WO1999004275A1 - Microwave reflectometer and method, and microwave oven equipped therewith - Google Patents

Abstract

The invention concerns a microwave reflectometer for monitoring the evolution of a microwave oven impedance, comprising a power directive feed (10) including two antennae penetrating into the waveguide (100) and mutually connected by a stripline (13), and a multi-port coupler comprising several coupling lines. The multi-port coupler co-operates with the power directive feed (10) to deliver an information concerning the reflected wave phase coming from the chamber with respect to the incident wave emitted by the magnetron. The selected distance between these two antennae is less by a quarter than the magnetron emitting wavelength. The invention is useful in microwaves for domestic and industrial purposes.

Description

"DEVICE AND METHOD FOR MICROWAVE REFLECTOMETRY, AND MICROWAVE OVEN THUS EQUIPPED"

The present invention relates to a microwave reflectometric device. It also relates to a process implemented in this reflectometric device, as well as a microwave oven equipped with this device.

Optimal use of a microwave oven, whether for domestic or professional use, means being able to follow the evolution of the impedance of this oven during cooking. A magnetron impedance diagram such as the Rieke diagram is usually used to study this evolution. One can use for this a reflectometric device composed of a directional power coupler fixed on the waveguide of the microwave oven and as disclosed in document US 4 211 911, and of a "six port" coupler as disclosed in the article entitled "A versatile easy to do six-ports based high power reflectometer" by M. Caron et al., Journal of microwave Vol.30, No4 1995. A directional power coupler used alone makes it possible to measure the power emitted by the magnetron of a microwave oven and the power reflected by this oven. It should be noted in this regard that the present invention also relates to a microwave reflectometric device including in its shortened version a power coupler used alone. There are in fact two complex voltages proportional to the microwave electric fields, incident and reflected respectively. The ratio of these two voltages provides a complex reflection coefficient seen by the magnetron with a module p and a phase φ. The function of the "six-port" coupler is precisely to recover this phase information.

Document US 4,211,911 teaches that the distance between the two antennas of a directional power coupler must be equal to a quarter of the wavelength. Now in the frequency range where operate normally microwave ovens, this distance constraint between antennas does not in practice allow the installation of such a coupler on a guide of a microwave oven for reasons of space. The object of the present invention is to remedy this drawback by proposing a reflectometric device having a smaller footprint than current devices while providing measurements of sufficient precision to allow monitoring of the evolution of the impedance of the furnace.

This objective is achieved with a reflectometry device for monitoring the evolution of the impedance of a microwave oven comprising a magnetron emitting an electromagnetic wave of predetermined wavelength and a waveguide to guide this wave from the magnetron to a heating or cooking chamber, comprising a directional power coupler comprising two antennas arranged in this waveguide and connected together by a conductive strip or stripline.

According to the invention, the distance between these two antennas is chosen to be less than a quarter of the emission wavelength of the magnetron.

Thus, it now becomes possible to place compact reflectometric devices on the waveguides currently fitted to the majority of microwave ovens, where conventional reflectometers could not be installed.

In a first embodiment, the reflectometry device according to the invention further comprises a multi-port junction circuit disposed downstream of the power directional coupler, this multi-port junction circuit cooperating with the power directional coupler to deliver , from signals delivered by said directional power circuit, information on the phase of the reflected wave coming from the enclosure with respect to the incident wave emitted by the magnetron.

In another embodiment, the reflectometry device according to the invention comprises two power-voltage converters each comprising an adaptation stage and a microwave detection stage, a first power-voltage converter cooperating with the directional power coupler for measuring the incident power and a second power-voltage converter cooperating with the directional power coupler to measure the reflected power.

According to a preferred version of the reflectometric device according to the invention, the antennas are arranged on either side of a center line of the waveguide respectively in phase planes distant by less than a quarter of the length of emission wave of the magnetron. The ribbon or stripline connecting the two antennas can then be designed to be substantially straight. This particular configuration has the effect of considerably reducing the radiation of the ribbon observed at the loops. With a straight ribbon, this radiation is minimum.

