EP1190239A2

EP1190239A2 - Stray field probe

Info

Publication number: EP1190239A2
Application number: EP00931258A
Authority: EP
Inventors: Jörg Tobias; Matthias Busse; Reinhard Prof. Dr. Knöchel
Original assignee: Hauni Maschinenbau GmbH
Current assignee: Koerber Technologies GmbH
Priority date: 1999-06-03
Filing date: 2000-05-23
Publication date: 2002-03-27
Also published as: PL352127A1; CN1353812A; DE19925468A1; WO2000075645A2; JP2003501655A; AU4924700A; CN1173168C; WO2000075645A3; US6411103B1

Abstract

The invention relates to a stray field probe for measuring the dielectric properties of materials. Said probe comprises generating elements for generating an electric field and shielding elements to shield the generated electric field. Said shielding elements are configured in such a way that the electric field is situated at least partially outside the shielding elements. The invention is characterised in that the shielding elements have at least two openings for decoupling the electric field in the external zone.

Description

Stray field probe

The invention relates to a stray field probe for measuring the dielectric properties of substances, with generating means for generating an electrical field, with shielding means for shielding the generated electrical field, the shielding means being designed such that the electrical field is at least partially outside the shielding means.

Stray field probes of this type are known from the prior art. Using a stray electromagnetic field, these stray field probes measure dielectric properties of substances that are located in the stray electromagnetic field. For example, with the help of such stray field probes, the effective dielectric constant of a substance can be measured, in order to in turn indirectly determine the type of substance, its moisture, its density and the like. close. For a better understanding of such stray field probes and in particular for a better understanding of the present invention, the general technical background of such stray field probes will be discussed first. The aforementioned stray field probes are more generally referred to as dielectric probes. Such dielectric probes preferably generate an electrical high-frequency alternating field. The substance to be examined is then brought into this high-frequency alternating field, which increases the displacement current. A capacitor formed in this way is preferably part of a resonant circuit. Because the resonance increase in the field strength increases the sensitivity of the dielectric probe for determining changes in the dielectric constant. A dielectric constant that increases as a result of the introduction of the substance to be examined lowers the resonance frequency, while losses in the substance to be examined dampen the resonance.

There are two ways to examine the substance. On the one hand, the substance to be examined can be introduced into the resonator. Such a procedure is known, for example, from our own patent application DE 1 97 05 260 A1 and the associated additional application DE 1 97 34 978 A1. In the devices shown there, the resonator of the dielectric probe is arranged in a housing, which housing each has an inlet and an outlet opening. A string of tobacco, in particular a string of cigarettes, is then passed through this inlet or outlet opening according to the device disclosed therein. Thus, in the device shown there, by passing the cigarette rod through the resonator, the resonance frequency of the resonance circuit is influenced and measurement of the properties of the cigarette rod, for example the moisture or the density of the cigarette rod, is possible. A precise measuring method for measuring these resonance shifts is also described in the cited documents, so that a new description is not required at this point and reference is expressly made to the measuring methods described in the cited documents.

The arrangements and methods described in the aforementioned publications for measuring the density of a substance, in particular for measuring the density of a tobacco rod, have proven themselves in the prior art. Because of the methods and arrangements disclosed in the above-mentioned documents, the density can be measured in a control loop which at the same time influences the density. With the aid of these devices, the density of a substance examined in this way, for example the tobacco rod mentioned several times, can be set precisely. The method described in the cited documents measures the resonance value at at least two points in the resonance curve of the resonator through which the substance is passed. In addition to the average of the measured values and the difference, the dielectric properties, for example the moisture of the substance and then the density of the substance can be calculated.

