WO1999017588A1

WO1999017588A1 - Controlling the feeding of microwave power through a waveguide

Info

Publication number: WO1999017588A1
Application number: PCT/EP1998/006149
Authority: WO
Inventors: Ann-Marie Andersson; P.-O. Risman; Sven Isaksson
Original assignee: Whirlpool Corporation
Priority date: 1997-09-29
Filing date: 1998-09-28
Publication date: 1999-04-08
Also published as: SE9703528L; SE9703528D0

Abstract

A method and a device for controlling the feeding of microwave power through a waveguide in a microwave heating appliance. A conductor member (31) is movably arranged in the waveguide (32) between an inactive position with no appreciable effect on the feeding of microwave power and an active, resonant position, in which resonance currents are generated, which give rise to a changed mode pattern which acts upon the transportation of the microwave power through the waveguide. The member (31) is preferably an elliptic ring member. In this connection, repositioning is effected by means of controlled rotation about an ellipse axis.

Description

CONTROLLING THE FEEDING OF MICROWAVE POWER THROUGH A WAVEGUIDE

Field of the Invention

The present invention relates to controlling the feeding of microwave power through a waveguide in a microwave heating appliance, in particular a microwave oven intended for domestic use which is provided with a waveguide-fed oven cavity.

Background of the Invention

It is well known to control the feeding of microwave power by means of a rotatable so-called flag member from a generator arm to two or more load arms of a multi- way waveguide. The flag member is usually a simple metal plate which can be considered a reflecting member.

In connection with microwave ovens, it is also well known to use so-called stirrers, which act on the various combinations of cavity modes which can exist in the oven cavity. As a rule, the stirrers consist of movably arranged metal plates or wings or combinations thereof, which are arranged adjacent to the feeding opening of the cavity, usually inside the latter. Furthermore, it is known to employ movable completely blocking or short-circuiting metal members, which are controllably insertable into a waveguide, in order to control the feeding of microwave power through the waveguide, something which has not been widely used because of the risk of microwave leakage and the flash-over risk. In the above applications, the metal members utilised are usually relatively large and require fairly large waveguide dimensions. In addition, it has been found that there are often problems with large currents in those parts of the metal members which are in galvanic but movable contact with a fixed waveguide part. On the other hand, if such galvanic contact is lacking, the intended action is usually unsatisfactory. The action achieved often turns out to be rather imprecise in the sense that several different actions are obtained simultaneously when the metal member is moved or rotated, for example impedance change with respect to both amplitude and phase, and change of electric length in a part of the system.

Finally, in feeding a circular waveguide in connec- tion with simultaneous mode transformation in the circular waveguide, it is known to use essentially two-dimensional annular resonators as filters to eliminate rotation asymmetry caused by defects in a waveguide transition . Object and Summary of the Invention

The object of the present invention is to permit the control of the feeding of microwave power through a waveguide in a manner such that the problems and limitations mentioned above are essentially obviated while, at the same time, other advantages are gained.

This object is achieved by a method, use, and a device exhibiting the features stated in the appended claims .

Thus, the invention is based on the insight that the transportation of the microwave power through a waveguide, i.e. the total microwave power, can be very advantageously controlled by the provision of resonance phenomena in the waveguide which create a mode-pattern change such that the desired action on the microwave power transportation is achieved. This can be effected by the controlled provision of a local resonance region in the waveguide for frequencies in the incoming microwave field, resulting in the generation of resonance currents, which counteract the incoming microwave field and give rise to a completely changed mode pattern which acts upon or affects the transportation of the microwave power through the waveguide.

With respect to the desired action upon the transportation of the microwave power, there are two advan- tageous main alternatives.

According to a first aspect of the invention, the controlled action means that the power feed through the waveguide is at least essentially completely blocked. In this connection, a mode pattern is created, i.e. a trans- formation takes place to a mode pattern in the waveguide, such that a blocking action is obtained for propagating waves in the waveguide. Such a mode pattern contains so- called evanescent modes which cannot propagate in the waveguide, the resonance location n the waveguide where they are generated being chosen so that the desired decay can occur. Accordingly, with respect to modes, said location should be separate from the connection of the waveguide to the subsequent work space, such as an oven cavity. In practice, this means that said location should be situated at least one or a couple of tenths of free space wavelengths from said connection.

It will be appreciated that this aspect involves an on-off control of the power feed.

