EP0022282B1 - Radiating devices producing circularly polarized microwaves and their use in the field of microwave applicators - Google Patents

Description

• - un générateur, alimenté sur le secteur, qui produit l'onde électromagnétique au moyen d'un ou plusieurs tubes hyperfréquences;
• - un applicateur, relié au générateur par un guide d'ondes ou un ensemble de guides, qui comprend la cavité métallique dans laquelle s'effectue le traitement, et le ou les systèmes rayonnants, qui permettent à l'énergie de sortir des guides et de rayonner dans la cavité.

The present invention relates to industrial microwave ovens. Such ovens schematically have two very distinct parts:

• - a generator, powered from the mains, which produces the electromagnetic wave by means of one or more microwave tubes;
• an applicator, connected to the generator by a waveguide or a set of guides, which includes the metal cavity in which the treatment is carried out, and the radiating system or systems, which allow energy to exit the guides and to radiate into the cavity.

There are many examples of the use of radiant slot arrays in microwave ovens. For example, in the system described by French patents FR-A-2 076 405 (GUERGA-HALLIER), FR-A-2 110 539 (HALLIER-COQUET) and FR-A-2 147 456 (POURRAT, ZWOBADA, DENIS ), by patent application FR-A-2 382 778 (FOUR-NET, FA VAS), by German patent applications DE-A-2 558 589 (FRITZ) DE-A-2 622 173 (KUSONOKI, YOSHIMURA ), or by American patent US-A-3,961,568 (JEPPSON).

It is also known from patent FR-1 126 260 an antenna, intended for the emission of microwaves with circular polarization, constituted by a metal helix extended by a capacitive plunger penetrating into an opening formed in a waveguide. The use of this type of antenna is fairly standard in radar or radio communications, with coaxial cable supply and counterweight. It is also known from US Pat. No. 3,705,283 to produce a radiating device comprising a waveguide having elongated openings made longitudinally in the wall.

The basic rule is to ensure a good adaptation, that is to say a coupling as complete as possible of the energy to the material to be treated. This means that the wave reflected back to the generator must be as weak as possible, so as not to harm the performance, as to protect the generator.

However, it turns out that adaptation is difficult in certain cases. In particular, when the material to be treated is contained in open metal containers, or when it itself contains metal parts, or even when it has an anisotropy of reflectivity with respect to the incident wave. In the first two cases, the metal surfaces reflect a significant fraction of the energy. In the latter case, the reflection will be more or less important, depending on the orientation of the incident electric field.

The object of the present invention is to provide a convenient means of achieving adaptation in these difficult cases.

We know that a plane wave consists of an electric field and a magnetic field, both normal to the direction of propagation.

The ratio of the two fields is constant at all points and at all times, but their respective amplitudes vary.

In the general case, the end of the vectors representing the two fields describes an ellipse. We then say that the wave is elliptically polarized.

There are two important particular cases: linear polarization, where the ellipse is reduced to its long axis, and circular polarization, where the end of the vectors describes a circle.

In the case of a linearly polarized wave, the fields always keep the same orientation. In particular, the electric field reflected by a flat metallic surface is parallel to the incident field, and is capable of traversing the radiating system in the opposite direction and of ascending the guide towards the generator.

In the case of a circularly polarized wave, the electric field reflected by a plane metallic surface rotates in the opposite direction to the incident field, and is rejected by the radiating system, which only accepts the opposite direction of rotation. The wave is trapped, and must, after a series of reflections, be absorbed by the material.

• - une bonne adaptation, qui traduit un faible couplage du système rayonnant avec l'onde qu'il a lui-même émise, et avec les ondes émises par d'autres systèmes rayonnants identiques, et ce, même lorsque le produit à traiter donne lieu à des réflexions importantes;
• - un chauffage régulier des milieux homogènes et isotropes, du fait de la rotation du champ électrique;
• - pour la même raison, un moyen commode de traiter de façon convenable des milieux hétérogènes ou anisotropes, lorsqu'il est matériellement impossible d'aligner un champ électrique polarisé linéairement sur la direction d'absorption maximale du produit.

