ITRE20130028A1 - MICROWAVE EMITTER SYSTEM - Google Patents

Description

DESCRIPTION

"MICROWAVE EMITTER SYSTEM"

TECHNICAL FIELD

The present invention relates to a microwave emitter system for microwave ovens, the related microwave oven and the operating method of the microwave emitter system.

More specifically, the invention relates to a microwave emitting system equipped with a slotted wave guide and intended for use in a continuous or discontinuous microwave oven, for example suitable for heating products containing asbestos for the inerting them, as well as the method of operation of the microwave oven itself.

PRE-EXISTING TECHNIQUE

Microwave ovens equipped with microwave generators are known, which include a microwave source, for example a magnetron, which, when electrically powered, generates electromagnetic waves which are channeled along a wave guide, for example made in suitable material reflecting the microwaves, for example of a tubular shape with a quadrangular, circular or any section.

On one wall of the wave guide there is one (or more) slot, that is a slot, open or shielded by a suitable window transparent to microwaves, through which the microwaves are emitted, so that they enter the cooking chamber microwave oven where the material to be heated is placed.

At the opposite end of the waveguide with respect to where the magnetron is placed there is a short circuit that occludes the end of the waveguide, the distance of which from the magnetron, for example, in certain applications it can be adjusted, so that the short circuit can be placed in various configurations which then remain fixed during the use of the oven, ie during the emission of microwaves by the magnetron.

A drawback found in these known microwave ovens lies in the fact that while the material to be heated remains or transits inside a cooking chamber and is exposed to one or a multiplicity of microwave generators, the heating of the material does not appear homogeneous and uniform.

This drawback is even more felt when the material to be heated is a large-sized product, with low thermal conductivity or containing a dangerous material to be thermally treated due to its inertization, as the lack of uniformity of heating results in necessarily in low heating efficiency of the entire volume of the product and in low inertization efficiency, with consequent failure to harm the material.

For example, the use of microwave ovens for the inertization of products containing asbestos is known.

The thermal methods for the inertization of asbestos provide for treatments at temperatures above 800 ° C in order to induce a transformation of the magnesium silicates into a form no longer dangerous for health and reusable in the cement production cycle within limits of 5%, in accordance with the UNI ENV 197/1 standard.

However, thermal methods collide with the low thermal conductivity of asbestos, which requires long process times for the heat to reach the internal portions of the material, especially if this is, as often happens in the case of products containing asbestos, conformed as large plates and also of considerable thickness.

The greater homogeneity of treatment can be achieved at the expense of thermal efficiency, however making the use of microwave ovens for the inertization of such materials uneconomical.

An object of the present invention is to overcome the aforementioned drawbacks of the known art, within the ambit of a simple, rational and low cost solution.

These objects are achieved by the characteristics of the invention reported in the independent claim. The dependent claims outline preferred and / or particularly advantageous aspects of the invention.

<EXPOSURE OF THE INVENTION>

The invention, in particular, makes available a microwave emitting system which includes a microwave source, a wave guide which is connected at a first end to this source in order to receive the microwaves generated by it and is equipped with a slot from which said microwaves are irradiated, and a short circuit slidingly associated with the second end of said waveguide and movable so as to vary the length of the waveguide itself.

According to the invention, the short circuit is mobile of reciprocating motion between a position close to a position away from the second end of the waveguide in a continuous or discontinuous way during the emission of microwaves by said source.

Thanks to this solution, it is possible to continuously alter, during the emission of microwaves by the microwave emitting system itself, the distribution of the electromagnetic field within the waveguide itself, and therefore within the cooking chamber of the oven. microwave. At the same time, thanks to this solution, possibly in combination with the variation of the operating parameters of the microwave emitter system (for example of the microwave source), it is possible to minimize the power reflected towards the microwave source, thus maximizing energy efficiency. of the microwave emitter system.

In this way it is possible to create a microwave emitter system that allows to obtain different configurations of the electromagnetic field and with consequent greater homogeneity of the heating of the product to be thermally treated in a microwave oven, due to the fact that an overlap of different configurations of the electromagnetic field that allows to compensate for any non-homogeneity of heat generation in the material of the product to be treated.

In practice, the product that is subjected to heat treatment by means of a microwave emitting system as described above is as if it were continuously brushed by a spatially variable electromagnetic field, which changes configuration continuously, allowing a more uniform, deep and fast of the artifact itself.