According to a second aspect of the invention, the controlled action means that the microwave power is transported onwards with the completely changed mode pattern, which in this instance is utilised in the feeding of power to a multimode space. In this case, the mode transformation should take place adjacent to the connec- tion of the waveguide to the space, so that the least possible decay of the new modes occurs in the waveguide, before the new modes can propagate in the space. Alternatively, or as a complement, the waveguide can be made with a certain increase m its a-dimension (e.g. funnel- or horn-snaped) after the location of the mode transfor- mation in order to optimise the coupling of the new modes to said space. It will appreciated that the latter is adapted to the different mode patterns, i.e. to the mode pattern which exists without the controlled action and the mode pattern which exists after the controlled action.

It will be appreciated that this aspect involves controlled change between different mode patterns in the work space, typically a multimode oven cavity, with no substantial action upon the power level.

According to the invention, the incoming microwave field is advantageously of the TEι₀ type, the resonance- controlled action advantageously consisting of a transformation to TE₃₀ type and/or TE₂₀ type. The invention is particularly advantageous in conjunction with a waveguide with a rectangular cross-section, especially one with a small height (i.e. b-dimen- sion) relative to its width (i.e. a-dimension) .

The preferred method according to the invention of providing a resonance phenomenon, i.e. a local resonance region in the waveguide, involves the controlled generation, in the waveguide, of a current path resonant at least for the frequencies in the incoming microwave field, which path can be completely separate from the walls of the waveguide but which can also partly comprise a waveguide wall part.

In this case, current path refers to the conditions necessary for a current to flow in a defined manner. The current path becomes resonant when the geometric charac- teristics of the path combined with the microwave field in question permit the generation of resonance currents in the current path.

According to the invention, current paths can be generated by means of a po≤itionable current path member, i.e. a member made of a conductive material whose confi- guration can define current paths, either alone or in combination with a waveguide wall part, and/or conditions enabling displacement currents. In a certain position, the current path member creates the conditions necessary for resonant current paths, which are advantageously loop-shaped and surround capacitance-creating cavities, as will be described in more detail below.

It may be advantageous to create the conditions necessary for resonant current paths, i.e. to bring the conductor member to the resonant position, during a temporary interruption of the microwave feed in the waveguide .

According to the invention, a resonant current path can be achieved by changing the position of a current path member in the waveguide from an inactive position to an active position. In the inactive position, the current path member does not create the conditions necessary for the generation of resonant current paths, but rather the microwave field can propagate past the current path mem- ber with no substantial changes. However, in the active position, by virtue of well-defined dimensions, either exclusively its own or in coaction with a waveguide wall part, the current path member creates the conditions necessary for resonant current paths with the attendant generation of resonance currents and the resulting mode transformation. The skilled person will appreciate that, in principle, generating resonant current paths is a matter of adjusting the inductances and capacitances provided by the geometrical conditions to the frequencies in question (typically 2450 MHz for conventional microwave ovens) . For loop-shaped current paths the appearance of the loop area is of lesser importance, but the circumferential dimensions should be in relation to the wavelength. Advantageously, in its active position, the conductor member is designed to generate resonant current paths in a waveguide cross-sectional plane, the current paths being loop-shaped seen in said plane. In this connection, it has been found to be preferable for the conductor member to have a flat but clearly three-dimensional configuration, the active position meaning that the flat plane is lying transversely of, preferably perpendicular to, the longitudinal direction of the waveguide. The three- dimensional configuration means that, in its active position, the current path member has a certain extent in the longitudinal direction of the waveguide, preferably in the order of a tenth of a free space wavelength. As a result, improved current distribution is achieved, the flash-over risk is reduced, and the power capacity is increased, making it possible to control high microwave power levels.

The flat configuration with a certain but limited "thickness" of the conductor member permits the easy achievement of an inactive position by the conductor member being placed with its flat plane parallel to the longitudinal or axial direction of the waveguide, i.e. in the case of a rectangular waveguide either parallel to the a-dimension or parallel to the b-dimension. In both cases, the position of the conductor member can easily be changed from an inactive or an active position (and vice versa) by a simple 90° rotation about an axis (preferably a symmetry axis) which is parallel to the a-direction and the b-direction respectively. It will be appreciated that, in its inactive position, the conductor member will exhibit a proportionately very small surface seen in the longitudinal direction of the waveguide, which is the main reason for its insignificant action upon the microwave field propagating past it. This insignificant action can be compensated for in a conventional manner with the aid of adjustment members in the waveguide prior or subsequent to the conductor member.

