Background of the Invention A substantial problem associated with known micro- wave ovens is a tendency to uneven heating of the load.

One reason for this tendency is the varying relative <BR> <BR> <BR> effective permittivity, herein designated E, of the load.

Another reason is the existence of what is usually called cold and hot spots. Yet another reason is the existence of an edge-heating effect.

In order to obviate the above-mentioned problem, various types of field stirrers and/or a rotatable load carrier in the oven cavity have been proposed.

Object of the Invention The object of the present invention is to provide improved microwave heating in a microwave oven, whereby, in particular, the need to use rotating load carriers or field stirrers is eliminated.

Summary of the Invention According to the invention, the above-mentioned object is achieved by a microwave oven, a heating method and a microwave usage as defined in the appended claims.

The invention is thus based on an understanding of the advantages of providing a resonant quarterwave mode, preferably of the TM type (in the load), in a preferably rectangularly parallelepipedal cavity of a microwave oven. It has been found that such a resonant quarterwave mode provides good heating throughout the surface of an extended load, when the load is placed somewhat spaced

from the bottom of the cavity in the usual manner. More- over, surprisingly, it has been found that there is no pronounced edge-heating effect. Thus, according to the invention, the field, seen vertically, ends in a quarter- wave in the lower part of the cavity. Above this, there may be one or more halfwaves. It has been found advanta- geous to provide the quarterwave without any halfwaves, i. e. with a vertical mode index which is 1/2.

It will be appreciated that, in order for the reso- nant quarterwave mode to occur, the oven cavity must have suitable dimensions therefor, and, in addition, the wave impedance of the mode inside the load must be higher than it is outside the load. The latter means that the reso- nant quarterwave is generated when the e of the load is low, typically between about 4 and about 40. Since a load <BR> <BR> which is deep-frozen has low c, this, in turn, means that the resonant quarterwave is particularly suitable for use in connection with defrosting. In connection with"dry- ing"heating or"expanding"baking, the load will have increasingly lower E, which means that in these cases, too, it can be particularly advantageous to utilise a resonant quarterwave mode.

It should be noted that, when the cavity is suitably dimensioned, the resonant quarterwave mode can, in the <BR> <BR> <BR> case of a load with higher E, i. e. typically greater than 40, be made to gradually become a Brewster mode, i. e. essentially reflectionless at the load because of impe- dance similarity. This means that e. g. in connection with defrosting and continued heating of a deep-frozen load, <BR> <BR> which initially has very low e and as the defrosting and<BR> <BR> <BR> heating continues acquires increasingly high E, one may first obtain a resonant quarterwave mode and subsequently a Brewster mode, i. e. a very favourable heating process.

Accordingly, very good coupling of the field in the cavi- ty to the load is obtained, as well as high efficiency.

Naturally, the opposite process is also possible, e. g. in connection with baking, when one may start out

with high E and a Brewster mode, which will become a resonant quarterwave mode as the baking process pro- gresses and E is reduced.

The skilled person will appreciate that it is not necessary for the resonant quarterwave mode/Brewster mode to supply all the power to the load, but that it can be combined with other modes that may exist in the oven cavity.

In particular, it has been found suitable to com- bine the quarterwave resonant mode/Brewster mode with an additional mode which is a halfwave resonant mode or a Brewster mode, with the surprising result that the com- bination provides to an essential degree quadrature field patterns at the load. In this context, the term"quadra- ture"refers to the fact that the field patterns are such that they are added arithmetically rather than vectorial- ly. It has been found that this leads to the heating effect being considerably more even than is normally the case, i. e. the problem of cold and hot spots can to an essential degree be eliminated without any requirement for the load to rotate in the usual manner. This is because a cold spot in the load for one mode can overlap a hot spot of the load for the other mode and vice versa.

It has been found particularly advantageous to have as said additional mode a mode which for a load with low <BR> <BR> <BR> e is a Brewster mode and for a load with high E is a nor- mally resonant halfwave mode, but with the same horizon- tal mode index. Just like in the case of the resonant quarterwave mode/Brewster mode, the additional mode will then automatically switch between said two modes when the E of the load increases and decreases respectively.

