A FEEDING ARRANGEMENT FOR A MICROWAVE OVEN

Abstract: An arrangement for feeding microwave energy from a microwave source (14) to the oven cavity (10 of a microwave oven comprises an elongated groove-shaped recess (16) in one of the conductive cavity walls (17), into which recess (16) microwave energy is fed at a centrally located feeding point (15). At its open side the recess (16) is covered by a rotatable conductive plate (22) arranged very close to the cavity wall (17) so that the recess (16) together with the plate (22) will form a waveguide-like structure. The plate (22) is shaped so that during its rotation time-varying feeding passages between the recess (16) and the oven cavity (10) are formed at both opposite ends of the recess, the passages allowing the transfer of microwave energy from the recess (16) to the cavity (10). These measures result in a very simple and space-saving arrangement of the "dual feed" type.

Description: The invention relates to a microwave oven including an arrangement for feeding microwave energy from a microwave source mounted external of an oven cavity, said cavity being bounded by a plurality of conductive walls into the interior of the oven cavity, the arrangement comprising a substantially plane, rotatable conductive plate arranged within the oven cavity. Such a microwave oven is known from US-A-4.327.266. More particularly, the invention relates to a feeding arrangement for an oven cavity that is excited in a conventional manner according to the wellknown "multi-resonance" principle. Several known feeding arrangments for multi-resonant oven cavities are based upon a movable metallic antenna that projects into the oven cavity and that is coupled, usualy through a waveguide of TE₁₀-type, to a microwave source in the form of a magnetron. On the one hand this antenna can be regarded as an antenna in a conventional sense, i.e. as an element that can be described as having a radiation diagram, and on the other hand as a structure cooperating with the resonant modes in the cavity between the microwave input power and the resonant modes in dependence on the antenna position. In both cases, the result is that a number of different resonant modes are excited in succesion, and that - in particular for large loads in the oven when the figure of merit (Q-factor) for resonant modes can be so low that it is not appropriate to consider the field as being resonant - a direct radiation into the load takes place. In order to achieve a good microwave energy distribution in the oven cavity and more uniform heating of the load it has also been proposed that microwave energy be fed into the cavity at two spatially separated feeding points, an arrangement known as "dual feed". Reference may be made to the following prior art documents: US Patent No. 3,939,320 describes an energy feeding arrangement that includes a resonant coupling structure disposed near a centrally located feeding point in the top of the oven cavity. This resonant structure is in the shape of a short hollow cylinder of conductive material which rotates eccentrically to spread an energy beam in many directions and which is contained in a "feed box" to produce energy reflections and facilitate protection by means of a cover permeable to microwaves. U.S. Patent No. 4,133, 977 describes an arrangement for feeding energy to the oven cavity by means of rotating stirrers contained in "feed boxes" that are mounted to each of two opposite side walls. This "dual feed" arrangement is used to achieve a more uniform heating of the load in the oven cavity. Microwave energy is supplied to the feed boxes through a waveguide from a single magnetron resulting in an arrangment that is complicated and expensive. U.S. Patent No. 3,993,886 describes another type of dual feed arrangement including a flared waveguide into which microwave energy from a magnetron is fed at its smaller end. Microwave energy from several resonant modes of the TE-type is fed from the flared waveguide into the oven cavity through two coupling apertures at its wider end, in which apertures small rotating stirrers are placed for further improving the energy distribution in the oven cavity. U.S. Patent No. 4,249,058 describes a similar dual feed arrangement that utilizes, however, rotating stirrers of a considerably larger diameter. An attempt at realizing a dual feed arrangement of more compact dimensions is found in U.S. Patent No. 3,439,143. This arrangement comprises a rectangular waveguide to feed microwave engery from a magnetron to the oven cavity through two coupling apertures on both sides of a reflector of conductive material located within the waveguide and mounted off-axis to a rotating support of material that is transparent to microwaves. However, this compact arrangement does not achieve the advantage of that according to U.S. Patent No. 3,993,886 producing several simultaneously existing resonant modes in the waveguide, nor the advantage of the arrangement according to U.S. patent no. 4,249,058 having rotating stirrers covering a substantial area of the top wall of the oven cavity. U.S. Patent No. 3,814, 890 describes an arrangement for feeding microwave energy from a magnetron directly into the oven cavity without using an intermediate waveguide. This arrangement comprises a large feed box in the shape of a truncated cone having an aperture at its truncated end through which the radiating portion of