GB2039200A - Microwave oven - Google Patents

Abstract

In a microwave oven a rotating radiator 36 is formed of a dielectric plate 38 carrying a plurality of transmission line members 40 which terminate in conductive radiating elements 42. The transmission line members 40 are spaced about and rotated about the axis of a conductor 20 extending through an aperture 22 in a grounded plane 24. The conductive radiating elements 42 are fed, from a microwave source 16, by a plurality of strip transmission lines formed from the transmission line members 40 and the adjacent portion of the oven wall forming the grounded plane 24. The radiator 36 may be rotated by a motor 26 or by cooling air (Fig. 7, not shown) impinging on wind vanes, and it may be either in the roof or in the floor of the oven. <IMAGE>

Description

SPECIFICATION Microwave oven Microwave ovens have been produced in which a radiator provides a multi-mode pattern which is stirred by reflective mode stirrers. However, since the heating patterns in such ovens are generally non-uniform, food bodies placed in the oven have to be periodically rotated or moved to produce uniform heating thereof. Furthermore, as shown, for example, in our copending United Kingdom application 7 909 337 (Serial No.

2,018,098), microwave ovens have been produced in which a number of differently shaped radiating apertures, fed by a common rotating plenum, radiate separate radiator patterns. However, such ovens are relatively expensive.

According to the present invention, there is provided a microwave oven comprising a conductive enclosure, and means for supplying the enclosure with microwave energy, comprising a conductor which extends through an aperture in a wall of the enclosure and is coupled to means for radiating the energy, which means comprise a plurality of transmission lines feeding separate metallic radiating elements. More specifically, the transmission lines are microstrip transmission lines formed between one of the enclosure walls, acting as a common ground plane, and a number of rotating conductive strips.

The radiating elements are preferably formed as extensions of the conductors, which extensions extend at an angle to the ground plane region of the oven wall and which are preferably spaced at different distances and directions from the axis of rotation of the radiating structure. In addition, while the axis of rotation of the structure is preferably perpendicular to the wall of the oven it may, if desired, be at another angle with respect thereto.

The radiating structure can be supported by a ceramic or a plastics type material depending upon whether or not additional conventional heating is used.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates a vertical sectional view of a microwave oven embodying the invention taken along line 1-1 of Fig. 2; Figure 2 illustrates a transverse sectional view of the oven of Fig. 1 taken along line 2-2 of Fig. 1; Figure 3 illustrates an embodiment of the invention wherein auxiliary heating elements, such as baking and/or grilling units, are incorporated in the oven; Figure 4 is a partial sectional view of the oven of Fig. 3 taken along line 4-4 of Fig. 3; Figure 5 is a transverse sectional view of the oven with the door open, taken along line 5-5 of Fig. 3;; Figure 6 is a vertical sectional view of an alternative embodiment of the invention, wherein the radiating structure, supported directly from a magnetron output structure, is rotated by air directed against paddles attached to the radiating structure; and Figure 7 is a transverse sectional view of the embodiment of Fig. 6 taken along line 7-7 of Fig. 6.

Referring now to Figs. 1 and 2 there is shown a microwave oven 10 comprising an enclosure 1 2 made, for example, of a conductive material such as stainless steel or steel coated with porcelain. The enclosure 1 2 has an access opening closed by a door 1 4 which is sealed to the walls of the enclosure 1 2 by a microwave choke type seal shown, for example, in U.S. Patent No. 3,767,884.Microwave energy, supplied by a magnetron 16, is fed through a waveguide 1 8 to a conductive element 20 which extends through an aperture 22 in the lower wall of the waveguide 1 8 and the upper wall 24 of the enclosure 1 2. The upper end of the conductor 20, which is contained in the waveguide 18, is attached to a motor 26 by a dielectric coupling member 28 which extends through an aperture in the upper surface of the waveguide 18 to the motor 26. The size of the aperture is substantially less than one-half wavelength of the microwave energy in the dielectric coupling member 28. A quarter wavelength choke (not shown) may surround the member 28 between the waveguide 1 8 and the motor 26. The dielectric coupling member 28 is rotated by the motor 26 and thereby rotates the conductor 20.

Energy from the magnetron 16, having a frequency of, for example 2.45 GHz, is propagated from the magnetron output probe 30 through the waveguide 1 8 and the conductor 20 into the enclosure 12. Impedance matching of the waveguide 1 8 to the magnetron 1 6 is selected by positioning a waveguide shorting end plate 32. This permits the magnetron 1 6 to operate at maximum efficiency in accordance with well-known practice. The other end of the waveguide 1 8 is impedance matched to the conductor 20 by selecting the position of a waveguide shorting plate 34.