When employing a conductor, member which coacts with a waveguide wall part in the active position in order to achieve resonant current paths, it is usually possible to achieve an inactive position by simply displacing the conductor member translationally a short distance away from the waveguide wall part in question. Alternatively, the conductor member can be rotatably arranged in rela- tion to the waveguide wall part, so that, by means of rotation (typically 90°), the position of the member is changed in relation to the microwave field in such way that resonances cannot occur.

The conductor member can advantageously have annu- lar or partly annular configurations, seen in its active position and in a waveguide cross-sectional plane, in view of the fact that it is loop-forming circumferential paths (particularly inner ones) which are active in achieving resonant current paths . Symmetrical configura- tions are preferred.

It has been found to be particularly advantageous to employ an annular conductor member which in its active position exhibits a generally elongated or flattened circumferential shape with rounded tapering ends, which cir- cumferential shape enables the formation of resonant current paths in an advantageous manner. An essentially elliptic circumferential shape is preferred. The skilled person will appreciate that a conductor member configuration of this kind is particularly well suited to use in a waveguide with a rectangular cross-section, it having been found that a small b-dimension of the waveguide is made possible without a risk of flash-over.

An alternative embodiment of a conductor member according to the invention, which is also particularly well suited to use in a waveguide with a rectangular cross-section, consists of a configuration in the form of a lying H, seen in the active position and in a waveguide cross-sectional plane, the two parallel parts of the conductor member being parallel to the a-direction of the waveguide cross-section. Advantageously, the rotation to the inactive position takes place about an axis parallel to the b-direction.

In a modification of the alternative embodiment mentioned above, the conductor member itself lacks one of the two parallel parts, i.e. it has a T configuration, and is spaced apart from the adjacent waveguide wall in its inactive position. In order to achieve the active position, the conductor member is moved to coaction with the waveguide wall so that the latter will form the iss- ing configuration part. The skilled person will appreciate that such a translatory movement forwards and backwards can be achieved very easily.

In cases like the above, when parts of the waveguide wall is utilised for the generation of resonant current paths, it is suitable to stop the microwave feed during the actual adjustment, i.e. during the movement of the conductor member, in order to avoid flash-over problems at the moment of electric coupling to or from the waveguide wall. In another modification of the above-mentioned alternative embodiment, which also has a current path member with a T-configuration, the current path member can be rotatably mounted in the coacting waveguide wall by means of the part of the member which is directed towards the wall. In this case, the current path member can easily be caused to rotate through 90° between its active position with its plane parallel to the cross- sectional plane of the waveguide and its inactive position with its plane in the longitudinal direction of the waveguide, i.e. about an axis parallel to the b-direc- tion .

By utilising the principles of the present invention it is possible to achieve a number of substantial advan- tages, such as

exceptionally good discrimination with at most about 5% transmission when blocking of the power feed is desired, and more than about 70% transmission (which can easily be increased to 100% by the utilisation of special, conventional adjustment members) when full power feed is desired;

essentially negligible losses when full power feed with mode transformation is desired;

small mechanical dimensions of the conductor member employed;

small mass of the conductor member so that its position can easily be changed, e.g. by means of an electromagnet or a stepping motor;

small "electric size", i.e. the field is acted upon only within a very small area around the conductor member, as a result of which very short waveguide pieces are sufficient for full functioning;

small risk of flash-over or a dangerous temperature increase despite high outputs and small waveguide height, the latter being very desirable in connection with microwave ovens in order to save space;

little action upon the microwave system; essentially no risk of microwave leakage.

The invention will be described in more detail below by way of exemplifying embodiments with reference to the accompanying drawings .

Brief Description of the Drawings Fig. 1 is a schematic sectional view of an oven cavity with an associated microwave feeding system in a microwave oven comprising an embodiment of the present invention.

Fig. 2 is a schematic view illustrating microwave feeding to an upper corner of an oven cavity utilising another embodiment of the invention.

Fig. 3 is a schematic cross-sectional view of a waveguide provided with a current path member according to a presently preferred embodiment of the invention, the current path member being in an active, resonant position .