Particularly in the tobacco processing industry, however, it is desirable that in particular the moisture of the substance to be examined, i.e. in the tobacco processing industry, the moisture of the tobacco to be processed can be measured very early. So it is desirable in the tobacco processing industry to examine the tobacco as bulk goods, i.e. even before the tobacco is introduced into the tobacco rod. Because only at this point can the moisture of the tobacco be sufficiently influenced. If the tobacco has already been processed into the tobacco rod, the moisture of the tobacco processed in this way can no longer be influenced sufficiently. From this it follows that it is necessary that the substance to be examined is not introduced as a sample into the resonator itself, but that it is guided past a resonator, from which resonator a stray field emerges. Such an embodiment of a resonator relates to the present invention, in particular the stray field probe of the present invention mentioned at the beginning.

With such stray field probes, the substance to be examined is thus passed through a stray field present on the outside of the actual resonator. For this purpose, the resonator should preferably be designed such that the electrical field of the resonator is at least partially outside a shielding means, while the rest of the electrical field and also the magnetic field are within the shielding means and thus not with the substance to be examined, for example, the bulk tobacco to be examined interacts. Thus, in the case of the stray field probes of the type mentioned at the outset, only a portion of the electric field which is located within the shielding means is coupled into the outside. There, the stray electrical field coupled into the exterior interacts with the substance to be examined. For the measurement of the moisture of substances to be examined, for example the moisture of a tobacco, the measurement in the microwave range has proven to be particularly advantageous.

In order to be able to precisely determine the above-mentioned losses in the substance to be examined with the help of such stray field probes, it is necessary that radiation losses from the stray field probe are avoided. This is necessary, because in addition to such losses in the material to be examined, in addition to ohmic losses in the conductors, which are known, only dielectric losses occur in the case of non-magnetic materials. The latter are the goal of the measurement. It is therefore important to suppress the radiation losses as much as possible. Suppression of the radiation losses in this way means that the far field of the electric field is suppressed, so that there is only a near field in the vicinity of the stray field probe. The aforementioned radiation losses are avoided in particular if the radiation components of the far field cancel each other out for all possible angular positions because of their phase opposition. Such mutual extinction of the radiation components in the far field is only possible if the diameter of the opening through which the stray field emerges from the shielding means of the stray field probe is significantly smaller than the wavelength used. However, this in turn has the disadvantage that, owing to the small diameter of the stray field openings in the stray field probe, measurements, for example, of the moisture of the substance to be examined can only be carried out in this area. Because of the stray field probes constructed in this way, ie in the case of stray field probes with small openings, only selective measurements of the dielectric constants of the substance to be examined are obtained. Again, this is disadvantageous if the substance to be examined is inhomogeneous. Such inhomogeneity is present, for example, in the case of bulk goods. For example, in the case of loose tobacco, as used by the tobacco processing industry, there is already a strong inhomogeneity in the loose tobacco due to the arbitrary arrangement of the individual tobacco fibers. The moisture content of such an inhomogeneous tobacco can therefore only be measured very inaccurately with the aid of scattered field probes.

To overcome the aforementioned disadvantage, it has now been proposed in the prior art to use a plurality of such stray field probes. With the help of several stray field probes or resonators it would then be possible, e.g. leaf tobacco, which is often also not in a climatic equilibrium, to approximately determine the moisture of the leaf tobacco even with an uneven moisture distribution in the individual leaf of the leaf tobacco, by using the number of stray field probes used is averaged. Such an arrangement of several probes is known, for example, from EP-A-0 558 759 A1.

A disadvantage of the arrangement of a plurality of resonators or However, it is stray field probes that - if, for example, measurements are carried out in the microwave range - the microwaves have to be divided by a power divider and have to be mixed together again behind the resonators or probes. This requires a high level of components. In addition, such a measuring arrangement requires that very high-quality resonators are used to ensure that all resonators operate with an identical resonance frequency. All in all, a high technical effort is therefore made in the prior art to overcome the problems of radiation.

It is therefore an object of the present invention to improve a stray field probe of the type mentioned at the outset in such a way that the aforementioned disadvantages are avoided and a stray field probe is provided which reliably tests even dielectric materials, in particular their moisture, on inhomogeneous substances or bulk material can, while at the same time it is simply constructed.

This object is achieved in a stray field probe of the type mentioned in the introduction in that the shielding means for coupling out the electrical field have at least two openings.