The position of the plate 34 is selected to match the impedance presented to the waveguide 1 8 by a radiating structure 36 which is supported in the enclosure ì 2 by the conductor 20. Any desired impedance matching structures may be selected by selecting the position of the plates 32 and 34. However the structures are preferably chosen so that the magnetron operating for example with 4,000 volts anode to cathode potential, will provide microwave power at high efficiency.

The radiating structure 36 comprises a dielectric support plate 38 supporting flat con ductive strips 40 which are, for example, between 5 mm and 25 mm in width and which extend radially outwardly from the conductor 20 and are electrically connected thereto. In this embodiment of the present invention three strips 40 are illustrated; however, any desired number can be used. In this embodiment the lengths of strips 40 are different and the angular separation between adjacent strips is the same; however, different separation angles could be used.

The outer end of the strips 40 are bent downwardly so as to form conductive radiating elements 42 which act as separate radiators. The length of each radiating element 42 is preferably substantially a quarter wavelength at the frequency of the microwave energy; however, radiators which are substantially multiples of one quarter wavelength long could be used.

The spacing of the strips 40 from the top wall 24 of enclosure 12, is preferably selected to produce micro-strip transmission lines. The impedance of each line is between 75 and 300 ohms so that at the conductor junction an impedance of between 25 and 100 ohms, is presented. Preferably the spacing of the strips 40 from the upper wall 24 is less than their average width. Thus, when the conductor 20 is rotated by the motor 26, the individual strips 40 and the radiating elements 42 fed thereby, rotate about an axis through the conductor 20. The dielectric member 28 provides a bearing against the upper wall of waveguide 1 8 to prevent sideways motion of the radiator 36.

The portions of the strips 40 together with the adjacent portion of the upper wall 24 act as individual micro-strip transmission lines.

The aperture 22 together with the conductor 20 act as a coaxial transmission line, feeding energy from the waveguide 1 8 into the enclosure 1 2. The microwave energy propagates radially outwardly along the micro-strip transmission lines. It has been found that the major portions of the microwave energy from the magnetron 16, are radiated from the radiating elements 42 as separated radiation patterns, each pattern having at all times a substantial component directed downwardly towards a load such as a food body 44 positioned in the enclosure 12. Rotating the structure 36 thus produces many additional modes in the enclosure 1 2, giving a heating pattern substantially without cold spots. Thus in this oven the food body 44 need not be turned or changed in position during a cook ing cycle.

In addition, since a substantial portion of the microwave energy being radiated from the elements 42 first impinges on the food body 44, prior to reflection from the walls of the enclosure 12, efficient coupling of the microwave energy from the magnetron 1 6 into the food body 44 occurs.

The radiating structure 36 simultaneously radiates three separate directing patterns into the enclosure 12, having different polarizations positioned at different distances from the axis of rotation of the radiating structure 36.

In this way different toroidal radiation patterns are formed in the enclosure 12, with preferably larger amounts of microwave energy being directed towards the central regions. While some of the energy is radiated directly from the members 40 it is possible, by making the spacing between the members 40 and the wall 24 substantially less than one quarter of a wavelength of the frequency of the microwave energy, the major portion of the microwave energy will be radiated from the radiating elements 42.

Thus, it may be seen that a relatively inexpensive radiating structure may be provided, which will radiate peak powers in excess of one kilowatt into the oven, while providing an improved heating pattern within the oven.

The conductive radiating elements 42 are protected by a dielectric shield member 46 which is positioned in the enclosure 1 2 below the radiating structure 36 to prevent accidental contact of the radiating structure 36 by dishes or food bodies as they are being positioned in the oven.

A blower structure 48 is used to blow air across the magnetron 16. A portion of the air is blown in through apertures in the end plate 32 and thence into the enclosure 1 2 via the aperture 22 and apertures in the dielectric shield 46, so that cooking vapours do not condense in the waveguide 18 or on the radiating structure 36. Air thus blown into the oven may be extracted therefrom through an aperture 50 covered by a screen 52 which prevents microwave energy from leaking out of the oven in accordance with wellknown practice.

An energy load, such as a glass plate 54, may be positioned in the bottom of the enclosure 1 2 to support the food body 44, and to provide a minimum load if the oven is energized when no food body is positioned therein. The plate 54 need not necessarily be used to prevent excess reflections of micro wave energy back to the magnetron 16.

Rather the micro-strip members 40 may have different lengths chosen such that the microwave energy, reflected by the enclosure 12 to the elements 42, is transmitted to the conductor 20 in different phases, thus producing substantial cancellation at the conductor 20 and thereby reducing the amount of microwave energy reflected back to the magnetron 1 6 from the enclosure 1 2 when light loads are being inserted.