Furthermore, it is possible to reduce the attenuation usually induced by the multi-port junction circuit by fixing at the coupling points of this junction circuit microwave SMD capacities of the order of picoFarad.

According to another aspect of the invention, a method is proposed for carrying out reflectometry measurements in a microwave oven provided with a reflectometry device according to any one of the preceding claims, in which signals are taken from the respective terminals of two antennas arranged inside a waveguide, these signals are applied to a multi-port junction circuit to deliver signals allowing after processing to provide a information on the phase of the wave reflected by the cavity formed by the box of the microwave oven, with respect to the incident wave generated by the magnetron.

According to the invention, the microwave signals are taken from the waveguide at two points located in phase planes separated by a distance less than a quarter of the emission wavelength of the magnetron.

According to yet another aspect of the invention, there is provided a microwave oven comprising at least one reflectometric device according to the invention.

Other features and advantages of the invention will appear in the description below. In the accompanying drawings given by way of nonlimiting examples: - Figure 1 is a perspective view of a part of the reflectometric device according to the invention arranged on a waveguide;

- Figure 2 is a partial sectional view of a directional coupler implemented in a reflectometric device according to the invention;

- Figures 3A and 3B illustrate two embodiments of a "six-port" junction circuit implemented in a reflectometric device according to the invention; - Figure 4 is a top view of a first embodiment of a directional coupler for a reflectometric device according to the invention;

- Figure 5 is a top view of a second embodiment of a directional coupler for a reflectometric device according to the invention;

- Figure 6 shows schematically a microwave oven provided with a reflectometric device according to the invention;

- Figure 7 schematically shows a microwave oven provided with two reflectometric devices according to the invention; - Figure 8 shows a power-voltage converter implementing a reflectometry device according to the invention; FIG. 9 represents an experimental configuration for measuring the incident and reflected powers using two power-voltage converters of the type shown in FIG. 8; FIG. 10 illustrates a reading of reflectometry measurement carried out with a device according to the invention, in the case of idling of a microwave oven; and

- Figure 11 illustrates a reflectometry measurement reading, illustrating the detection of the boiling of a cup of water in a microwave oven.

We will now describe several embodiments of a reflectometric device according to the invention, with reference to the aforementioned figures.

A reflectometric device 1 according to the invention comprises, with reference to FIGS. 1 and 2, a directional power coupler 10 fixed on the upper face 11 of a waveguide 100, and a "six-port" junction circuit 30 as described for example in the article "A Versatile Easy to do Six-Port Based High-Power Reflectometer" by M.Caron et al. published in the journal 3International Microwave Power Institute "Vol.30, No4, 1995. The reflectometric device 1 is positioned on the top of the waveguide 100, and makes it possible to measure, on the one hand, the incident power, and on the other hand, the reflected power.

The waveguide 100 shown in Figure 1 typically corresponds to an application to a microwave oven 60 as shown in Figure 6. At one of its ends, this waveguide 100 communicates with a housing 13 provided to contain a transmitting antenna 62 of a magnetron 61, while its other end opens into a cavity or enclosure 63 intended for receive a load 64. The upper wall of the housing 13 contiguous to the waveguide 100 may for example have a boss 5 shaped so as to optimize the operation and the arrangement of the transmitting antenna 62 located below. The cavity 63 receives incident radiation 01 generated by the magnetron 61 and channeled by the waveguide 100, and returns reflected radiation OR in return. The directional coupler 10 of the reflectometric device 1 according to the invention is fixed on the upper part 11 of the waveguide and is connected to the "six-port" junction circuit 30 which delivers a set of four microwave signals which are filtered and processed in a processing and control module which controls the magnetron 61. The power directional coupler 10 comprises two antennas 22, 23 penetrating inside the waveguide 100 and connected to two connectors 2, 3 accessible on the upper face of the directional coupler 10. The two antennas 22, 23 are interconnected by a deposited conductive tape or stripline 13 disposed on an upper plate 6 of insulating material, for example epoxy resin, itself disposed on a lower plate 7 forming a plane massive. A metal sheet 8, for example made of aluminum, is advantageously disposed between the lower plate 7 and the upper part 11 of the waveguide 100. The power directional coupler 10 is securely fixed to the waveguide 100 by means of a set of screws (not shown). The antennas 23, 22, fixed to the connectors 3, 2 extend inside the waveguide 100 through two holes formed in the latter, and of larger diameter than that of the antennas so as to avoid short-circuiting. antenna-guide circuit.