Surprisingly, it has been found that in the case of a mode combination of the kind mentioned above, the quad- rature relationship is substantially maintained during a heating process when the E of a load goes from a low value to a high value or vice versa. This results in

particularly uniform heating of the load throughout the heating process, as well as high efficiency.

Also surprisingly, it has been found that for a mode combination according to the invention, the heat- ing effect becomes less sensitive than usual to the load height, i. e. the thickness of the load.

When a mode combination according to the invention is utilised, it is preferable that it supplies a substan- tial part, preferably the main part, of the power to the load, but it will be appreciated that other modes could also exist in the oven cavity, in particular where small loads are concerned.

It has been found possible to excite the two modes of the mode combination according to the invention simul- taneously with the aid of a single means, preferably a single cavity feed port, which in this connection should be slot-shaped and can be located in the connecting area between a cavity side wall and the cavity ceiling.

However, it is preferred to excite the two modes of the mode combination according to the invention with the aid of two separate cavity feed ports, which are located at the top and the bottom respectively of the oven cavi- ty. The upper port is used as the main feed port and the lower port is used for transmitting power in ways which favour the modes excited by the upper port. This requires the desired modes generated by the respective port to be suitably phase-shifted in relation to each other. Accord- ing to the invention, this can advantageously be achieved by the utilisation of a resonant area in the device feed- ing the two feed ports. In this way, the required phase difference between the mode fields excited by the ports can be obtained by means of a suitable difference in the feeding route from the resonant area feeding the two ports.

It has been found that the utilisation of double feed ports makes it possible advantageously to influence the power balance of the modes in the special mode combi-

nation in the oven cavity and consequently also the uni- formity of the heating effect achieved. Such influence can take place by temporary controlled blocking of the excitation from the lower feed port, while maintaining the feeding of microwaves to the upper feed port.

As stated above, the dimensioning of the oven cavity will affect the achievement of desired modes in the same.

It has been found that the cavity dimensions are rela- tively sensitive when it comes to achieving the effects according to the invention, and the skilled person may be obliged to carry out a number of attempts with varying dimensions before obtaining modes with the desired rela- tive strength.

To sum up, it is evident that there are a number of aspects of the present invention.

According to a first aspect of the invention, a microwave oven is thus provided comprising a preferably at least essentially rectangularly parallelepipedal oven cavity with a cavity bottom for receiving a load which is to be heated, and with means for feeding microwaves into the oven cavity, which means comprise a feed port in the ceiling and/or side wall of the oven cavity, the oven cavity being dimensioned to provide a resonant quarter- wave mode therein when the wave impedance of the mode inside the load is higher than it is outside the load.

The oven cavity is preferably dimensioned to also provide a resonant halfwave mode or a Brewster mode in a quadra- ture field pattern in combination with the quarterwave mode at the load.

In a preferred embodiment, the microwave oven has two feed ports arranged in a cavity side wall, of which feed ports a first one is arranged at the ceiling of the cavity and a second one is arranged adjacent to the bot- tom of the cavity, and a resonant waveguide device is arranged for feeding microwaves to said two feed ports, the waveguide device being dimensioned to provide such a phase difference between the microwave feeds from the two

feed ports that the mode field excited from said second feed port favours the mode field excited by said first feed port, in order to provide the desired mode balance.

According to a second aspect of the invention, a method is provided for heating a load in the cavity of a microwave oven with the aid of microwaves, comprising the generation in the cavity of a first mode which, in the <BR> <BR> <BR> case of a load with low e, typically lower than about 40, is a quarterwave resonant mode. A preferred embodiment of the method also comprises the generation in the cavity of an additional mode, which is a halfwave resonant mode or a Brewster mode, so that the combination of it with the quarterwave mode provides quadrature field patterns at the load. Said additional mode is preferably a Brewster <BR> <BR> <BR> mode in the case of a load with low c. The Brewster mode is advantageously generated so that in the case of a load with higher e, it will be a normal resonant halfwave mode.