Referring now to Figs. 3, 4 and 5 there is shown a further embodiment of the present invention wherein a stove 60 has a conductive enclosure 62 with a door 64 through which a food body 66 may be positioned in the enclosure 62. The food body 66 may be supported, for example, on a metal rack 68 whose position may be adjusted by selecting the level of different supporting protuberances 70 on the side walls of the enclosure 62. A ceramic plate 72 is positioned in the middle of the rack 68 and a dish 74, which is preferably transparent to microwave energy, rests on the plate 72 and supports the food body 66.

A radiating structure 76 positioned adjacent the bottom of the oven surface, comprises a plurality of metal conductive members 78 connected respective to a plurality of metal radiating members 80 which may be extensions of the conductive members 78. A central conductor 82 of a coaxial line 84, is connected to the conductor members 78 which extend radially outwardly to the radiating members 80. The coaxial line 84 extends through the bottom wall of the enclosure 62 to a waveguide 88. The conductor 82 extends through the waveguide 88, where it is supported by a dielectric bearing and conductive choke assembly 90. An extension of the conductor 82, which extends below the waveguide 88, is rotated by a motor 92 to rotate the radiator 76.

Microwave energy at a frequency of 2.45 GHz is generated by a magnetron 94 and is propagated through the waveguide 88 and the coaxial line 84, to the stripline conductive members 78 which extend radially outwardly at different angles, in a plane substantially parallel with the bottom wall of the enclosure 62, to the radiating elements 80. The conductive members 78 are preferably in a plane spaced from the conductive stationary bottom wall of the enclosure 62 by a distance which is less than an effective quarter wavelength of the frequency of the microwave energy in the enclosure 62. The conductive members 78 are preferably flat strips whose width is between 5 mm to 25 mm, forming separate transmission lines whose impedances are determined by their effective electrical spacing from the bottom wall of the enclosure 62.The outer ends of the conductive members 78 may be bent upwardly at obtuse angles of, for example, 45 degrees to form the radiating elements 80. The elements 80 may be, for example, substantially one quarter wavelength long and directively radiate separate patterns towards the food body 66. As the radiator 76 is rotated by a motor 92 the radiation patterns from the radiating elements 80 form toroidal patterns whose axes lie along the axis of rotation of the structure 76. Due to the different distances of the radiating elements 80 from the axis of rotation and the different angles at which the radiating elements 80 lie with respect to the axis of rotation, the toroidal radiation patterns are of different sizes and/or shapes.By selecting the distances of the elements 80 from the axis of rotation, and their angles with respect thereto, the average intensity of the heating pattern in the centres of the toroids may be selected. Preferably the average heating pattern intensity is somewhat greater in the centre than towards the oven side walls.

A ceramic cover 96 is accurately positioned over the structure 76, by supporting it on bumps 98 formed in the bottom wall of the enclosure 62 so that the radiator 76 is protected from spills of food or damage from dishes which might otherwise be dropped onto the radiator 76. Air from a blower 100 is blown through the fins of the magnetron 94 to cool the anode. The air then passes through apertures 102 in the side wall of the waveguide 88 and into the enclosure 62, through the coaxial line 84, and under the edges of the cover 96 between the bumps 98. Air entering the oven is allowed to escape through a screened exhaust vent 104 in the upper wall of the oven, connected to a pyrolitic cleaning element 106, so that oven vapours are pyrolized before being exhausted into the room.

Auxiliary heating is provided for the oven by a grilling element 108 supported adjacent the upper wall of the oven and by a baking element 110, surrounding the dish 74, and supported from the floor of the oven. Thermal insulation 11 2 is positioned around the oven to prevent loss of heat. Preferably, the blower 100 operates whenever either the conventional heat is energized or the microwave heat is energized so that air is blowing by the cover 96 to maintain it below its softening temperature of 370"C or 425"C even though the baking element 110, spaced therefrom by a distance of about 25 mm, may be heated to temperatures above 540'C.

Preferably the door 64 has a microwave choke 11 4 formed thereon which may be, for example, of the type illustrated in U.S. Patent No. 3,767,884. A high temperature vapour seal 11 6 is positioned outside the choke region so that cooking vapours are exhausted to the room only through a vent 1 04. A latch prevents the door 64 for opening during operation in accordance with well-known practice.

Auxiliary equipment such as a light 1 20 is positioned to illuminate the oven through a translucent ceramic window 1 22 and a microwave impervious screen 1 24. Hob rings 1 26 and associated controls may also be provided.

It has been found that the directive heating pattern of the radiator 76 may be used in combination with conventional heating provided by the grilling and baking elements to reduce the cooking time and to produce any desired degree of cooking and/or surface browning of a food body.