Fig. 4 is a view of the same kind as in Fig. 3 but with the current path member in an inactive position. Fig. 5 is a schematic cross-sectional view of a waveguide provided with a current path member according to another embodiment of the invention, the current path member being shown in its active, resonant position. Fig. 6 is a schematic cross-sectional view of a waveguide provided with a current path member according to yet another embodiment of the invention, the current path member being shown in its inactive position.

Fig. 7 is a view of the same kind as in Fig. 6 but with the current path member shown in its active, resonant position.

Fig. 8 is a schematic cross-sectional view of a waveguide provided with a current path member according to a further embodiment of the invention, the current path member being shown in its active, resonant position. Description of Embodiments

Fig. 1 schematically shows the microwave system of a microwave oven, which system is provided with a waveguide-blocking device 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 device is a bulge 3 with a hole for inserting the coupling antenna 9 of the microwave source, which consists of a standard magnetron 8 with a frequency of 2.45 GHz (not shown in detail) . In the load zone of the cavity, there is a bottom plate 5 which rotates with the aid of wheels 12 and on which the load 11, e.g. a food item or a vessel containing a liquid, is placed and rotates during cooking. A microwave oven also comprises a power unit, which is actuated by mains voltage and which generates high tension voltage for the magnetron, and control means for controlling the power unit with respect to cooking time and power levels, for example. The power unit and said control means are of a type common in microwave ovens and have been omitted for the sake of simplicity since they lie outside the scope of the invention.

Thus, Fig. 1 shows a part-sectional side view of the cavity 1 with the waveguide device 2 on which a magnetron 8 with a coupling antenna 9 is mounted. In the embodiment shown, the waveguide device 2 and the cavity are adjoining, whereby the long side of the waveguide which faces 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 is a lower and an upper coupling opening 16 and 17 respectively, which are connected to the waveguide device 2, for feeding microwaves from the magnetron 8 to the cavity 1.

The waveguide device 2 is dimensioned so that it is resonant for the purpose of providing advantageous field patterns in the cavity. More detailed information on this subject is disclosed in the United States Patent Application No. 5,237,139, to which reference is hereby made.

The waveguide device comprises a vertically arrang- ed, rectangular waveguide 21 which feeds microwave power to the opening 16. In the waveguide, a blocking conductor member 23 is rotatably arranged spaced from the opening 16. Advantageously, the member is of the type shown in Figs 3 and 4, it being appreciated that, in this case, the axial direction of the member 23 is perpendicular to the plane of the Figure and that the member is indicated in its position blocking the transportation of microwave power through the waveguide 21.

The member 23 is suitably rotated about its hori- zontal axis with the aid of a stepping motor or the like (not shown in Fig. 1), which can suitably be controlled by the control means of the microwave oven as desired. It may, for example, be suitable to block the microwave feeding to the opening 16 when the load 11 is not rotat- ing.

Fig. 2 schematically illustrates another utilisation of a conductor member according to the invention. In the example shown, a magnetron 8 feeds, by the intermediary of its coupling antenna 9, a rectangular waveguide 25 leading to a coupling opening 27 situated at the top of the cavity 1. The final part of the waveguide has a decreasing height dimension, i.e. a wedge-shaped b-dimen- sion decrease for the purpose of adaptability. A conductor member 29 according to the invention is arranged immediately adjacent to the opening 27. In this case, too, the member 29 is preferably of the type shown in Figs 3 and 4. The axis of the member 29 is horizontal, but here the member is shown in its inactive position, i.e. in the position in which it does not have any appre- ciable effect on the transportation of power through the waveguide into the cavity 1. When the member 29 is rotated through 90° to its active position, something which is suitably effected with the aid of a stepping motor controlled by the control means (not shown) of the microwave oven, it achieves a complete change of the mode pattern which is fed into the oven cavity without any appreciable change of the power level by virtue of the fact that the member is located so close to the opening that there is no time for any noticeable decay to occur. It may be suitable to design the opening 27 so that it is also easier for the new modes to pass through it into the oven cavity. For the same purpose, the waveguide part in question, i.e. the final part, could also be provided with a gradually increasing a-dimension. It will be appreciated that, by means of controlled switching between mode patterns in the oven cavity, by means of a controlled change of position for the member 29, it is possible to achieve an improved heating result in the oven cavity. A number of configurations for the conductor member and their arrangement in a waveguide according to the invention will be described in more detail below with special reference to Figs 3-8.

