US 3745292 A
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United States Patent 1 Couasnard July 10, 1973  HEATING DEVICES FOR CARRYING OUT 3,670,133 6/1972 Admiraal et al. 219/ 10.55 HIGH FREQUENCY HEATING BY 3,127,494 3/1964 Kellough et al 219/ 10.55
DIELECTRIC LOSSES GENERATOR 7 Primary Examiner-J. V. Truhe Assistant Examiner-Hugh D. Jaeger Attorney-Cushman, Darby & Cushman [5 7 ABSTRACT The dielectric loss heating devices of the invention apply high-frequency energy to a load circuit in which the component to be heated is placed, through the medium of a 3 dB directional coupler. The two outputs of said coupler, which carry equal energy fractions, are coupled to said load circuit in order that there propagate through same two independent waves whose polarisation directions are perpendicular to one another. The waves reflected by the object being heated are dissipated in a matched load connected to the fourth channel of the coupler, thus protecting the highfrequency generator.
6 Claims, 4 Drawing Figures Patented July 10, 1973 2 Sheets-Sheet 1 Patented July 10, 1973 2 Sheets-Shoot. 2
HEATING DEVICES FOR CARRYING OUT HIGH FREQUENCY HEATING BY DIELECTRIC LOSSES The present invention relates to improvements in high-frequency devices for heating components of dielectric material, said components, which are arranged in a high-frequency electromagnetic field, being rapidly heated as a consequence of energy dissipation within their mass.
The invention relates more particularly to a device which, whilst achieving a high-quality heating as for example a uniform heating, prevents waves reflected by the component being heated, from damaging the highfrequency generator.
High-frequency heating by dielectric loss, is a well known technique which is utilised in numerous applications such for example as the processing of certain materials, th heating of components for moulding, forming, welding, drying, etc. etc.
Various kinds of devices are used depending upon the dimensions, shape and nature of the components being processed. In a general way, microwave energy produced by a tube generator such as a klystron or magnetron for example, is supplied to the component being heated, by a waveguide. Depending upon the case, component can be placed in the waveguide itself, in a cavity supplied by the waveguide or may be arranged in front of a radiating horn which terminates the waveguide. In virtually all cases, the introduction of the component into the electromagnetic field results in a reflection coefficient and a not inconsiderable reflected energy fraction which returns to the generator and can disturb its operation, possibly even damaging it.
One object of the present invention is to create a device for carrying out heating by dielectric loss while protecting the generator against the energy reflected by the component being heated.
Another object of the invention is to produce a heating device of this kind which-in respect of components which are of small dimensions vis-a-vis the wavelength of the energy used, achieves particularly uniform heating. Many other arrangements for carrying out heating or other kinds of components are in the scope of the invention; All these arrangements make use of a generator being protected against the reflected energy according to the invention.
In a device for heating components by dielectric loss, as proposed in accordance with the invention, the highfrequency energy is applied to a load high-frequency circuit (for example a waveguide or a cavity) through the medium of a 3 dB directional coupler which splits the power supplied to it by the generator, into two equal fractions. These two energy fractions are coupled to the load circuit in order that there shall propagate through same, two waves whose polarisation directions are at right angles to one another. The component being heated, receives these two waves and the energy which it reflects is wholely directed by the directional coupler to a matched load, so that the generator'is quite protected. For proper operation of such a device, and efficient protection of its generator, the circuits where the two waves propagate should have the same structures and in particular must have the same electrical length; as to the component being heated it should be sufficiently symmetrical in relation to the two waves not to modify the phase-shift between the two reflected waves.
Other objects, features and results of the invention will become apparent from the ensuing description which is given by wayof non-limitative example and illustrated in the attached figures where:
FIG. 1 is a plan view, schematic and partially in sec tion, of one embodiment of a heating device in accordance with the invention;
FIG. 2 is a perpspe'ctive view of part of the device shown in FIG. 1;
FIGS. 3 and 4 are schematic sectional views of the load waveguide and the component being heated, in two kinds of applications.
The three first figures discussed hereinafter relate to one and the same embodiment, which has been described in more detail here, whilst the fourth relates to a different embodiment. It goes without saying that in these figures similar references indicates similar elements.