An aspect of the invention provides that the microwave emitting system comprises actuator means associated with the short circuit and able to move it alternately between said approached position and said removed position.

Such actuator means, for example, are placed externally to the wave guide.

Thanks to this it is possible in a simple and effective way to control the movement of the short circuit according to a predetermined and appropriate law of motion according to the heat treatment needs.

Advantageously, the microwave emitting system comprises a control unit operatively connected to said actuator means and configured so as to command the actuation of said actuator means continuously or discontinuously during the emission of microwaves by said source of microwave.

For example, the actuator means comprise at least one invertible electric motor connected to means for transforming the drive shaft of the electric motor into translational motion of the short circuit from rotary motion.

Alternatively, the actuator means comprise at least one double-acting jack on whose movable stem the short circuit is keyed. Advantageously, a further aspect of the invention provides that the waveguide comprises a hollow tubular body with a substantially quadrangular section and a straight longitudinal axis, equipped with a single elongated slot made on one of the walls (eg one of the wide walls ) of said tubular body and arranged with a longitudinal axis substantially parallel to the central longitudinal axis of the waveguide and eccentric (or not coaxial) with respect to it.

Thanks to this configuration of the waveguide it is possible to generate a microwave beam that is emitted by the waveguide that is strongly directed and concentrated, and which is substantially lying on a prevailing plane depending on the geometry of the slot .

For example, the slot has a straight longitudinal axis.

Alternatively, the slot can have a substantially curvilinear longitudinal axis, for example with a wavy course.

Advantageously, the width of the slot can vary along its length, thus it is possible that the microwave emitter system is configured to vary the power density along the longitudinal axis of the slot.

A further aspect of the invention makes available a microwave oven which comprises at least one cooking chamber delimited by walls designed to contain the electromagnetic radiation inside the cooking chamber, at least one of which can include at least one window transparent to radiation electromagnetic, and at least one microwave emitter system, as described above, fixed to at least one of said walls, for example so that said slot is substantially superimposed on said transparent window, so that the microwave beam emitted by the microwave emitter system enter the cooking space, for example through the transparent window.

Thanks to this solution it is possible to create a microwave oven with high performance, high thermal efficiency and high homogeneity of heating, especially for the heating of large-sized products and / or thermal inertia, as well as particularly suitable for heat treatment for € ™ inerting of products containing asbestos.

Furthermore, advantageously, the oven comprises a plurality of said microwave generators fixed to at least one of said walls of the cooking chamber.

Thanks to this solution it is possible to further increase the heating efficiency and the variation of the electromagnetic field to which the product to be heated is subjected.

Even more advantageously, the microwave oven can be of the modular and continuous type, i.e. it comprises a plurality of cooking chambers side by side and communicating with each other, capable of defining a longitudinal cooking channel as a whole, and means of transporting the products to be heated. along the longitudinal canal.

In this way it is possible to vary, according to the needs, the length of the cooking channel and to treat in a rapid and continuous way a large quantity of products at a time, without increasing the thermal inertia.

A further aspect of the invention makes available a method of operation of a microwave emitter system, as described above, which comprises the step of electrically powering the microwave source so as to create a microwave beam which passes through the microwave guide. wave and is emitted through the slot.

According to the invention, the method comprises the step of controlling the variation of the configuration of the electromagnetic field relative to the microwave beam emitted from slot i continuously during the phase of electrical supply of the microwave source.

In particular, the step of controlling the variation of the electromagnetic field takes place by controlling the movement of the short circuit during the phase of electrical supply of the microwave source.

Alternatively or in addition, the step of controlling the variation of the electromagnetic field takes place by controlling the variation of the operating frequency of the microwave source during the phase of electrical supply of the microwave source.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the invention will become evident by reading the following description provided by way of non-limiting example, with the aid of the figures illustrated in the attached tables.

Figure 1 is an axonometric top view of a microwave emitting system according to the invention.

Figure 2 is an axonometric view from below of Figure 1.

Figure 3 is a side view of Figure 1.

Figure 4 is a top plan view of Figure 1.

Figure 5 is a view of an enlarged detail of Figure 4 with the moving short circuit associated with a first variant of actuator means.

Figure 6 is a view of the enlarged detail of Figure 4 with the moving short circuit associated with a second variant of actuator means.