the magnetron extends, but it does not comprise rotating antennas or stirrers for improving the energy distribution in the oven cavity. The above cited U.S. 4,327,266 discloses a microwave oven having a feeding system for feeding microwave energy into the cavity via a feeding opening in one of the cavity walls. In order to improve the energy distribution in the cavity a rotating field stirrer is provided in front of the feeding opening, said stirrer being so shaped that it covers the opening. The field stiffer comprises a conductive plate having on one hand a radiation slot positioned close to the center of rotation of the plate and consequently close to the feeding opening in the cavity wall, and on the other hand a number of vains positoned far away from the feeding opening. The plate is positioned in the cavity at a distance from the cavity wall, such that the microwave energy may propagate between the cavity wall and the plate towards the circumference of the plate. Accordingly, a part of the energy radiates directly into the cavity via said centrally positioned slot. The remaining part of the energy propagates in the space between the plate and the cavity wall towards the circumference of the plate, at which position also this part radiates into the cavity. The energy propagation of the last mentioned part in the space between the plate and the cavity wall takes place radially in all directions from tile centrally positoned feeding opening. This known microwave oven accordingly has a feeding arrangement having a relatively simple and inexpensive construction, but does not offer the advantages of the above descriped "dual feed". It is an object of the present invention to provide a mechanically simple, inexepensive and spacesaving arrangement for feeding energy from a microwave source into the microwave oven cavity having the advantages of a dual feed arrangement without using conventional intermediate waveguides. According to the invention a microwave oven to this effect is characterized in that the arrangement further comprises: an elongate groove-shaped recess in one of said conductive cavity walls having a microwave energy feeding point directly connected to the microwave source; and that the substantially plane, rotatable conductive plate is arranged in the oven cavity facing the recess and immediately adjacent to said one conductive cavity wall, said plate being shaped and dimensioned so that, at least during a part of a single rotation feeding passages a part formed at both ends of the recess, which feeding passages allow a periodically varying transmission of microwave energy from the recess into the oven cavity. The combination of the elongate groove-shaped recess with the conductive plate arranged immediately adjacent to the cavity wall will form a TE₁₀-type waveguide-like structure that is supplied with microwave energy at a centrally located feeding point and, owing to the fact that the plate is shaped so that feeding passages providing a periodically varying transmission of microwaves are formed at both ends of the elongated recess, a dual feed arrangement has been obtained in a very simple manner without using any waveguide system proper. Suitably, the recess has a substantially rectangular cross section and is terminated at both ends by oblique portions that terminate at and extend to the cavity wall. By these oblique portions at the ends of the recess, where the energy radiates into the cavity, the energy transfer from the recess to the cavity will be facilitated. The essentially plane plate of conductive material is preferably rotatably mounted about an axis of rotation that is located centrally in the recess and substantially coincides with the microwave engery feeding point in the recess, and this plate has at least two diamatrically opposite sections which, when these sections pass each end of the recess, provide for mutually different passages between the recess and the cavity that vary periodically with the rotation of the plate. Besides serving as rotating antenna, the rotating plate will also cause a resonance balance variation and a wobbling of certian cavity resonant modes because the radiating feeding zones will move somewhat with the antenna rotation. Suitably, the conductive plate or antennae can be shaped so that one of the said sections is bounded by an outer edge which is closer to the axis of rotation than the ends of the elongated recess, resulting in that a passage is formed between the said outer edge and the end of the elongated recess when the relevant section passes the recess. Preferably, the plate has also at least one feeding aperture which is close to the axis of rotation and through which microwave energy will continuously be fed into the cavity. This way of realizing the plate with two diamectrically opposite feeding passages, one of which is simply bounded partially by an outer edge of the plate, and with a centrally located feeding aperture will result in a maximally simple antenna plate construction, which nevertheless has proved to give a very good energy distribution in the cavity. If the antenna of the magnetron projects into the recess, then reactive pins, known as "stubs", can be arranged in the bottom of the recess on each side of the feeding point. In a preferred embodiment, the reactive pins are formed by the fastening screws for the magnetron or extensions of these screws. The invention will