By decoupling the stray field according to the invention at two points of the shielding means, it is advantageously possible for inhomogeneous substances to have their dielectric properties even at small openings / points through which the stray field emerges or comes into contact with the substance to be examined examine. The moisture of the investigated substance can then be determined from these dielectric properties. It is thus possible thanks to the invention, even with inhomogeneous substances, such as leaf tobacco, which has an inhomogeneous moisture and density distribution, by a sufficiently large number of points for decoupling the electrical field or for coupling the electrical field to the one to be examined To carry out averaging of the individual measured values and thus to obtain a more precise measured value than was previously possible in the prior art. In particular, the disadvantages resulting from the arrangement of a large number of independent stray field probes according to EP-A-0 558 759 A1 are avoided by the invention. This is because the complex construction of the individual probes required there or the necessary division and merging of the electromagnetic waves before and after the probe is no longer necessary due to the invention. Because the invention uses only a single stray field probe or a single stray field resonator while it at the same time, the stray field is coupled into the outside at several points. It is thus possible according to the invention to cover a large surface area with the aid of a resonator according to the invention without having to accept the disadvantages of large openings, ie radiation. The invention is therefore particularly advantageously suitable for use in the tobacco processing industry. Because - as already mentioned several times - it is particularly important to analyze loose tobacco or leaf tobacco, which is inhomogeneous in terms of density and moisture. Such an analysis is now possible thanks to the invention in a simple manner with high accuracy.

The electromagnetic field generated by the generating means is preferably a high-frequency alternating field. The generating means are preferably designed so that the stray field probe forms a resonator, i.e. that the high-frequency alternating field is in resonance. The preferred measurement method in this context and the exact measurement setup is described in the above-mentioned German patent applications DE 1 97 05 260 A1 and DE 1 97 34 978 A1. A renewed explanation is therefore omitted here, since the method described in these publications can be transferred to the present invention by the person skilled in the field of microwave measurement technology without problems.

In a preferred embodiment of the invention, the shielding means are designed as a housing for the generating means. The openings are designed as holes in the housing. The housing is preferably a metallic housing.

A particularly preferred embodiment decouples the electric field at locations of the same amplitude of the electric field by arranging the openings in the shielding means or in the housing such that they are located at the locations of the same amplitude. It is particularly preferred if the locations of the same amplitude are the maximum electric field of the same name. Thus, by interrupting the shielding of the resonant electric field at points of the half-wave maxima of the electric field with the same name, a part of the electric field is decoupled as a stray field to the outside of the shielding means in such a way that each decoupling by itself does not radiate, ie none Far field is present. This is how it occurs in the present embodiment of the invention particularly advantageous for limiting the stray field to the immediate vicinity of the stray field probe constructed in this way. Because of the polarity of the same name of the electrical stray field components that emerge from the stray field probe, no long connecting field line can occur between the different openings in the shielding means, which field line would lead to radiation, since there is no potential difference between the individual openings in the shielding means in the outer field.

In this embodiment, it is therefore particularly advantageous that a series of stray field probes or stray field resonators can be completely dispensed with. Rather, thanks to the present invention, in this embodiment, a single stray field resonator is sufficient to provide several measurement points with the aid of the plurality of openings, so that inhomogeneous measurement objects can also be measured. Thus, with the aid of this embodiment, measurement of leaf tobacco or loose tobacco, for example, which has an inhomogeneous density and moisture distribution, is also possible without any problems.

In a further preferred embodiment of the invention, the generating means for generating the resonant electric field are designed as waveguides. The waveguide is preferably designed as a line resonator by short-circuiting at both ends. The length of the waveguide is particularly preferably selected so that it is an integral multiple of half the wavelength. It is particularly preferred if the length of the waveguide is three times half the wavelength, even more preferably at least ten times half the wavelength at resonance. In these embodiments, any number of measuring points can therefore advantageously be provided by simple means. For example, to provide an entire area with measuring points, the line resonator can be designed as a meander. In this way, a maximum number of measuring points per area can be accommodated. The surface is then preferably the surface of a wall on which the bulk material to be measured, for example the loose tobacco to be measured, is guided past.