Figure 7 is a bottom plan view of Figure 1.

Figure 8 is a bottom plan view of a second embodiment of the microwave emitter system, according to the invention.

Figure 9 is a bottom plan view of a third embodiment of the microwave emitter system, according to the invention.

Figure 10 is a schematic isometric view of a microwave oven, according to the invention.

<BEST WAY TO IMPLEMENT THE INVENTION>

With particular reference to these figures, the numeral 10 globally indicates a microwave emitting system, suitable for generating a microwave beam adapted to be directed inside a cooking chamber of a microwave oven, globally indicated with the reference number 100.

The microwave emitter system 10 particularly comprises a microwave source, for example a magnetron 20, or a solid state generator or a klystron, which when powered by electric current emits a microwave beam.

The magnetron 20 (or the solid state generator or the klystron) is for example of the type that allows you to vary the operating frequency. The magnetron 20 is connected to a wave guide 30, which comprises a tubular body 31 with longitudinal development, in the example with a rectangular section, made of at least one metal material, whose internal cavity is suitable for receiving the microwave beam emitted by the magnetron 20 and convey it along the longitudinal axis A of the wave guide 30 itself.

It is not excluded that the section of the waveguide 30 may alternatively be of another quadrangular or circular shape or of any shape according to the construction requirements.

Furthermore, the internal cavity of the waveguide 30 could be empty or filled or partially filled with a transparent material with respect to microwaves, so that the microwave beam emitted by the magnetron 20 is reflected by the internal walls of the tubular body 31 and conveyed along the longitudinal axis A of the same tubular body 31.

The tubular body 31 has a first open end 311 constrained to the magnetron 20, by interposing a first connecting portion 32 of the wave guide 31 to the magnetron itself, and a second free end 312.

The first connection section 32 is filled with a dielectric material which acts as a barrier.

Between the first connection section 32 and the waveguide 31, moreover, impedance matching means 33 are interposed, for example comprising a triple Stub and / or a tuner E-H, as known to the skilled in the art.

In one wall of the tubular body 31, for example in one of its major walls, a longitudinal slot 34 is made, which extends for example substantially for part or for the entire length of the tubular body 31 from the first end 311 to second end 312.

In practice, a microwave beam exits from the slot 34 which, previously conveyed by the wave guide 30, is emitted through the slot itself substantially on a plane dependent on the geometry of the slot itself.

Slot 34 is large enough to allow the microwave beam emitted by the magnetron 20 to escape, in practice the slot 34 is the only part of the microwave emitter system 10 in communication with the outside of the same, that is the only part of waveguide 30 that radiates microwaves.

Slot 34 can be opened or occluded by a window of material transparent to microwaves.

Slot 34 is arranged with the longitudinal axis B substantially parallel to the longitudinal axis A of the waveguide 30 and, for example, eccentric (not coaxial and not aligned in plan) with respect to it, in practice the slot 34 is It is made in a decentralized area of the wall of the tubular body 31.

In a first embodiment of the microwave emitter system 10, shown in Figures 1 to 7, the slot 34 has a straight longitudinal axis and has a substantially rectangular shape.

In a second and alternative embodiment of the microwave emitter system 10, shown in Figure 8, the slot 34 always has a straight longitudinal axis (in the example shown misaligned in plan with the central longitudinal axis of the tubular body 31) and it has a substantially trapezoidal shape, with long sides curved according to a logarithmic profile.

In practice, in this second embodiment the amplitude (width) of the slot 34 varies along its length.

In the example, the tapered end of the slot 34 is placed in correspondence with the first end 311 of the tubular body 31, it is not excluded as an alternative that it may, on the contrary, be placed in correspondence with the second end 312 of the same.

In a third embodiment of the microwave emitter system 10, shown in Figure 9, the slot 34 has a substantially curvilinear longitudinal axis B.

In the example, the longitudinal axis of the slot 34 has a substantially wavy course, for example irregular with one or more curves with differentiated radius or regular substantially sinusoidal or similar.

It is not excluded that also in this third embodiment the width of the slot 34 may be variable along its length or that the slot 34 is globally substantially centered (median axis substantially centered) in the wall of the tubular body 31 or eccentric with respect to to her.