As already briefly mentioned hereinbefore, in the heating devices in accordance with the invention the energy supplied by a high-frequency generator 1 is applied to an input channel 2 of a 3 dB directional coupler 3; each of the two output channels 4 and 5 of said coupler 3 delivers half the energy supplied at 2. The fourth channel 6 of this coupler is terminated in a matched load 7.
The 3 dB coupler can be designed in accordance with one or other of the conventional principles used for these devices. In the example described here, the waveguides constituting the channels of the coupler are coupled by a hole 8 formed in their adjacent shorter sides.
A well known property of 3 dB directional couplers is that incident energy supplied to an input channel, channel 2 in this case, is split equally between two output channels, in this case channels 4 and 5, the waves propagating through these two channels being in phase quadrature; if these two waves are then reflected without any change in relative phase, the whole of the reflected energy will be picked up in the fourth channel,
in this case channel 6.
The invention exploits this property in order to protect the generator 1 against the waves reflected by the object being heated thanks to a special arrangement an embodiment of which will now be described.
The output channels 4 and 5 of the coupler 3 are connected to a load circuit, which in the example hereindescribed is a load waveguide 11, by two curved waveguide sections 9 and 10 which pivot the directions of propagation of the two waves they transmit, respectively through +45 and 45 in relation to the direction of propagation of the two waves through the coupler channels. The waveguide sections 9 and 10 are constituted in this example by I-I-plane corners in which the plane containing the magnetic component of the wave and its direction of propagation, does not change.
The dimensions of the waveguide 11 and its arrangement in relationship to the waveguide sections 9 and 10, are such that said waveguide 11 simultaneously and independently transmits the two waves, polarised at rightangles to one another, which come from the waves propagating through the waveguide sections 9 and 10.
In the example described here, the waveguide 11 is a square-section waveguide which can transmit both the TE, and the TE, mode. It is disposed in such a fashion that its direction of propagation is at rightangles to the directions of propagation of the waves in waveguide sections 9 and 10. The ends of the waveguide sections 9 and 10 are coupled to it through two openings respectively formed in two of its adjacent faces, in a manner shown in FIG. 2. The result is the production in the waveguide 11 of two independent waves polarised at rightangles to one another. If, and this is the case on most occasions, the electromagnetic waves are propagated through the waveguides which supply energy to the load waveguide 11, in the fundamental mode TE their electric components e and e are converted in the waveguide 11 respectively into E, and E These two electrical components of the two waves propagated through the waveguide 11, are thus at rightangles to one another. They are moreover in phase quadrature. In other words, the waves supplied at 4 and 5 by the coupler 3 are in phase quadrature and the waveguide sections 9 and 10 are symmetrical in order not to disturb this relative phase condition.
The fact that these two waves are polarised at rightangles to one another, furthermore makes it possible to prevent any coupling between the waveguide sections 9 and 10, the wave having the electric component E being unable to propagate through the waveguide section 10, whilst the wave having the electric component E cannot propagate through the section 9.
As far as the quadrature relationship between their phases is concerned, this not only serves to protect the generator because of the property of the coupler, but also to render the heating uniform as will be explained hereinafter.
A short-circuiting piston 12 closes off the waveguide 1 l at one of its ends; it makes it possible, in accordance with conventional techniques, to match the transition consititued by the junction between the three waveguides 9, l0 and 11 in order in particular to reduce in the best possible way any parasitic modes which might appear.
The load waveguide 11, in the example described, is coupled by an aperture 13 to a resonant cavity 14 whose-resonance frequency is matched to that of the generator 1 by a short-circuiting piston 15. That part 16 of an object 17 (FIG. 3) which is to be heated, is introduced into the cavity 14 through an opening 18 formed in the centre of the piston 15.
For optimum operation of the device in accordance with the invention, as will be explained the component 16 to be heated must be located symmetrically inside the cavity 18. Moreover, the protection afforded to the generator 1 will be the better because the symmetry of said component 16 approaches to the symmetry of revolution about the axis of the waveguide 11.