According to a particularly preferred embodiment of the method, the excitation of the quarterwave resonant mode and the additional mode is effected with the aid of two separate cavity feed ports located at the top and the bottom respectively of the oven cavity, the upper feed port being utilised as the main feed port and microwaves being fed to the lower feed port with such a forced phase difference in relation to the upper feed port that modes excited by the upper feed port are favoured. In this con- nection, there may advantageously be controlled and intermittent blocking of the feeding of microwaves to the lower feed port for influencing the power balance of the modes in the oven cavity.

A third aspect of the invention comprises the use of a resonant quarterwave mode in the cavity of a micro- wave oven for heating a load with low E, such as initial defrosting of a deep-frozen load or final heating of a load with decreasing E as a result of the heating.

A fourth aspect of the invention comprises the use of a resonant quarterwave mode in combination with a resonant halfwave mode or a Brewster mode in phase quad- rature in the cavity of a microwave oven for heating a load with at least temporarily low e.

The invention will be described in more detail below by means of exemplifying embodiments thereof with refe- rence to the accompanying drawings.

Brief Description of the Drawings Fig. 1 is a schematic perspective view showing an outline of a microwave oven with an oven cavity and its associated microwave feeding system according to an embo- diment of the present invention.

Fig. 2 is a schematic vertical sectional view of the oven cavity with the associated microwave feeding system in Fig. 1.

Fig. 3 is a schematic horizontal view of the ceil- ing area of the oven cavity illustrating an alternative arrangement of a cavity feed port and providing an out- line of the H field patterns of a mode combination according to the invention at the top of the cavity.

Fig. 4 is a schematic horizontal view of the bottom area of the oven cavity with an outline of the H field patterns of the same mode combination at the bottom of the cavity. In addition, the existence of cold spots is indicated.

Fig. 5 is a simplified schematic vertical view of the same kind as in Fig. 2 with an outline of a momentary pattern of the vertical E field with areas of maximum and minimum field strength respectively indicated.

Description of Embodiments Thus, Figs 1 and 2 schematically show a microwave oven and its associated microwave system according to an embodiment of the invention. The microwave system comprises an oven cavity 1, a waveguide device 2 applied

to one side wall 10 of the oven cavity, on one side of which waveguide device there is a bulge 3 with a hole for inserting a coupling antenna 9 of a microwave source comprising a standard magnetron 8 with a frequency of 2.46 GHz. In the load zone of the cavity there is a bot- tom plate 5 transparent to microwaves on which the load 11, e. g. a foodstuff in a preparation container 13, is placed raised above the cavity bottom 15 during cooking.

A microwave oven also comprises a power-supply unit driven by line voltage and generating high tension vol- tage to the magnetron, as well as control means for con- trolling the power-supply unit with respect to, inter alia, cooking time and power levels. The power-supply unit and said control means are of the standard type in microwave ovens and have been omitted for the sake of simplicity since they lie outside the scope of the inven- tion.

Fig. 2 thus shows a part-sectional side view of the cavity 1 with the waveguide device 2 upon which a magne- tron 8 with a coupling antenna 9 is mounted. In the embo- diment shown, the waveguide device 2 and the cavity are integrated, whereby the broad dimension of the waveguide facing the cavity is formed by a corresponding part of the side wall 10 of the cavity.

In the side wall 10 of the cavity there are a lower and an upper feed port 16 and 17 respectively, which are slot-shaped and communicate with the waveguide device 2, for feeding microwaves from the magnetron to the cavity 1.

The waveguide device is dimensioned so that it is resonant for the purpose of providing advantageous field patterns in the cavity. More detailed information in this regard is provided in Applicant's US Patent Spe- cification 5,237,139, which is herewith incorporated by reference. The waveguide device is dimensioned so that there is a phase difference between the field which is excited by the feed port 17 and the feed port 16 respec-

tively. In the present example, the difference can be in the order of 45°.

The waveguide device comprises a vertically arrang- ed, rectangular waveguide 21 which feeds microwave power to the port 16. In the waveguide, a controllable micro- wave-blocking member 23 is rotatably arranged spaced from the port 16. Advantageously, the member is of the type shown in Applicant's Swedish Patent Application 9703528-1, which is herewith incorporated by reference.