The use of circularly polarized waves therefore offers several advantages:

• - a good adaptation, which translates a weak coupling of the radiating system with the wave which it itself emitted, and with the waves emitted by other identical radiating systems, and this, even when the product to be treated gives rise important reflections;
• - regular heating of homogeneous and isotropic media, due to the rotation of the electric field;
• - for the same reason, a convenient means of suitably treating heterogeneous or anisotropic media, when it is materially impossible to align an electric field linearly polarized on the direction of maximum absorption of the product.

The device which is the subject of the invention is a radiating system, producing circularly polarized waves, by means of an antenna or a network of antennas, as defined by claim 1. The energies radiated by the antennas can be equal to each other, or, on the contrary, modulated so as to produce different power levels in such and such a place of the applicator, as required. Said device cannot be reduced to conventional networks of slots, nor to simple helical antennas. It represents an innovation, in the sense that it associates a network of slots with propellers, and that the properties of the assembly are neither those of the slots alone, nor those of the propellers alone.

The device in question may be integral with the wall of the applicator, the antennas then plunging into it, or else be placed inside it.

Figure 1 shows a device according to the invention.

FIGS. 2 and 3 illustrate two examples of industrial applicators, of the tunnel type with conveyor belt, produced from devices as described in relation to FIG. 1.

The device shown in Figure 1 is a guide of waves 1, that is to say of a hollow metal cylinder of a priori arbitrary section, pierced with longitudinal openings 2, carrying metal propellers 3. The propellers are rigidly fixed in the center of a metal bar 4 , which is placed transversely to the axis of the guide, outside and at a distance from the wall of the latter. The two ends of the bar are mechanically linked to the wall of the guide, for example by means of welded head screws 5. Each propeller extracts part of the energy propagating in the guide by means of a capacitive metallic plunger 6, starting from the point of attachment of the propeller to its support bar, and penetrating a certain length in the guide through the opening. The openings are oblong in shape and generally referred to as slots. They can, for example, be shunt slots pierced in the axis of the largest wall of a rectangular waveguide.

In one embodiment of the device, the waveguide is of rectangular section, and the slots are drilled on the axis of the long side, separated from each other by a distance equal to an integer multiple of λg / 2, generally at λg, λ being the propagation wavelength in the guide format considered. Such slots alone do not radiate. This would require that they be spaced from the axis, and all the more that they are further from the feed point. In addition, to avoid resonance problems, the distances between the slits should alternately be slightly greater and slightly less than λg / 2.

The addition of propellers to this network of non-radiating slots regularly spaced allows them to radiate, and to obtain a circular polarization with a maximum of field in the axis of the propeller. This provided that the perimeter of the slit, as well as that of the turns of the propeller, are close to the wavelength in vacuum o. This condition is fulfilled in the case of the present invention. The number of antennas per radiating device can vary between the unit and values of the order of several tens.

The waveguide of the device is supplied directly or indirectly by the generator, for example in the middle or at one of its ends. The ends are closed by fixed short-circuits, or by mobile short-circuit pistons. Furthermore, the radiating devices considered may be part of a larger assembly, consisting of several of these devices, in particular aligned or in parallel, and which may be supplied by different microwave tubes, or several by the same tube, the power then being equally distributed between devices by means of conventional methods.

The assembly generally designated by the term antenna, constituted by the opening; the propeller, the fixing bar and the capacitive plunger, presents numerous adjustment possibilities, to which it is necessary to add, at the level of the complete device, the position of the mobile short-cooked pistons and the distance between antennas. We can in particular adapt the dimensions of the slots, the location of the propellers along the slots, the pitch of the slots, the diameter, the section, the number of turns, the winding direction and slope of the propellers, the dimensions of the bars. transverse and their distance from the guide, the diameter of the plungers and their depth of insertion into the guide; it is also possible to end the divers with obstacles of various sizes, for example blind nuts, which modify their impedance.