As set forth above, it is advantageous to have con- ductor member configurations which in a well-defined manner permit the formation of resonant current paths. Moreover, symmetrical configurations are preferred.

In this connection, for use in a rectangular waveguide, a simple option is a conductor member in the shape of a short piece of a square tube with dimensions adapted to the a- and b-dimensions of the waveguide. It will be appreciated that this results in a configuration in the form of a ring or a closed loop surrounding a cavity, which provides desirable conditions for such inductance and capacitance as required for the formation of resonant current paths.

However, it is advantageous to depart from the angularly shaped configuration and provide the elongated con- ductor member with tapering and rounded ends . A preferred shape is essentially elliptic, as illustrated in Figs 3 and 4.

Figs 3 and 4 show an essentially elliptic or annular conductor member 31 rotatably arranged in a TEo waveguide 32, the major axis of the ellipse being centred in the waveguide and parallel to its a-direction and the minor axis of the ellipse also being centred in the waveguide and parallel to its b-direction. The member 31 is metallic and fixed to a bipartite metallic shaft 33 with its respective short ends 34, 35. The shaft 33 is made to protrude through matching holes in the opposite side walls of the waveguide and on one side it is mounted in a bearing 36 and on the other side it is coupled to a stepping motor 37 for controlled rotation of the member 31 with increments of 90°.

Fig. 3 shows the member 31 in its active, resonant position with the annular or flat plane transversely of and perpendicular to the longitudinal direction of the waveguide. Fig. 4 shows the member 31 in its inactive position achieved after a 90° rotation.

In the resonant position, the H field of the microwave field induces resonance currents which will travel in two loops, as shown in Fig. 3. Each current loop comprises current I which travels in half the conductor mem- ber ring and is closed by the displacement current δD/δt which in the middle of the ring bridges the free space between the upper and the lower part of the ring. In this connection, good resonance is ensured by the circumference of the ring being approximately equal to the wave- length of the microwave field. From the point of view of mode theory, the resonance phenomenon can be interpreted as follows. The resonance currents provide large vertical (in relation to Fig. 3) currents in the major axis ends of the elliptic loop, which give rise to two co-rotating H fields. In the middle, between these two, a third counter-rotating H field is created, since the distance between the major axis ends is "right" therefor. Thereby, a TE₃o mode is formed. However, the mode is highly evanescent in the waveguide in question, which creates a power-blocking action if the required decay distance is present. Alternatively, the new mode can keep propagating if the conditions after the mode transformation location are right therefor, e.g. the member 31 being located immediately adjacent to the cou- pling opening, i.e. as close to it as possible taking into consideration the fact that it should be possible to rotate the member 31 to its inactive position without interfering with the microwave-transparent cover which is usually present over a coupling opening. In the inactive position of the member in Fig. 4, no loop-end currents are induced because a balancing is achieved.

A substantial advantage of the configuration shown in Figs 3 and 4 is that no substantial currents are induced in the metallic shaft parts 33, and as a result there is no risk of microwave leakage. The reason why no current is induced is that the shaft is perpendicular to the E field direction of the microwave field.

The flattened ellipse configuration of the annular conductor member provides further substantial benefits.

The member is efficient, since it has greater capacitance with the attendant greater quality factor. By virtue of the resonance currents I being concentrated to the inside of the ring, a more "confined" resonance is obtained which lowers the risk of flash-over. The resulting total geometry is very suitable for a TEι₀ mode, i.e. the waveguide cross-section is well filled without the risk of flash-over increasing. The small height of the member permits the b-dimension of the waveguide to be small, and also makes it possible to place the conductor member very close to a coupling opening.

The following typical dimensions for a TEι₀ mode and a microwave frequency of 2450 MHz are provided by way of an example:

a-dimension of the waveguide: about 100-105 mm b-dimension of the waveguide: about 20-25 mm Length of the member: about 70-75 mm Height of the member: about 10 mm Depth of the member: about 10 mm

Wall thickness of the member: about 2 mm

The dimensions of the member relate to Fig. 3. With respect to the wall thickness, it may be suitable to increase it somewhat in the major axis ends because of the high current intensity present there.

