. comprising a sector-shaped panel 121 with a lateral wing 122 spaced from the sector-shaped panel 121 via a side wall 123, except that the rotatable antenna 220 comprises a top opening 126, e.g. a rectangular aperture, at the top of the sector-shaped panel 121 from which microwaves may exit the antenna 220. Providing an additional top aperture 126 at the top of the sector-shaped panel 121 is advantageous in that it provides an additional supply of microwaves to the browning region 130. Further, as illustrated in FIG. 2, the electric field lines are substantially parallel to the plane defining the browning region 130 in or close to the top aperture 126 but, further away from the aperture, inclined towards a perpendicular direction (as indicated by direction V1 in FIG. 2) relative to the plane defining the browning region 130 due to the boundary conditions and, as a result, a very effective crisp function is provided from the top aperture 126. The rotatable antenna 220 may then be designed such that the right balance in power for the microwaves emitted from the main opening 124 of the rotatable antenna and the top aperture 126 is obtained.

(21) Advantageously, the rotatable antenna 220 may be equipped with a spring-loaded piece (or sheet) of high-epsilon ceramic, for example titanium dioxide (TiO.sub.2), arranged at the top of the sector-shaped panel 121 of the antenna. This spring-loaded piece may be arranged to either cover the aperture arranged at the top of the sector-shaped panel 121 or to be on the side of it. Such a spring-loaded piece is advantageous in that the power transmission through the aperture 126 can be altered depending on the position of the spring-loaded piece wherein the energy transmitted through the aperture 126 when the spring-loaded piece covers (or partially covers) the aperture 126 is significantly lower than (or at least different from) the power transmitted when the spring-loaded piece does not cover the aperture 126. The movement of the ceramic sheet between the two (or more) positions is, for example, accomplished via the elastic property of the spring-loaded piece in combination with different antenna rotation speeds wherein the “spring” property of the piece causes a release of the ceramic sheet if the rotation speed is above a specific threshold, thereby covering the aperture 126.

(22) Further, for obtaining the crisp function, the microwave heating apparatus may comprise a browning plate 135 arranged to receive the piece of food to be browned and being arranged in the browning region 130. The browning plate may comprise a first part or layer 136 adapted to absorb microwave energy and transform the absorbed microwave energy into heat and a second part or layer 137 arranged in thermal contact with the first part. The second part may advantageously be arranged to receive a piece of food and have relatively good thermal conductivity.

(23) The first part or layer 136, i.e. the microwave-absorbing layer, corresponds to the underside (or the sole) of the crisp or browning plate 135 and the piece of food can be browned on the second part 137, i.e. the thermally conductive layer, at the upper side of the browning plate 135. Generally, the upper side of the crisp or browning plate 135 may consist of an aluminum (or steel) plate which has small thermal mass and good thermal conductivity and possibly a non-stick coating. As already mentioned, in the present specification, no particular distinction is made between a crisp plate and a browning plate and reference to a crisp plate could equally be made to a browning plate and vice versa.

(24) According to the present embodiment, the second part 137 being made of a thermally conductive material provides a uniform browning effect in the browning plate 135. A sufficiently good heat conduction is usually achieved with a metal plate made of e.g. aluminum or steel and the second part 137 enables therefore dissipation of heat in the browning plate. Aluminum has the advantage of having a relatively high heat conduction as compared to steel. However, steel is a more economic alternative.

(25) The underside of the crisp plate (i.e. the first layer 136) may be a ceramic such as rubber-embedded ferrite (in a proportion of about 75% ferrite and 25% silicon dioxide). The ferrite material has a Curie point at which absorption of microwaves in the material ceases. The characteristics for absorption of the microwaves in the ferrite material may be varied by altering the thickness of the layer and/or the composition of the material. Generally, the temperature of the upper side of the crisp plate that comes into contact with the piece of food stabilizes in a temperature range of 130-230° C.

(26) As the antenna is rotated and a uniform crisp function is provided thereof, the browning plate does not necessarily require being circular and could for instance be rectangular. This is advantageous since the largest user benefit is reached with a rectangular browning plate.

(27) In the present embodiment, the rotatable antenna 120 may provide microwaves to both the sole (i.e. the first part or layer 136) of the crisp plate 135 for energizing a crisp function in the browning region 130 and/or to the cavity for excitation of cavity modes (with or without any browning plate).

(28) In addition to, or as an alternative to, the above mentioned crisp plate arranged to receive a piece of food, the microwave heating apparatus may further comprise holding means for holding a container in which the piece of food is located. The holding means may be arranged such that the container is positioned in the browning region 130. The container would then advantageously comprise a first part or layer adapted to absorb microwave energy and transform the absorbed microwave energy into heat and a second part or layer arranged in thermal contact with the first part, such as described above for the crisp plate. In other words, it may be envisaged that a container comprises an integrated crisp plate and that such a container may be arranged in the cavity by means of specific holding means.

