Electric heating – Microwave heating – Field modification
Reexamination Certificate
2000-10-27
2001-12-25
Leung, Philip H. (Department: 3742)
Electric heating
Microwave heating
Field modification
C219S748000, C219S695000, C333S230000
Reexamination Certificate
active
06333496
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to microwave feeding of a cavity in a microwave oven employing an elongated coupling slot made in a cavity wall, which slot is fed from a waveguide externally connected thereto.
TECHNICAL BACKGROUND
The dimensioning of coupling slots for microwave feeding of the kind mentioned above often causes a number of problems. The achievement of good fixing, that is, good galvanic contact, between the feeding waveguide and the cavity wall can be problematic because of inaccessibility for spot welding means which at the time of manufacture have to be inserted into and behind the slot opening in connection with the spot welding of the waveguide to the cavity. As will be appreciated, this leads to constructive limitations.
For technical microwave reasons it is often desirable that the width (so-called a-dimension) of the feeding waveguide be maintained in the whole slot area, despite the fact that the width (a-dimension) of the selected coupling slot is significantly smaller than the a-dimension of the waveguide. This can cause problems with the required spot welding in view of the plate bending desired in connection with manufacturing, as well as with concentrations of conduction current in the cavity wall plate adjacent to the slot in the area between the respective slot ends and the side wall connection associated with the waveguide. The latter is particularly pronounced in connection with simple coupling slots of the so-called H-type and when TE
10
mode fields are employed.
OBJECTS OF THE INVENTION
A main object of the present invention is to make possible microwave feeding employing a relatively small coupling slot while achieving essentially the type of field pattern which can be provided by a slot with a small a-dimension, despite the fact that the feeding waveguide employed has a considerably larger a-dimension than the coupling slot employed.
Another object of the present invention is substantially to obviate the problems described above.
A further object of the present invention is to make possible microwave feeding with increased flexibility with respect to the constructive design of the coupling slot in the cavity wall and the connection thereto of the feeding waveguide.
Yet another object of the present invention is to make possible microwave feeding which provides an additional degree of constructive freedom with respect to the action on the wall current distribution in the slot area and thereby the excitation of the cavity and the heating pattern for certain loads.
SUMMARY OF THE INVENTION
The above-mentioned objects are achieved by a device and a method which exhibit the features stated in the appended claims.
The basis of the invention is thus that it has been found possible with simple means to affect the current distribution in the cavity wall areas on each side of the microwave radiating slot part out towards the respective side walls of the feeding waveguide. Such action can be achieved according to the invention by the slotting of said areas from the radiating slot part, whereby such current paths as would exist in the absence of slotting are cut off, which leads to a more even current distribution without current concentrations occurring in the areas in question.
It has been found that such a changed current distribution does not affect the excitation of the cavity fields unfavourably, but that the risk of overheating and major current losses in the areas in question is significantly reduced. In addition, said action can be utilised to favourably adjust the excitation and the heating pattern of the cavity, as well as the impedance matching, with a basic configuration as the starting point. In other words, an extra degree of freedom can be obtained.
The slotting according to the invention can be seen as the arrangement of an extension of the radiating coupling slot, that is, of its main part, and is effected in such a way that no appreciable microwave radiation into the cavity is obtained directly from the extra slot areas achieved as a result of the slotting. In other words, these additional slot areas will have the nature of secondary surfaces to the radiating main surface of the coupling slot. The slots are thus made narrow, that is, with a small height- or cross-dimension. This is possible because the flash-over risk will be minimal, since the electric field (in the slot plane) is at its lowest level in the areas in question and increases towards the radiating main part of the coupling slot, something which applies to, for example, TE
10
mode fields.
Behind each secondary slot surface a “pocket” is created in the feeding waveguide which, depending on the geometry, will constitute an inductive or capacitive member, as well as a capacitive stretch between the cavity wall at the secondary slot surface and the opposite waveguide wall. Seen from the waveguide, the pocket will function as if it were at least substantially closed, that is, as if the extra slotting did not exist. This makes it possible to affect both the field image of the coupling slot and the wave impedance for the propagating mode of the waveguide.
With respect to the required galvanic contact between the cavity and the waveguide it has been found that the invention provides better constructive opportuities for spot connections with a mutual spacing which eliminates microwave leakage between the waveguide and cavity plates. In this connection, it is also important to note that when microwave fields of the TE
10
type are concerned, the power flux density which can arise at the secondary slot areas is relatively speaking very small, which further obviates any leakage problems.
According to a first aspect of the invention, a device for microwave feeding of a cavity in a microwave oven is provided, comprising an elongated coupling slot made in a cavity wall and a waveguide externally connected thereto, the coupling slot at the respective slot ends having a slot extension with a substantially smaller height- or cross-dimension in relation to the b-dimension of the main part of the coupling slot. In this connection, it is a preferred feature that the a-dimension of the main part of the coupling slot together with the length dimension of the slot extensions essentially correspond to the a-dimension of the connected waveguide.
Advantageously, the coupling slot has an essentially rectangular main part, from which a slot extension originates at each short end. The slot extensions can have an essentially constant cross-dimension, that is, have an essentially rectangular shape, or alternatively, in the direction away from the main part of the coupling slot, have a decreasing cross- or height-dimension.
Preferably, the slot edge of the cavity wall is rounded in the wall plane at least in an upper transition between the main part of the coupling slot and the respective slot extensions. This results in the formation of a configuration which can be compared to a “curtained” slot.
Although it has been found suitable that the respective slot extensions form an extension, projecting directly outwards, of the lower edge area of the main part of the coupling slot, the skilled person will appreciate that the slot extensions can be arranged at a different height relative to the slot, e.g. at the top (reversed “curtain configuration”) or symmetrically on the halfway point of the slot.
For a 2450 MHz microwave oven for domestic use, an effective slot cross-dimension, that is b-dimension, which typically is about 10 times greater than the cross-dimension of the slot extensions, has been found suitable.
According to a second aspect of the invention, a method is provided for feeding microwaves into the cavity of a microwave oven through an elongated coupling slot, which is fed by an external waveguide, employing a coupling slot whose effective a-dimension is significantly smaller than the a-dimension of the waveguide, which method comprises the adaptation of the coupling slot to the waveguide and the desired cavity excitation by the connection of a slot extension, whose cro
Carlsson Dan
Nordh Ulf
Risman Per Olov
Leung Philip H.
Rice Robert O.
Roth Thomas J.
Van Winkle Joel M.
Whirlpool Corporation
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