EP0022282A1 - Radiating devices producing circularly polarized microwaves and their use in the field of microwave applicators - Google Patents

Abstract

A radiation device, operating in the range of microwaves or hyperfrequencies, providing circularly polarized waves, comprising a wave guide (5) and antennas carried by such guide, which combine the use of slots and spirals. This device may be part of a set of similar devices. It allows to provide levels of radiation power equal from one antenna to another (4), or, on the contrary, different from one antenna to another, according to the needs. Its utilization in an industrial applicator makes possible the microwave treatment of products causing strong reflexions, either because the loss factor thereof is insufficient, or because they are contained in open metal containers (3), or because they contain themselves metal portions, or because they present heterogeneities or anisotropies.

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.

Industrial microwave 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 the energy to exit the guides and radiate into the cavity.

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 towards the generator must be as weak as pos- _s ible, as not to affect performance, for protecting 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 again, 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 crossed electric field and a magnetic field, both carried by the plane perpendicular 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 major axis, and circular polarization, where the ends of the vectors describe 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 emitted itself, and with the waves emitted by other identical radiating systems, and this, even when the product to be treated gives rise to 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 an array of antennas, the description of which will be found below. 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.

The device in question, a possible embodiment of which is shown in FIG. 1, may be integral with the wall of the applicator, the antennas then plunging into it, or else be placed inside it itself.

It is a waveguide (1), that is to say a hollow metal cyclinder of a priori arbitrary section, pierced with longitudinal openings (2), carrying metal propellers (3) . The propellers are rigidly fixed to the center of a small metal bar (4), which is placed transversely to the axis of the guide, outside and at a certain 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.

, généralement à λg, λg étant la longueur d'onde de propagation dans le format de guide considéré. De telles fentes, seules, ne rayonnent pas. Il faudrait pour cela qu'elles soient écartées de l'axe, et d'autant plus qu'elles soient plus éloignées du point d'alimentation. De plus, pour éviter des problèmes de résonance, il faudrait que les distances entre fentes soient alternativement un peu supérieures et un peu inférieures à λg/2.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

, generally at λg, λg 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 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 the vacuum λo. This condition is fulfilled in the case of the present invention.

The number of antennas per radiating device can vary between the unit and high 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 movable short circuit pistons.

Furthermore, the radiating devices considered may be part of a larger assembly, consisting of several of these devices, in particular placed end to end or in parallel, and which may be supplied by different microwave tubes, or even several by the same tube, the power then being equally distributed between devices by 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 must be added, at the level of the complete device, the position of the mobile short-circuit pistons and the distance between antennas. We can in particular play on the dimensions of the slots, on the location of the propellers, on the pitch, the diameter, the section, the number of turns, the winding direction and slope of the propellers, on the dimensions of the transverse bars and their distance from the guide, over the diameter of the lengths and their depth of insertion into the guide; we can also finish the divers with obstacles of various sizes, for example blind nuts, which modify their impedance.

By the interplay of these various adjustment possibilities, is achieved by irradiating each antenna the desired amount of power, for exam- pl _e the same power for all antennas, or the powers v i _ar-able along the device. On the other hand, it is possible to obtain a good broadband adaptation, that is to say an overall level of power reflected by the device sufficiently low over a frequency range covering the entire extent of the allocated band used.

In practice, it is convenient to reduce the number of parameters by considering a certain number as fixed. 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.

The device which is the subject of the invention cannot be reduced to conventional fertile networks, nor to simple helical antennas.

The latter do not seem to have been used in the field of microwave ovens. Their use is fairly standard in radar or radiocommunications, with supply by coaxial cable and counterweight.

On the other hand, there are many examples of the use of networks of radiating slits. For example, in the systems described by French patents 70 01187 (GUERGA - HALLIER) and 70 37965 (HALLIER _- COQUET), 71 27618 (POURRAT, ZWOBADA, DENIS), 7706546 (FOURNET, FAYAS), by German patents 25 58 589 (FRtTZ), and 26 22 173 (KU-SONOKI, YOSHIMURA), or by US patent 3,961,568 (JEPPSON).

The device which is the subject of the present invention represents an innovation, in 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. .

This device has made it possible to produce several industrial microwave ovens: in particular, a tunnel oven intended for the coagulation of sardines in their unclosed metal bundles, and an applicator intended for the treatment of textile fibers representing low charges.

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 weak charges, that is to say essentially of products in small quantity or with low loss factor, or 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.

FIGS. 2 and 3 illustrate two examples of industrial applicators, of the tunnel type with conveyor belt, produced from the devices described above.

In these figures, we see in (1) the cavities receiving the "microwave" radiation. The product '(3) is transported by the conveyor belt (2) along the radiating devices (5) supplied with (6) by a "microwave" source, for example a magnetron. A movable short circuit piston (7) closes the waveguide at the end opposite the source.

A dielectric separation (4) isolates the radiating devices from the emanation of vapors, greases, dusts, produced during treatment.

A door (8) 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 and one behind the other.

Claims (6)

1) Radiant device in the microwave range, composed of a waveguide carrying an antenna which produces a circular polarization, characterized in that said antenna comprises a slot or _{a long} -length opening pierced longitudinally in the wall of the guide, and completed by a metal propeller supported by a small bar mounted outside the guide, transversely and at a certain distance from it, the ends of which are made integral with the outer wall of the guide by adjustable metal spacers ; the propeller itself being extended by a capacitive plunger, entering the guide through the opening. 2 °) device composed of a waveguide comprising an alignment of two or more antennas similar to that described in claim 1, each radiating equal, or, on the contrary, different power levels, according to the needs of the application. Said device being adjustable by means of various geometrical parameters: dimensions and location of the slots, geometrical characteristics of the antennas and their position on the openings, dimensions of the transverse bars and of the latter to the guide, shape, dimensions and depth of penetration capacitive plungers, position of the mobile short-circuit pistons ending the guide. 3 °) Device according to claims 1 or 2, characterized in that the slots are longitudinal shunt slots pierced on the axis of the long side of a rectangular waveguide. 4) microwave applicator, comprising one or more electromagnetic cavities, each equipped with one or more radiating devices similar to that described in claims 1 or 2, characterized in that the level of power reflected towards the generator is weak whatever the charge contained in the cavity, and in particular when the product subjected to the radiation is contained in open metal receptacles or itself contains metallic parts, or when the charge is weak, or even when it presents heterogeneities or anisotropies. 5 °) microwave applicator of elongated shape and rectangular or circular section, equipped with a plurality of radiating devices according to claims 1, 2 and 3, characterized in that all the devices are modular and aligned one after another, and mounted along the longitudi axis nal of one of the walls of the applicator or of a generator. 6 °) microwave applicator of elongated shape and rectangular section or having the shape of a circular segment closed by a straight line, equipped with a plurality of radiating devices according to claims 1, 2 and 3, characterized by the fact that these devices are modular and mounted along one of the walls perpendicular or oblique to the longitudinal axis of this wall or perpendicular or oblique to the axis of the applicator along the flat wall of the applicator to partially circular section.

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