(29) With reference to FIG. 3, there is shown a microwave heating apparatus according to another embodiment of the present invention. The microwave heating apparatus may be identical to the microwave heating apparatus described with reference to FIG. 1 or 2 above. In particular, FIG. 3 shows a schematic view of a rotatable antenna 320 and the electric field lines at the bottom of the cavity 150.

(30) The rotatable antenna 320 is identical to the rotatable antenna 220 described above with reference to FIG. 2 except that it does not comprise an aperture at the top of the sector-shaped panel 121.

(31) As shown in FIG. 3, a specific browning plate 335 configured to limit or eliminate the coupling of microwaves to the cavity 150 is provided. For this purpose, the size of the browning plate 335 may advantageously be large as compared to the size of the rotatable antenna 320.

(32) In the present embodiment, the special browning plate 335 preferably fulfills some suitable boundary conditions for efficiently limiting or quenching the power transmitted to the cavity 150. Thus, in addition to considerations wherein the ferrite/silicone mixture may advantageously be adapted to be highly absorbing in the frequency band of interest, e.g. 2400-2500 MHz, and, have a suitable Curie point and heating time derivative, the overall geometry of the structure may be designed for tuning/limiting the level of power transmitted from the antenna to the cavity. Specific parameters include the distance from the cavity bottom to the browning plate, the distance between the cavity bottom and the rotatable antenna and the own geometry of the browning plate. Additional parameters include the ferrite content and the ferrite chemical and heating properties of the browning plate. It will be appreciated that, although it is advantageous to have a browning plate which is as large as possible, a too large browning plate may result in undesirable arcing between the plate and the cavity walls.

(33) The lines in FIG. 3 illustrates the electric field lines of the microwaves wherein in zones indicated as B and B′, the power is significantly lower than in zone A due to strong losses (absorption) in zone C of the first layer 336 of the browning plate 335. Zone C corresponds to an area wherein there is a strong heating caused by the horizontal magnetic field and the vertical electric field of the microwaves in this zone. The power of the microwaves in zone B′ depends on dimensions of the structure and, e.g., the distance between the rotatable antenna 320 and the browning plate 335.

(34) The microwave heating apparatus shown in FIG. 3 differs also from the microwave heating apparatuses described above with reference to FIGS. 1 and 2 in that it comprises an additional feeding port 190 for feeding microwaves in an upper part of the cavity 150. The provision of such an additional feeding port 190 is particularly advantageous in combination with the special browning plate 335 described above (i.e. for limiting transmission of microwaves to the cavity 150) but this could also be envisaged in combination with any one of the embodiments described above with reference to FIGS. 1 and 2.

(35) The additional feeding port 190 may be fed via a separate feeding structure connected to a separate microwave source (not shown) or via a transmission line 161 connected to the same microwave source 110 as the microwave source connected to the transmission line 160 feeding microwaves to the rotatable antenna 320. Although the additional feeding port 190 is shown to be arranged at a side wall in an upper part of the cavity, it may also be envisaged to arrange the additional feeding port 190 in the cavity ceiling.

(36) If the same microwave source 110 supplies microwaves to both the rotatable antenna 320 and the additional feeding port 190, the available power from the microwave source 110 may then be divided between the two transmission lines 160 and 161 (or between branches of transmission line).

(37) In combination with the special browning plate 335 described above, all power radiated by the rotatable antenna 320 is in principle used for browning while all power radiated via the feeding port 190 of the separate feeding structure serves for excitation of modes in the cavity volume. Thus, the microwave heating apparatus may further comprise a controlling unit (not shown) for controlling the balance in power transmitted via the rotatable antenna 320 at the bottom of the cavity for energizing the browning function and via the feeding port 190 in the upper part of the cavity for excitation of cavity modes. Optionally, for further improving the cooking performance via the additional feeding port 190 for excitation of the cavity volume mode, the microwave heating apparatus may also comprise a stirring device (not shown) for stirring the microwaves within the cavity volume.

(38) Further, it will be appreciated that, for a microwave heating apparatus comprising the additional feeding port 190 but having a standard browning plate 235 such as described with reference to FIG. 2 (i.e. without the particular characteristic of limiting the transmission of microwaves to the cavity 150), cross-talk and unwanted field cancellation or enhancement may occur. Thus, for reducing this effect, the microwave heating apparatus may advantageously be equipped with a switch (not shown) arranged in the transmission line feeding the additional feeding port such that feeding of microwaves to the additional feeding port may be blocked, in particular when the oven is in browning mode. Such a switch may be activated by a control system of the microwave heating apparatus.

(39) According to yet another embodiment, any one of the above mentioned microwave sources may be a solid-state microwave generator (based on e.g. semiconductor elements) or a magnetron. The advantages of a solid-state microwave generator comprise the possibility of controlling the frequency of the generated microwaves, controlling the output power of the generator and an inherent narrow-band spectrum.

(40) While specific embodiments have been described, the skilled person will understand that various modifications and alterations are conceivable within the scope as defined in the appended claims.

(41) For example, although a cavity having a rectangular cross-section is shown in the figures, it is also envisaged to implement the present invention in a cavity having a geometry describable in any orthogonal curve-linear coordinate system, e.g. a cavity having circular cross-section.