In other words, the two waves of electric components E, and E used to heat the component 16, are partially reflected by the latter. In order that the recombination property of the coupler 3 shall efficiently protect the generator 1 against the thus reflected waves, it is necessary for the reflected waves entering the waveguide sections 9 and 10 to have the same strength and to have retained a relative phase-shift of 90. This is achieved when the two waves striking the component 16 are reflected identically, that is to say in particular when said component is symmetrical in relation to the two waves. 6
In this case, virtually no relfected energy returns to the generator 1; it is directed instead to the matched load Thus, the device described makes it possible to heat a component or a part of a component, without the microwave generator running any risk of damaging.
Moreover, the device makes it possible to achieve particularly uniform heating of all points on the component. The electromagnetic vibration reaching the component is, in effect, the resultant of two vibrations containing the rectilinear components E, and E which are perpendicular to one another and in phase quadrature. It is well known, from polar theory, that this kind of re sultant is a circular vibration in the plane of the two directions E and E about an axis perpendicular to said plane, in this case the axis of the waveguide 11.
Other, different embodiments of the device enable these same results to be achieved. Some of them are mentioned briefly here by way of non-limitative example; their implementation, having in mind the device which has just been described, is within the scope of the person skilled in the art.
As already indicated, the 3 dB directional coupler can be designed in any known manner, and the two waveguides which constitute it can in particular be coupled through their longest faces.
The load waveguide 11 may be cylindrical. In this case, the two waveguide sections 9 and 10 are couupled to it at two ends of two perpendicular radii, and the two waves whose polarisation directions are at rightangles to one another and are propagated through this waveguide 11 in the TE, mode.
The cavity 14 can be cylindrical itself, whether the load waveguide 11 is cylindrical or square in section.
The coupling between the waveguide sections 9 and 10 and load waveguide 11 can be efiected in different manners, the large dimension of the cross-section of the waveguide sections 9 and 10, being parallel to the direction of propagation through the waveguide 11.
Moreover, heating devices of this kind which exploit dielectric loss, can be used in other ways than that described in relation to the example given here; the cavity 14 can, for example, be replaced by a radiating horn before which the object to be heated is placed.
FIG. 4 schematically illustrates this kind of application in which the load waveguide 11 is terminated in a radiating horn 20. The object 21 being heated is placed in front of said horn. Th object is for example a block of construction material such as concrete, which is to be broken up. Heating at different points of the block renders it brittle and makes it possible to break it up more readily, using a known technique. A heating device such as that of FIG. 4, is particularly well suited for producing this kind of heating, without any risk of damaging the high-frequency generator.
What I claim is:
1. A heating device for carrying out high-frequency heating of a component by producing dielectric losses within the body of said component and comprising:
a high-frequency generator,
a 3 dB directional coupler having four channels, one of said channels, called input channel receiving the high-frequency energy from said generator, two other of said four channels, called output channels, each delivering one half of said energy and the fourth of said four channels being connected to a matched absorbing load,
and a load high-frequency circuit within which is disposed said component to be heated, said load circuit being coupled both to said output channels of said directional coupler thereof producing within said load circuit two electromagnetic waves the electric component of which are polarised perpendicularly to one another, are phase-shifted by 90 and are perpendicular to the axis of said load circuit.
2. A heating device as claimed in claim 1, wherein said load circuit is a waveguide, said component to be heated being disposed along the axis of said waveguide, and wherein said two electromagnetic waves are both propagated through said waveguide in the same direction, said direction being perpendicular to the plane containing said electric components of said two waves, and being parallel to the axis of said waveguide.
3. A heating device as claimed in claim 2, wherein the coupling between said output channels of said directional coupler and said load waveguide, is provided by means of two waveguide sections which modify the direction of propagation of the electromagnetic which they transmit, so that said two directions are at rightangles to one another at the outputs of said waveguide sections.
4. A heating device as claimed in claim 3, wherein said load waveguide is a square-section waveguide, said two waveguide sections are rectangular-section waveguides and are connected respectively to said load waveguide at two of the latters adjacent faces, and the cross-sections of said waveguide sections are orientated in the same way.
5. A heating device as claimed in claim 2, wherein said load waveguide is terminated at one of its ends in a resonant cavity to which it is coupled, said component to be heated being arranged inside said cavity, and is terminated at its other end in a short-circuit tuning stub. I
6. A heating device as claimed in claim 2, wherein said load waveguide terminates at one of its ends in a radiating horn in front of which said component to be heated is located, its other end being terminated in a short-circuit tuning stub.