now be described in more detail, by way of non-limitative example, with reference to the accompanying drawings, in which: Fig. 1 shows a vertical sectional view of a microwave oven comprising a feeding arrangement according to the invention; Fig. 2 shows a plan view of the same oven seen from above; Fig. 3 shows a sectional view of a part of the cavity top walls in which a groove-shaped recess is made as component of the feeding arrangement according to the invention; and Fig. 4 shows a plan view in enlarged scale of the rotatable antenna plate. In Figs. 1 and 2, reference numeral 10 designates an oven cavity bounded by conductive walls, reference numeral 11 designates a bottom shelf on which food 12 to be heated is placed, reference numeral 13 designates a door which gives access to the oven cavity 10 and reference numeral 14 designates a magnetron having an antenna 15. According to the invention, the magnetron 14 is mounted on the outside of an elongated groove-shaped recess 16 in the cavity top wall 17 so that the antenna 15 projects into the central part of the recess. As is apparent from Fig. 2, the recess 16 has a substantially rectangular cross section, but alternatively the recess 16 can have a cross section with essentially oblique side walls 18 and 19, as shown in Fig. 3. At both ends, the elongated recess 16 is terminated by two oblique end portions 20 and 21 that continue in the top wall 17, as shown in Fig. 1. In front of the recess 16 there is a plane conductive plate 22 that is rotatably journalled about a rotation shaft 23 protruding from aprotective envelope 24 permeable to microwaves. The plate 22 is driven by cooling-air for magnetron 14 that supplied through entrance apertures 25 and exhausted through exit apertures 26 in the cavity top wall 17 (Fig. 2). For that purpose the plate 22 provided with microwave-transparent wings 27 on which the air-flow impinges so as to cause the plate 22 to rotate. In the example shown, the axis of rotation coincides with the centre of the recess 16 and thus with the feeding point for the microwave energy that is formed by the magnetron antenna 15. The conductive plate 22 is arranged very close to the cavity top wall 17 and covers the open side of the groove-shaped recess 16. The combination of the recess 16 with the plate 22 will therefore form a waveguide-like structure that fed centrally via the antenna 15 and in which the microwave energy propagates towards both ends of the recess 16. The plate 22 is shaped so that, during each revolution, time-varying coupling passages are formed between the recess 16 and the oven cavity, through which passages microwave energy is fed into the cavity. In the example shown, the fastening screws 28, 29 for the magnetron are provided with extensions 30, 31 and these screws are positioned and dimensioned so that they serve as reactive impedance matching elements, known as "stubs", for the magnetron 14. An example of a suitable shape of the antenna plate 22 is shown in Fig. 4. According to Fig. 4, the antenna plate, which is suitably manufactured by punching of a conductive sheet, consists of four wings 32, 33, 34, 35 which all produce coupling between the groove-shaped recess and the cavity when passing the recess but which are all mutually different. The two wings that are substantially responsible for the energy transfer to and energy distribution in the cavity are the wings that, at a given moment, are situated opposite to the ends of the recess 16. With the antenna position shown in the figure, where the longitudinal axis of symmetry of the recess 16 is indicated by a broken line 36, it is the wings 32 and 34 that are responsible for the energy transfer. One of these wings, wing 34 in the figure, consists simply of a short piece of sheet, the outer edge 37 of which is so close to the centre 38 of rotation that, when this wing 34 coincides with the elongated recess, a feeding passage will be formed between the edge 37 and the relevant end of the elongated recess. The opposite wing 32 has a feeding passage 39 that extends approximately to the relevant end of the elongated recess when the wing 32 coincides with the recess. At the central part of the plate, this feeding passage 39 continues in an arcuate slot 40 that curves around the center 38 of rotation and that also serves as a feeding aperture. The two other wings 33 and 35 are provided with mutually different feeding passages 41, 42 which are located relatively close to the periphery of the plate 22 and will lie relatively close to the ends of the elongated recess when these wings 33 and 35 coincide with the recess. In all positions of the antenna plate 22, the feeding arrangement operates so as to propagate microwave energy in the elongated recess 16 from the centrally located feeding point, which coincides with the axis of rotation (shaft 23), towards the ends of the recess 16, where the microwave energy will be transferred to the oven cavity to the extent that any of the openings in the plate 22 coincides with the elongated recess. Owing to the rotation of the plate 22, the size and position of the feeding passages will vary continuously with time, resulting in a complex excitation of the cavity and thereby in a good energy distribution. The central feeding slot 40 continuously radiates energy into the cavity, but also this feeding is time-variable owing to the changing position of the slot during the rotation.