Fig. 5 schematically illustrates an example of a conductor member 51 which is flat in shape with a lying H configuration. The member is symmetrically arranged in the waveguide rotatably about a vertical shaft 53 (i.e. a shaft parallel to the b-direction) . The shaft is made of a microwave-transparent and low loss dielectric material, such as a ceramic material. The direction of the shaft coincides with the direction of the web part 54 of the member. The lower part of the shaft 53 is made to protrude through the bottom of the waveguide 52 and is mounted in a bearing 55 and the upper part of the shaft is made to protrude through the top of the waveguide and coupled to a stepping motor 57, which functions in a way similar to that of the stepping motor 37 in Fig. 3 and which is employed to rotate the member from the active, resonant position shown to an inactive position aligned with the longitudinal direction of the waveguide.

As will be seen from Fig. 5, the member 51 forms two counter-directional U-shaped partial current paths I which can be closed for achieving resonant loop current paths by means of the displacement current δD/δt at the respective ends of the member.

Figs 6 and 7 schematically illustrate a modification of the embodiment according to Fig. 5, wherein a T-shaped conductor member 61 is employed which in its active, lowered position (Fig. 7) coacts with the waveguide bottom 62 in order to provide the desired lying-H-shaped configuration which permits resonant current paths. Accordingly, in the active position, the web part 63 of the member 61 connects with the waveguide bottom 62 in such a way that a current can flow between them. It will be appreciated that this can be achieved by metallic contact being ensured, it probably being suitable to effect such contact during an interruption in the feeding of microwave power in order to avoid flash-over effects.

However, in this connection, in the embodiment shown, it is also a matter of a displacement current, since the lower end of the web part 63 is fixedly mounted in a control member 65, which is vertically displaceably arranged in a corresponding hole in the waveguide bottom. The control member 65 is made of a microwave-transparent dielectric material, such as a ceramic material. Because of the fact that, here, the displacement current distance is substantially shorter than at the ends of the member 61 and that, in addition, it travels through a dielectric, the conditions for resonance are ensured.

For the purpose of achieving a controlled movement of the member 61 between its raised, inactive position and its lowered, active position, the control member 65 is connected to a control pin 67, which can be moved forwards and backwards with the aid of a controlled electromagnetic drive mechanism 69 coupled thereto.

Fig. 8 schematically illustrates a modification of the embodiment according to Figs 6 and 7, wherein the

T-shaped conductor member 71 is not translationally displaceably arranged but rather rotatably arranged in the waveguide bottom 72 with the lower part of the web part 73 in electric contact with the bottom 72. The lower part of the web part 73 is attached to a control member 75, also of a ceramic material, which is mounted on the drive shaft 77 of a stepping motor 79. The member 77 is thus brought to an inactive position by being rotated through 90° to a position aligned with the longitudinal direction of the waveguide.

Claims

1. A method for controlling the feeding of microwave power through a waveguide in a microwave heating appliance, in particular a microwave oven intended for domestic use, c h a r a c t e r i s e d by the controlled provision of a local resonance region in the waveguide for frequencies m the incoming microwave field, resulting in the generation of resonance currents, which counteract the incoming microwave field and give rise to a changed mode pattern wnich acts upon tne transportation of tne microwave power through the waveguide.

2. A method according to claim 1, c h a r a c - t e r i s e d in that in order to block the feeding of power through the waveguide, resonance currents are generated whicn give rise to mode patterns m the waveguide, which provide a blocking action for propagating waves in the waveguide .

3. A method according to claim 2, c h a r a c t e r i s e d in that evanescent modes are generated by means of the resonance currents, the resonance region being achieved in a location in the waveguide such that the evanescent modes can decay in the waveguide.

4. A method according to claim 1, c h a r a c t e r i s e d in that the changed mode pattern is created m connection to the coupling of the waveguide to a multimode cavity, so that propagating waves of the changed mode pattern can propagate into the multimode cavity with at least essentially unchanged power.

5. A method according to any one of the preceding claims, c h a r a c t e r i s e d in that the incoming microwave field is of the TEio type.

6. A method according to any one of the preceding claims, c h a r a c t e r i s e d in that said resonance region is achieved by generating, in the waveguide, at least one current path resonant at least for said frequencies, preferably essentially in a waveguide cross- sectional plane and with an extent in the longitudinal direction of the waveguide taking into consideration the controlled microwave power and the free space wavelength of the microwave field.

7. A method according to claim 6, c h a r a c t e r i s e d in that a resonant current path is gene- rated which is separated from the walls of the waveguide.