The second end 312 of the tubular body 31 that makes up the waveguide 30 is fixed to a second hollow connection section 40, which for example is substantially shaped like a polygonal section glass (in the example rectangular equal to that of the tubular body 31), whose open end is fixed to the second end 312 of the tubular body 31 (in practice forming a single body with it), its internal cavity substantially axially extends the internal cavity of the wave guide 30 and has a bottom wall 40 suitable for closing the wave guide itself.

The second connection section 40 is also made of metallic material able to contain the microwave beam emitted by the magnetron 20 inside.

The second connection section 40 has a substantially rectilinear longitudinal development in the example shown, however, it can have a substantially folded conformation, like a â € œLâ € or have other suitable shapes.

A short circuit 42 is slidingly associated inside the second connection section 40, as shown in figures 5 and 6.

The short circuit 42, for example, comprises a metal block having the shape of a parallelepiped with a rectangular base (or in any case homologous to the internal shape of the cavity of the second connection section 40), which is able to slide (by virtue of a small radial clearance), for example without turning, inside the second connection section 42.

The microwave emitting system 10 then comprises actuator means 50 suitable for translating along the axis of the second connection section 40 (or at least a portion thereof) the short circuit 42, for example of reciprocating motion between a position approached to the second end 312 of the tubular body 31 to a position removed from the second end 312 itself (i.e. approached to the bottom wall 41 of the second connection section 40), for example continuously or discontinuously during the emission of microwaves from part of the magnetron 20.

In practice, the short circuit 42 defines a movable closing wall of the wave guide 30, so as to allow a variation, in a continuous and controlled way according to a predeterminable law of motion (which can foresee phases of motion in the two the length of the waveguide itself between a minimum length, when the short circuit 42 is in the near position, and a maximum length, when the short circuit 42 is in the away position.

The variation of the position of the short circuit 42 at the same time as the generation of the microwave by the microwave source 20 defines a continuous variation of the configuration of the electromagnetic field generated by the microwave beam and emitted by the wave guide 30 through the slot 34.

In a first variant, shown in Figure 5, the actuator means 50 comprise an invertible electric motor 51, on whose drive shaft 510 a driving pulley 511 is keyed, which is adapted to transfer the motion to a driven pulley 512 by means of a flexible transmission member closed on itself in a ring and wound on the driven pulley 512 and on the driving pulley 511 themselves, for example a belt 513.

The driven pulley 512 is keyed onto an external end of a worm screw 514 which is rotatably inserted, for example supported by suitable bearings, into a hole made in the bottom wall 41 of the second connection section 40.

The short circuit 42 includes a threaded hole into which the worm screw 514 is screwed.

The rotation in the two directions of the worm screw 514 imposed by the electric motor 51 gives the translation of reciprocating motion of the short circuit 42 inside the second connection section 40. In a second variant, shown in figure 6, the actuator means 50 comprise a double-acting jack 52, for example pneumatic or mechanical or other, equipped with a cylindrical body 520 within which a stem 521 slides, the free end of which is inserted into a hole made in the bottom wall 41 of the second connection section 40 and it is fixed (directly or by means of extension and connection elements) to the short circuit 42, so as to impart the reciprocating motion translation of the short circuit itself inside the second connection section 40.

It is not excluded that the actuator means 50 may be different and equivalent with respect to those illustrated and described and in any case capable of giving the short circuit 42 an alternating forward / backward motion along the second connection section 40 or equivalently along the guide rail wave 30.

The microwave oven 100 comprises a cooking chamber 110 delimited by a plurality of metal walls 111, 112 suitable for containing the electromagnetic radiation inside the cooking chamber, for example, as is known to those skilled in the art.

In particular, the cooking chamber 110 is delimited by a lower wall (not visible in the figures), able to be arranged substantially horizontal, a parallel and homologous upper wall 111 and two (equal) lateral sides 112 joined at the bottom and at the top, respectively , to the lower wall and to the upper wall 111 and substantially squared with respect thereto.

In practice, the walls 111, 112 are suitable for delimiting and defining a substantially tunnel-shaped cooking chamber.

Furthermore, the microwave oven 100 comprises a continuous conveyor, for example a belt conveyor 120, which is placed with an axis of advancement substantially parallel to the longitudinal axis of the cooking chamber 110 (tunnel) and is adapted to internally go through the cooking chamber itself.

In practice, the belt conveyor 120 defines a mobile support surface along the longitudinal axis of the cooking chamber 110 placed slightly above the lower wall of the cooking chamber itself at a distance from the upper wall 111.