We can consider various configurations of the ribbon or stripline inter-antenna within the directional coupler, as shown in Figures 4 and 5. In a first embodiment illustrated by Figure 4, the conductive tape 43 connecting the connectors 2, 3 of the coupler directive 40 is of substantially sinusoidal or more generally wavy shape and corresponds to an arrangement of the ports PI, P2 on the longitudinal and median line of the waveguide. Such a configuration is described in particular in document US 4,211,911.

In a second exemplary embodiment shown in FIG. 5, the connectors 3, 2 of the directional coupler 50 are offset on either side of the center line so that it becomes possible to provide a tensioned conductive tape 13.

The "six-port" junction circuit 30 includes, with reference to FIG. 3A, two directional couplers and a resonance loop. The port PI constitutes a first input port and is connected for example, via the connector 3, to the antenna 23 of the directional coupler 10. The port P2 constitutes a second input port and is connected, via the connector 2, to the antenna 22 of the directional coupler 10. The port P3 constitutes a reference port providing a power level proportional to the input power level of the port PI. A suitable load is connected to port P'3.

As illustrated in FIG. 3B, the performance of the "six-port" junction circuit 30 ′ can also be significantly improved by inserting microwave capacities C13, C13 ', C24, C26, of a fraction of picoFarad, at the points of coupling of this junction circuit. In addition to the significant reduction in the attenuation of the six-port circuit thus obtained, the addition of these capacities has the effect of much better defining the coupling points and of improving the directivity. Thus, the addition of a microwave capacity of 1 pF brings the coupling to 3dB instead of 30dB, with a satisfactory directivity of 25dB.

We will now show why the distance between the two antennas can be chosen to be less than a quarter wave. Let: L, the inter-antenna distance (to be determined) Ls, the length of the inter-antenna ribbon or stripline, λs, the wavelength in the ribbon, λg, the wavelength in the guide, βs = 2π / λs βg = 2π / λg

Cl, C2, coupling coefficients within the directional coupler.

The electrical length of the strip or stripline must respect the following phase condition: βs.Ls = βg.L + π

We now consider the complex amplitudes at the different ports of the directional power coupler, with reference to FIG. 2, taking as reference the incident voltage normalized to 1:

Port 1 (corresponding to antenna 23) 1

Port 2 (corresponding to antenna 22) _e -jβg.L Port 3 l / 2 (Cl.e ^"jps - ^s + C2. E ^{" jβg} - ^L )

port 4 l / 2 (Cl + C2.e- ^jβs - ^Ls * e ~ ^jPg - ^L )

In practice, it can be considered that the coupling coefficients C1 and C2 are substantially equal. By taking into account this equality and the aforementioned condition on the electrical length, it is easily shown that the complex amplitude of the signal on port 3 of the directional coupler is zero. Furthermore, the complex amplitude of the signal on port 4 can be expressed as follows: V = 1/2. C. (1-cos (2. βg.L) + j sin (2.βg.L) Similarly, the power of the wave on port 4 is given by the expression:

VV * = C ^2/2. (1 - cos (2βg.L)) If we choose for example a length L equal to λg / 8 (i.e. 2.167 cm), we obtain on port P4 a complex amplitude equal to l / 2C (l + j), this that corresponds to an amplitude of 0.707C phase-shifted by 45 ° with respect to port 1, and to a power equal to C ^2/2, quite sufficient to track the evolution of the impedance. It is important that the contribution of the incident signal be zero or negligible on port 3, while the contribution of the reflected signal will be zero on port 4. As a practical example of embodiment, a directional coupler having an inter-antenna distance is produced. equal to 1/8 of the wavelength of the waveguide. For example, consider a waveguide having a height of 36.6 mm and a width of 92 mm. If the wavelength of the magnetron is λ = 122.36 mm, the wavelength of the waveguide is then λg = 163.84 mm and the distance between phase planes is for example chosen to be equal to λg / 8 , namely 20.48 mm.

The characteristics of the strip or stripline placed on the power directional coupler are for example the following: relative permittivity εr = 4.3 (epoxy) wavelength in the triplate structure

The length of the ribbon is for example chosen to be 5.λt / 8 = 36.88 mm. More generally, the length of the ribbon Ls can be determined by the following relation: Ls = L / λg .λs + k. λs / 2 with k = l, 3, 5 etc.

The length Ls of the ribbon depends on the dielectric constant of the support of this ribbon. The more this dielectric constant is increased, the shorter this length; and the thinner the thickness of the dielectric plates, the narrower the strip.

Compact reflectometric devices according to the invention can also be implemented in ovens provided with a magnetron and two injection slots, as illustrated diagrammatically in the figure 7. Thus, a microwave oven 70 comprising a magnetron 71 and two injection slots 721, 72S respectively lower and upper, is equipped with two reflectometric devices II, IS respectively lower and upper arranged on waveguides 731 , 73S on either side of magnetron 71. In this configuration, it is possible to measure reflection coefficients greater than 1 because one wave can exit through a slot and enter through the other. It should be noted that, within the framework of the present invention, it is possible to envisage a reflectometry device comprising a power coupler used alone without a six-port coupler, but with two detectors. This coupler provides the amplitude of the reflection coefficient, which is sufficient in simple cases such as idling safety.

Thus, with reference to FIG. 8, it is possible to design a power-voltage converter 80 composed of an adaptation stage 81 produced in micro-strip technology comprising at the input, for example, an SMA connector 811 or the end of a stripped coaxial cable, and a 50Ω ribbon 812, a microwave detection stage 82 comprising a Schottky detection diode 822, a 50Ω ribbon 821 and a radial ferrite 823, and a follower stage or equivalent 83 for computer processing of the voltage measurements obtained. This type of detector can also be used in the case of the implementation of a six-port coupler in P3, P4, P5, P6.

In order to measure the incident and reflected powers, two power converters 91, 92 are placed in a structure 90 illustrated in FIG. 9. These converters measure an incident power and a reflected power by recovering two proportional electrical signals Si, S2, picked up by the reflectometer R antennas inside the waveguide. By calibration, it is possible to determine the incident power emitted and the power reflected by the load and the cavity. We deduce the power consumed from the difference between the incident power and the reflected power.

The present invention can be implemented in any type of microwave oven, for any sector of activity, domestic or professional. In particular, microwave ovens used in chemistry experiments can advantageously be provided with reflectometric devices according to the invention.

Thanks to the reflectometry device according to the invention, it becomes possible to identify idle phases of a microwave oven as illustrated in FIG. 10, in which the continuous curve represents the time evolution of the coefficient of reflection for a cup of water, while the dotted curve represents the evolution of this reflection coefficient in the event of idling. We can also detect situations when a cup is boiling, as shown in Figure 11.

Of course, the invention is not limited to the examples which have just been described and numerous modifications can be made to these examples without departing from the scope of the invention. Thus, the dimensions and shapes of the waveguides and of the components of the reflectometric devices may vary depending on the type of oven equipped. It is the same for the frequencies and wavelengths considered in the present invention.