Claims: A microwave oven including an arrangement for feeding microwave energy from a microwave source (14) mounted external of an oven cavity (10), said cavity being bounded by a plurality of conductive walls into the interior of the oven cavity, the arrangement comprising a substantially plane, rotatable conductive plate (22) arranged within the oven cavity, characterized in that the arrangement further comprises: an elongate groove-shaped recess (16) in one of said conductive cavity walls (17) having a microwave energy feeding point (15) directly connected to the microwave source (14); and in that said substantially plane, rotatable conductive plate (22) is arranged in the oven cavity facing the recess (16) and immediately adjacent to said one conductive cavity wall (17), said plate being shaped and dimensioned so that, at least during a part of a single rotation feeding passages (37, 39, 40, 41,42) are formed at both ends of the recess, which feeding passages allow a periodically varying transmission of microwave energy from the recess into the oven cavity. A microwave oven as claimed in claims 1, characterized in that the recess (16) is terminated at both ends by oblique portions (18,19) that terminate at and extend to said one cavity wall (17). A microwave oven as claimed in claims 1 or 2, characterized in that the microwave energy feeding point (15) is located centrally in the recess. A microwave oven as claimed in any of the claims 1-3, characterized in that the recess (16) has a substantially rectangular cross section. A microwave oven as claimed in any of the claims 1-4, characterized in that said substantially plane conductive plate (22) is rotatable mounted about and axis of rotation (23) located centrally in the recess (16) and substantially coinciding with the microwave energy feeding point (15), and in that said plate comprises at least two diametrically opposite sections (33, 35; 32, 34) having mutually different feeding openings (41, 42; 30) which are positioned and dimensioned so as to allow transfer of microwave energy from the recess to the oven cavity at both ends of the recess (16) when said two sections coincide with the recess during the rotatory movement of said plate. A microwave oven as claimed in claim 5, characterized in that one of said sections (34) is bounded by an outer edge (37) which is closer to the centrally located axis of rotation (23) than the ends of the elongated recess, so that a feeding passage is formed between said outer edge and the relevant end of the recess when said one section passes the recess. A microwave oven as claimed in claims 5 or 6, characterized in that said conductive plate also has at least one feeding aperture (40) which is close to the axis of rotation (23) and through which microwave energy is continously fed into the oven cavity. A microwave oven as claimed in any of the claims 1-7 in which the microwave source is formed by a magnetron characterized in that impedance matching stubs in the shape of reactive pins are arranged in the bottom of the recess on opposite of the microwave energy feeding points. A microwave oven as claimed in claim 8, characterized in that said reactive pins are formed by tile fastening screws for the magnetron or extensions of said screws.

Inventors: CLAESSON, JAN SIGVARD, RISMAN, PER OLOV GUSTAV
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN, PHILIPS NORDEN AB
Application year: 1988
Main classification: H05B6
IPC codes: H05B6