The articles M to be subjected to heat treatment by means of the microwave oven 100 rest on the conveyor belt 120, which then pass through the volume inside the cooking chamber 110 with a speed that can be set, or with possible permanence without motion within the oven 100, suitable for to ensure the completion of the desired heating cycle for the product M itself.

At least one microwave emitting system 10 is fixed to at least one of the walls 111, 112 of the cooking chamber 110, so that the slot 34 faces the inside of the cooking chamber itself.

In practice, the wave guide 30 is fixed externally to one of the walls 111, 112 with the wall which comprises the slot 34 substantially supported and mating with the wall 111, 112 of the cooking chamber 110.

The wall 111,112, in correspondence with the area intended to be superimposed on the slot 34, has a window (not shown), open or occluded by an elongated protective element transparent to electromagnetic radiation, for example homologous to the slot 34 itself, which it allows the introduction of the microwave beam emitted by the microwave emitting system 10 inside the cooking chamber 110, as known to the skilled in the art.

For example, the wave guide 30 can be arranged with a longitudinal axis A substantially orthogonal to the longitudinal axis of the cooking chamber 110 or with a longitudinal axis A substantially parallel to the longitudinal axis of the cooking chamber itself.

Furthermore, it is possible to provide that on a wall 111, 112 of the cooking chamber 110 are fixed, as described above, more than one microwave emitting system 10, for example placed with the longitudinal axes A parallel to each other and, for example, equidistant.

In the example illustrated, the microwave oven 100 comprises a plurality of microwave emitting systems 10 fixed to the upper wall 111, in the example 4, placed with longitudinal axis A orthogonal to the longitudinal axis of the cooking chamber 110. The adjacent microwave emitting systems 10 are offset from each other, ie they have the magnetron 20 placed on opposite sides with respect to the side walls 112 of the cooking chamber 110.

Furthermore, the microwave oven 100 comprises a plurality of microwave emitting systems 10 fixed to at least one of the side walls 112 of the cooking chamber 110, in the example in number of 2, placed with the longitudinal axis A parallel to the longitudinal axis of the cooking chamber 110 (for example staggered).

It is not excluded that the arrangement and number of microwave emitting systems 10 may be different from that illustrated and whatever depending on the construction needs and / or the type of heat treatment that the microwave oven 100 is intended to provide as well as the dimensions of the article M to be heat treated. For example, the microwave emitter systems 10 could also be arranged inside the cooking chamber 110.

The microwave oven 100, for example, can be of the modular type, i.e. be composed of a plurality of cooking chambers 110, as described above, fixed and aligned along a longitudinal axis of common development to form a cooking channel more or less long depending on your needs.

Furthermore, the microwave oven 100 comprises a control and command unit, not shown, which is operatively connected to each magnetron 20 and to each of the actuator means 50, as well as for example to the belt conveyor 120, for the automated management and controlled heating / cooking thermal cycles imparted by the microwave oven itself to the M products.

Finally, the microwave oven 100 can be equipped with infrared radiation emitters, laser sources, electron sources, plasma sources, radio frequency sources, gas or liquid combustion groups or other heating systems of a known type, suitable for assisting the heating of the artifact M.

In particular, the microwave emitting system 100 can comprise injection means of at least one ionizable gas inside the waveguide 30.

For example, the injection means comprise at least one nozzle (not shown) fixed to at least one wall of the tubular body 31, in which the delivery mouth of the nozzle opens into the interior of the tubular body itself.

The nozzle is then in communication with an ionizable gas supply circuit.

In the light of what has been described above, the operation of the microwave emitter system 10 and, therefore, of the microwave oven 100, is as follows.

Each microwave emitting system 10, when the magnetron 20 is electrically powered, generates a microwave beam which passes through the waveguide 30 and is emitted through the slot 34.

Simultaneously with the power supply of the magnetron 20, a system of variation of the configuration of the electromagnetic field relative to the microwave beam emitted from slot 34 is activated.

In practice, it is possible to control the variation of the operating frequency of the magnetron 20 during the power supply phase of the magnetron 20, thus varying the electromagnetic field caused by the microwave beam inside the cooking chamber 110, or downstream of slot 34.

Alternatively or in addition to this, it is possible to activate the actuator means 50 so that they control the alternating motion of the short circuit 42, according to a predetermined law of motion, during the electrical supply phase of the magnetron 20.