Claims

1. Microwave reflectometry device (1) for monitoring the evolution of the impedance of a microwave oven (60) comprising a magnetron (61) emitting an electromagnetic wave of predetermined wavelength (λ) and a waveguide (100) for guiding this wave from the magnetron (61) to a heating or cooking chamber (63), comprising a power directional coupler (10) comprising two antennas (23, 22) arranged in said guide wave (100) and connected by a conductive strip or stripline (13, 43), characterized in that the distance (L) between these two antennas (23, 22) is chosen to be less than a quarter of the wavelength ( λ) emission of the magnetron (61). 2. A reflectometry device according to claim 1, characterized in that it further comprises a multi-port junction circuit (30, 30 ') disposed downstream of the directional power coupler (10), this multi-junction circuit ports (30, 30 ') cooperating with the power directional coupler (10) to deliver, from signals delivered by said power directional circuit (10), information on the phase of the reflected wave (OR) coming from of the enclosure (63) with respect to the incident wave (01) emitted by the magnetron (61). 3. Reflectometry device (90) according to claim 1, characterized in that it comprises two power-voltage converters (91, 92; 80) each comprising an adaptation stage (81) and a microwave detection stage (82 ), a first power-voltage converter (91) cooperating with the directional power coupler (R) to measure the incident power and a second power-voltage converter (92) cooperating with the directional power coupler (R) to measure the reflected power. 4. Reflectometry device according to one of the 5 preceding claims, characterized in that the antennas (23, 22) are arranged on either side of a longitudinal and median line of the waveguide (100) respectively phase planes distant by less than a quarter of the emission wavelength (λ) of the 0 magnetron (61). 5. A reflectometry device according to one of claims 1 to 3, characterized in that the antennas are substantially aligned on a longitudinal and median line of the waveguide. 6. A reflectometry device according to one of claims 4 or 5, characterized in that the strip or stripline (13) connecting the two antennas (23, 22) is 0 substantially rectilinear. 7. A reflectometry device according to claim 6, characterized in that the length Ls of the stripline strip (13) is determined by the following relation: Ls = L / λg. λs + k. λs / 2 with k = 1, 3, 5 etc. where 1 represents the difference between the two antennas, λs represents the wavelength associated with the directional coupler of the three-plate structure. 8. A reflectometry device according to claim 7, characterized in that k is chosen equal to 1 and 1 is chosen equal to λg / 8. 9. A reflectometry device according to any one of the preceding claims, characterized in that the multi-port junction circuit (30 ') comprises at each coupling point a microwave capacity (C13, C13 ', C24, C26). 10. A reflectometry device according to claim 9, characterized in that these microwave capacities are of the order of picofarad. 11. A reflectometry device according to any one of the preceding claims, characterized in that the directional power coupler is produced in the form of a three-plate circuit. 12. A reflectometry device according to claim 11, characterized in that it further comprises a metal foil or a conductive plate (8) inserted between the lower ground plane of the directional power coupler (10) and the upper part (11 ) of the waveguide (100). 13. Method for carrying out reflectometry measurements in a microwave oven (60) provided with a reflectometry device (1) according to any one of the preceding claims and claim 2, in which two signals are sampled antennas (23, 22) penetrating inside a waveguide (100), these signals are applied to a multi-port junction circuit (30) to deliver signals allowing after processing to provide information on the phase and modulus of the reflected wave (OR) by the enclosure or the cavity with respect to the incident wave (01) generated by the magnetron (61), characterized in that the antennas (23, 22) are distant by a distance less than a quarter of the emission wavelength of the magnetron (61). 14. Microwave oven (60) comprising a ref lectometry device (1) according to any one of claims 1 to 12. 15. Microwave oven (70) comprising at least two reflectometry devices (IS, II) according to any one of claims 1 to 12 arranged on waveguides (73S, 731) on either side of a magnetron (71).