8. A method according to claim 6, c h a r a c t e r i s e d in that a resonant current path is generated which partly comprises a waveguide wall part.

9. A method according to any one of claims 6-8, c h a r a c t e r i s e d in that a resonant current path is generated by rotating a current path member in the waveguide from an essentially inactive position to an active resonant position, preferably through about 90┬░ and preferably about an axis parallel to the a-direction of a rectangular waveguide.

10. A method according to claim 8, c h a r a c t e r i s e d in that a resonant current path is achieved by displacing a current path member in the waveguide from an essentially inactive position to an active position with resonant coaction with a waveguide wall.

11. A method according to any one of claims 6-10, c h a r a c t e r i s e d in that one or more loop-shaped resonant current paths are generated.

12. A method according to any one of claims 6-11, c h a r a c t e r i s e d in that the feeding of microwave power in the waveguide is temporarily stopped during the generation of the resonant current path.

13. Use of a repositionably arranged, resonant conductor member in a waveguide in a microwave heating appliance, in particular a microwave oven intended for domestic use, in order to control the feeding of the microwave power through the waveguide.

14. Use according to claim 13 for controlled blocking of the feeding of the microwave power through the waveguide.

15. Use according to claim 13 for the controlled achievement of a mode transformation in the microwave field in the waveguide in and for the feeding of a multimode space included in the appliance with different mode patterns depending upon the position of the conductor member.

16. A device for controlling the feeding of microwave power through a waveguide in a microwave heating appliance, in particular a microwave oven intended for domestic use, comprising a conductor member arranged in the waveguide, c h a r a c t e r i s e d in that, in an active position, the conductor member is adapted to be resonant to microwave field frequencies coming into the waveguide, so that, as a result, resonance currents are generated, which counteract the incoming microwave field and give rise to a changed mode pattern which acts upon the transportation of microwave power through the waveguide, the conductor member being controllably reposi- tionable between an essentially inactive, non-resonant position and the active, resonant position.

17. A device according to claim 16, c h a r a c t e r i s e d in that, in the active resonant position, the conductor member is designed to generate one or more loop current paths .

18. A device according to claim 17, when the microwave field is of the TE╧ç₀ type, c h a r a c t e r i s e d in that, in an active resonant position, the conductor member is designed to generate said loop current paths in a waveguide cross-sectional plane.

19. A device according to any one of claims 16-18, c h a r a c t e r i s e d in that the conductor member has a flat three-dimensional configuration, the member being adapted to be rotatable about an axis in a waveguide cross-sectional plane between an inactive position, with the flat plane parallel to the longitudinal direction of the waveguide, and an active, resonant position with the flat plane perpendicular to the longitudinal direction of the waveguide.

20. A device according to claim 19, when the waveguide has a rectangular cross-section, c h a r a c t e r i s e d in that the conductor member has an essentially elliptic circumference and is adapted to be rotatable about an axis which is parallel to an ellipse axis and extends between two opposite waveguide walls of the waveguide .

21. A device according to any one of claims 16-20, c h a r a c t e r i s e d in that the conductor member has an annular configuration.

22. A device according to claim 19, when the waveguide has a rectangular cross-section, c h a r a c t e r i s e d in that the conductor member has a lying H configuration and is adapted to be rotatable about an axis which lies in the member plane and extends between two opposite waveguide walls.

23. A device according to any one of claims 16-19, c h a r a c t e r i s e d in that the conductor member has a T configuration and is adapted to be displaceable from an inactive position spaced from the waveguide walls to an active, resonant position in coaction with a waveguide wall, preferably the bottom or top wall of the waveguide, to form a conductor member configuration in the shape of a lying H.

24. A device according to any one of claims 16-23, c h a r a c t e r i s e d in that, in the active, resonant position, the conductor member has an extent in the longitudinal direction of the waveguide in the order of 0.1 free space wavelength of the incoming microwave field.

25. A device according to any one of claims 16-24, c h a r a c t e r i s e d in that the conductor member is arranged in the waveguide spaced from the connection of the waveguide to an appliance cavity in such a way that, with respect to modes, the conductor member is separated from said connection.

26. A device according to any one of claims 16-24, c h a r a c t e r i s e d in that the conductor member is arranged adjacent to the connection of the waveguide to an appliance cavity of the multimode type.