It is possible to program the microwave oven 100 so that each microwave emitting system 10 is substantially independent of one another.

Furthermore, it is possible that in a single microwave oven 100 there are microwave emitting systems 10 which are different from each other, for example of the type described above according to the first, second and third embodiments.

The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept.

Furthermore, all the details can be replaced by other technically equivalent elements.

In practice, the materials used, as well as the contingent shapes and dimensions, may be any according to requirements without thereby abandoning the scope of the protection of the following claims.

Claims (17)

CLAIMS 1. Microwave emitter system (10) which comprises a microwave source (20), a longitudinal wave guide (30) connected to a first end (311) to this source in order to receive the microwaves generated by it and equipped with a slot (34) from which said microwaves are irradiated and a short circuit (42) slidingly associated with the second end (311) of said waveguide and movable so as to vary the length of the waveguide ( 30) itself, characterized by the fact that said short circuit (42) is mobile with reciprocating motion between a position close to a position away from the second end of the wave guide (30) during the emission of microwaves by said microwave source (20). 2. Emitter system (10) according to claim 1, characterized in that it comprises actuator means (50) associated with said short circuit (42) and able to move it alternately between said approached position and said removed position. 3. Emitter system (10) according to claim 2, characterized in that it comprises at least one control unit operatively connected to said actuator means (50) and configured so as to control the actuation of said actuator means continuously or continuously at times during the emission of microwaves by said microwave source (20). 4. Emitter system (10) according to claim 2, wherein said actuator means (50) comprise at least one invertible electric motor (51) connected to means for transforming (511-514) from a rotational motion of the drive shaft (510 ) of said electric motor (51) in translational motion of said short circuit (42). 5. Emitter system (10) according to claim 2, wherein said actuator means (50) comprise at least one double-acting jack (52) on whose movable stem (521) said short circuit (42) is fixed. 6. Emitter system (10) according to claim 1, wherein said waveguide (30) comprises a hollow tubular body (31) with substantially quadrangular section and straight longitudinal axis (A), provided with a single slot ( 34) elongated made on one of the walls of said tubular body (31) and arranged with longitudinal axis (B) substantially parallel to the central longitudinal axis (A) of the waveguide (30) and eccentric with respect to it. Emitter system (10) according to claim 6, wherein said slot (34) has a straight longitudinal axis (B). 8. Emitter system (10) according to claim 6, wherein said slot (34) has a substantially curvilinear longitudinal axis (B). Emitter system (10) according to claim 8, wherein said slot (34) has a substantially wavy longitudinal axis (B). Emitter system (10) according to claim 6, wherein the width of the slot (34) varies along its length. 11. Emitter system (10) according to claim 1, wherein impedance adjustment means (33) are interposed between said microwave source (20) and said wave guide (30). 12. Microwave oven (100) which includes: - at least one cooking chamber (110) delimited by walls (111,112) able to contain electromagnetic radiation inside them, at least one microwave emitting system (10), according to one or more of claims 1 to 11, fixed to at least one of said walls (111,112) so as to emit a microwave beam inside the cooking chamber (110). 13. Oven (100) according to claim 12, characterized by comprising a plurality of said microwave emitting systems (10) fixed to at least one of said walls (111,112) of the cooking chamber (110). 14. Oven (100) according to claim 12 or 13, characterized in that it comprises a plurality of cooking chambers (110) side by side and communicating with each other, able to define a longitudinal cooking channel as a whole, and transport means (120) of artifacts (M) to be heated along the longitudinal channel. Method of operation of a microwave emitting system (10), according to one or more of claims 1 to 11, which comprises the step of electrically powering the microwave source (20) so as to create a microwave beam which crosses the wave guide (30) and is emitted through the slot (34), characterized in that it comprises the step of controlling the variation of the configuration of the electromagnetic field relative to the microwave beam emitted from the slot (34) continuously during the power supply phase of the microwave source (20). Method according to claim 15, in which the step of controlling the variation of the electromagnetic field takes place by controlling the movement of the short circuit (42) during the phase of electrical supply of the microwave source (20). Method according to claim 15 and / or 16, in which the step of controlling the variation of the electromagnetic field takes place by means of controlling the continuous variation of the operating frequency of the microwave source (20) during the phase of electrical supply of the source of microwave itself.