By playing with these various adjustment possibilities, we manage to radiate the desired amount of power from each antenna, for example the same power for all the antennas, or else variable powers along the device: On the other hand, a good broadband adaptation is achieved, that is to say a global power level reflected by the device sufficiently low over a range of frequencies covering the entire extent of the allocated band used.

In practice, it is convenient to reduce the number of parameters by setting a number. As a non-restrictive example, one can impose the dimensions of the openings, those of the bars, their distance from the guide, the geometric characteristics of the propeller and the distance between antennas, and carry out the adaptation by varying the depth. the plungers, the location of the propellers on the slots, and the position of the mobile short-circuit pistons.

This device has made it possible to produce several industrial microwave ovens: in particular, a fourtunnel intended for the coagulation of sardines in their non-closed metal cans, and an applicator intended for the treatment of textile fibers representing low loads .

These two embodiments work perfectly satisfactorily, and would not have been possible without the device described.

In general, the device in question is capable of making possible the treatment by microwaves of low charges, that is to say essentially of products in low quantity or with low loss factor, of bodies comprising metallic parts, or heterogeneous or anisotropic materials, without this list being exhaustive.

In particular, an embodiment of the device consists of a straight rectangular guide, carrying one or more antennas located on the axis of the long side, fed at one of its ends, the other end being closed by a movable short-circuit piston.

The device defined above, associated with a certain number of other identical devices, is in particular capable of equipping a microwave applicator in the form of an elongated cylinder of any section, for example: 'rectangular, trapezoidal, circular, elliptical, in a segment of a circle or other; said devices being modutaires and aligned one after the other, and mounted along the longitudinal axis of one of the walls of the applicator of a generator, or even perpendicularly, or obliquely to this axis or generator.

In FIGS. 2 and 3, we see at 7 the cavities receiving the “microwave” radiation. The product 8 is transported by the conveyor belt 9 along the radiating guides 1, arranged perpendicular to the axis of the applicator and symmetrically in relation port to it, supplied at 10 by “microwave” sources, for example magnetrons. Movable short-circuit pistons 11 close the waveguides at the end opposite the source.

A dielectric separation 12 isolates the radiating devices from the emanations of vapors, greases, dusts, produced during treatment.

A door 13 allows access to the treatment cavity for cleaning, maintenance, etc. needs.

Figure 3 shows another possibility of using the devices in question, which are mounted on the cavity no longer transversely, but longitudinally, one behind the other, and in the axis thereof.

Claims (5)

1. Radiating device in the field of microwaves, composed of a wave guide holding an antenna which produces a circular polarization, the antenna composed by a slot in the wall of the guide and completed by a metal spiral, which prolongates a capacitive probe, plunging into the guide through the slot, characterized in that the antenna has a slot or elongated opening (2) pierced longitudinally into the wall of the guide (1), and in that the metal spiral (3) is supported by a metal rod (4), mounted on the outside of the guide (1) and crosswise to the latter, the ends of the above-mentioned rod (4) being fixed to the outside wall of the guide by adjustable metal braces.

2. Device, of the kind as described in claim 1, characterized in that the wave guide (1) includes an alignment of two or several antennae, each one radiating at power levels defined in advance by the needs of the application, the power of emission of each antenna being adjusted by varying either the position of the spiral (3) alongside the slot (2), or the penetration into the guide and/or the diameter of the capacitive diver (6).

3.. Device following claims 1 or 2, characterized in that the slots (2) of the antennae are shunt slots, pierced into the axis of the biggest wall of a rectangular wave guide.

4. Microwave applicator of elongated shape and of rectagular or circular section, epuipped with a plurality of radiating devices following claims 1, 2, and 3, characterized by the fact that all devices are modular and aligned one after the other, and mounted alongside the longitudinal axis of one of the walls of the applicator or af a generatrix of the circular section applicator.

5. Microwave applicator of elongated shape and of rectagular section or having the shape of a circular segment, equipped with a plurality of radiating devices following claims 1, 2 and 3, characterized by the fact that these devices are modular and mounted alongside the walls, either perpendicular or obliquely to the longitudinal axis of said walls or perpendicular or obliquely to the axis of the wall of the applicator with partially circular section.

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