The axial direction of the member 23 is thus perpendicu- lar to the plane of the Figure and the member is indicat- ed in the position in which it does not block the trans- portation of the microwave power through the waveguide 21.

The member 23 is suitably rotated about its horizon- tal axis to the blocking position with the aid of a step- ping motor or the like (not shown), which suitably can be controlled by the usual control means of the microwave oven as desired for influencing the power balance in the cavity. The skilled person will appreciate that any other suitable controllable blocking means could be utilised for controlling the feeding of microwaves from the feed port 16 into the oven cavity.

According to a preferred embodiment of the inven- tion, the at least essentially rectangularly parallel- epipedal oven cavity 1 is dimensioned to provide a reso- nant quarterwave mode of the TM43 type in the load in com- bination with an additional TM51 mode. With a microwave frequency of 2.46 GHz and a desired phase difference at the load of about 90° in order to provide an optimal quadrature field pattern, the oven cavity should have an effective height of about 180-200 mm. The effective height refers to the height between the load and the ceiling of the oven cavity. Said height dimension corre- sponds to the wavelength of the TM43 mode, which is about 760 mm, i. e. about 4 times the effective height, and to the wavelength of the TM51 mode, which is about 380 mm,

i. e. about twice the effective height. In other words, the height measurement corresponds to a quarter wave- length for the TM43 mode and half a wavelength for the TM51 mode ; i. e. the conditions are met for a 90° phase difference at the load.

With respect to the horizontal cross-sectional dimensions of the oven cavity, it has been found that a width of about 330 mm and a depth of about 280 mm pro- vide the desired conditions for the two modes. Said width dimension and depth dimension should be considered guid- ing values, which can be averages in the event that one cavity wall slopes somewhat in accordance with the de- scription in Applicant's Swedish Patent Application 9700448-5, which is herewith incorporated by reference.

The H fields associated with the modes of the TM43 and TM51 types are schematically illustrated in Figs 3 and 4 by means of the associated field pattern at the cavity ceiling (Fig. 3) and at the load (Fig. 4) respectively.

The mode (4 ; 3) is represented by circles 31 and the mode (5 ; 1) by ellipses 33.

Fig. 4 also illustrates by means of shaded areas 35 the locations where cold spots are obtained. As can be seen, these spots are relatively faint and, in the cen- tral area of the oven cavity in particular, the heating pattern is exceedingly even. Moreover, in the case of very low E, it has been found that the vertical E field provides a heating effect which further reduces the occurrence of cold spots.

Furthermore, Fig. 3 illustrates an alternative to the double feeding shown in Figs 1 and 2. In this alter- native, there is a single feed port in the form of a rec- tangular feeding slot 41, which is centrally located in the ceiling of the cavity directly adjacent to the cavity side wall and has its longitudinal extent in the depth direction. The slot can be fed in the usual manner by the intermediary of a feeding waveguide located on the ceil- ing of the oven cavity, through which the two desired

modes have thus been excited simultaneously. The slot 41 can typically be about 70 mm long and 15 mm wide.

Fig. 5 very schematically shows a pattern of the momentary vertical E field in a central vertical section in connection with double feeding of microwaves according to Figs 1 and 2 and under time-harmonic conditions. Areas 51 with maximum field strength and areas 53 with minimum field strength are indicated. The Figure clearly shows the phase difference between the field excited by the upper feed port 17 and the field excited by the lower feed port 16.

In addition to the blocking control made possible with the aid of the member 23, the special double feed- ing of the oven cavity with a forced, specific phase dif- ference between the microwaves leaving the two feeding slots 16 and 17 also makes it possible to influence the balance between the excited modes. A reduced phase diffe- rence has been found to provide stronger excitation of the TM43 mode, and an increased phase difference has been found to provide stronger excitation of the TM51 mode.

This can, especially in combination with controlled tem- porary blocking of the feeding through the lower feeding slot, be used for achieving the desired distribution of the relative intensities of the modes in the load which is most favourable to heating. A suitable distribution of the intensities is that they be about equal.

The invention is not limited to the embodiments described above. Changes and modifications are possible within the scope of the appended claims.