Referring now to Figs. 6 and 7 there is shown a further embodiment of the present invention wherein an oven 1 30 comprises a conductive enclosure 1 32 having a door 1 by means of which a food body 138 may be positioned within the enclosure 1 32. A source of microwave energy having a frequency of 2.45 GHz is provided by a magnetron 140 whose output 142 is inserted through the upper wall 144 of enclosure 1 32. The output 142 comprises a conductor 146 inside a ceramic cylinder 148 connected to an output end cap 150 in accordance with well-known practice.

A radiating structure 1 52 is supported in the enclosure 1 32 from the output cap 1 50.

The radiating structure 1 52 comprises a dielectric plate 1 54 supporting, on its lower surface, a plurality of conductive members 1 56 which feed metal radiating elements 1 58. The conductive members 1 56 are connected together at their inner ends and rest on a thin dielectric washer 1 60 which in turn rests on a metallic washer 1 62 attached to a metal conductor 1 64. The conductor 164 extends through the dielectric plate 1 54 and is attached by a spring clip 166 to the conductive cap 1 50 of the output structure 142.

As a result, the disc 154, carrying the conductors 1 56 and the radiating elements 158, is free to rotate about the conductor 1 64.

Effective electrical conduction between the output cap 1 50 and the metal conductive members 1 56 occurs since the dielectric washer 1 60 acts as a low impedance capacitive coupler at microwave frequencies. If desired, a metal to metal contact may be used between the support washer 1 62 and the members 156; however, better wear characteristics can be achieved if a dielectric material such as Mylar (Trade Mark) or nylon is used for the thin bearing washer 1 60.

Air, from a blower 168, is blown through fins attached to the anode of the magnetron 140 and thence, via a duct 1 70 and apertures 1 72 in the upper wall 144 of the enclosure 132, to a plenum 1 74. The plenum 1 74 has a nozzle 1 76 which directs the air tangentially against the periphery of the dielectric disc 54.The air impinges on a plurality of dielectric paddles 1 78 attached to the rim of the disc 1 54 causing it to rotate about the conductive member 1 64. The plenum 1 74 is shown attached to the upper wall 144 by screws 1 80. However, if desired, it could be formed integral with a dielectric cover member 182 which is supported on projections 1 84 so as to protect the radiating structure. The air passes through apertures 1 86 in the cover 182 into the oven enclosure 1 32 and thence out via a screen vent (not shown) so that oven cooking vapours may be vented from the oven.The conductive members 1 56 are flat strips lying in a plane spaced parallel to the upper wall 144 by a distance which is less than one quarter of an effective electrical wavelength of the microwave energy. The width of the members 1 56 is selected to provide an impedance match to the output structure 142 of the magnetron 140 enabling it to operate with optimum efficiency. For example, the width is preferably less than the spacing of the members 1 56 from wall 144.

The food body 138, as illustrated herein, rests on a glass tray 1 88. However, if desired, it may be supported on a metal rack similar to that of the embodiments of Fig. 4. Additional heat from hot air, or from a browning element positioned in the oven, could also be used.

Among many possible modifications a wide variety of sizes, shapes, and orientations of the radiating elements could be used. Furthermore more than three micro-strip lines and radiating elements could be used.

Claims (11)

1. A microwave oven comprising a conductive enclosure, and means for supplying the enclosure with microwave energy, comprising a conductor which extends through an aperture in a wall of the enclosure and is coupled to means for radiating the energy, which means comprise a plurality of transmission lines feeding separate metallic radiating elements.

2. A microwave oven according to claim 1, wherein each radiating element has an effective electrical length which is substantially an integral number of quarter wavelengths of the frequency of the energy.

3. An oven according to claim 1 or 2, comprising means for rotating the means for radiating the energy about an axis through the conductor.

4. An oven according to any of claims 1 to 3, wherein the transmission lines are microstrip transmission lines, comprising conductive members spaced from a stationary ground plane by a distance less than an effective electrical quarter wavelength of the energy.

5. An oven according to claim 4, wherein the conductive members are connected to the radiating element and rotate therewith.

6. An oven according to claim 4 or 5, wherein the conductive members are flat and are spaced parallel to the ground plane.

7. An oven according to any of claims 4 to 6, wherein the conductive members extend at different angles from the conductor.

8. An oven according to any of claims 4 to 7, wherein the conductive members have a width in the range between 5 mm and 25 mm.

9. An oven according to any of claims 4 to 8, wherein the ground plane is a wall of the enclosure.

1 0. An oven according to any of claims 1 to 9, wherein the transmission lines have different lengths.

11. An oven according to any of claims 1 to 10, wherein each of the radiating elements radiates a differently polarized beam of microwave energy.

1 2. A microwave oven substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.