Another preferred embodiment has a waveguide designed as a cross. It is particularly preferred if the integer multiple of half the wavelength, which forms the total length of the cruciform waveguide, is an odd integer multiple of half the wavelength. Because at In such an embodiment, the electrical field, for example a microwave field, can be coupled into the waveguide in the middle of the line. Such an arrangement, which can also be selected with a meandering arrangement of the waveguide, prevents additional resonances occurring at frequencies other than the desired resonance frequency if the waveguide is long. Because such resonances can lead to mutually interfering resonances with a long length of the waveguide or with many openings in the shielding means, for example in the housing around the resonator serving as generating means. The choice of the target frequency as an odd multiple of half the wavelength is an advantageous countermeasure in this regard. Because then there is a field maximum in the middle of the line resonator. If the field is now coupled into the line resonator at this point, all resonances at which n is even are suppressed, since those there have a field zero. Such suppression of undesired resonances is also ensured in other embodiments which provide any coupling of the electrical field into the line resonator which is symmetrical with respect to the center of the waveguide.

In the cross-shaped arrangement of the line resonator already mentioned above, it is particularly preferred that each leg of the cross has a length which corresponds to five times half the wavelength of the resonance. In this way, including a measuring point provided in the middle, five measuring points or openings can be provided in the housing for the line resonator. This embodiment was therefore a particularly simple and compact embodiment of the invention, which at the same time provides a sufficient number of measuring points.

In the above-mentioned cruciform embodiment of the waveguide, the distance between the actual line resonator and the housing which surrounds the line resonator is preferably provided by a pin connected to the line resonator at the points at which the electrical field is coupled out into the outer space, which pin is perpendicular to the Line resonator stands. In this way, the stray field can be coupled out particularly easily in order to interact with the material to be measured. It should be noted that the line length of the pins, which lead from the line resonator to the respective measuring window or opening in the shielding means, are included in the length of the waveguide or line resonator. It is particularly preferred if the waveguide is designed with high resistance. This design of the waveguide results in a particularly high quality of the line resonator thus formed, so that the measurement results also have a special quality.

In a further embodiment of the invention, the electrical field is coupled into the housing through openings in the housing surrounding the line resonator, which openings lie opposite the actual measurement openings in the housing. The coupling and decoupling is carried out with the help of appropriate antennas. As already mentioned above, the coupling of the electric field into the resonator is preferably carried out in the middle of the resonator. The decoupling can - but this is not absolutely necessary - be provided in the vicinity of the line ends of the line resonator, since there the conversion into the suppressed interference modes is low. In this regard, it should be noted that a symmetrical arrangement of the coupling antennas, i.e. a symmetrical decoupling completely avoids such conversion.

Another preferred embodiment uses a cavity resonator as the generating means for generating the electrical resonant field. Here, too, the electric field is coupled into the cavity, preferably in the middle of the cavity. The coupling of the generated electrical resonant field in the cavity resonator to the outside or to the measurement object takes place via antennas. These antennas are preferably designed as cylinders made of a dielectric, more preferably made of plexiglass cylinders.

Further preferred embodiments of the invention are set out in the subclaims.

Preferred embodiments of the invention are described below in the figures of the drawing. The drawing shows:

1 shows the principle on which the invention is based;

2 shows an embodiment of the invention with a meandering waveguide;

Fig. 3 shows the embodiment of Figure 2 in a side view.

Fig. 4 shows an embodiment of the invention with a cross shape arranged waveguide;

Fig. 5 is an exploded view of the embodiment of Fig. 4;

Fig. 6 is a side view of the embodiment of Fig. 4;

7 shows a schematic plan view of a further embodiment of the invention with a cavity resonator;

Fig. 8 shows the electrical field distribution of the cavity resonator

Fig. 7; and

Fig. 9 shows a disturbing field distribution of the cavity resonator

Fig. 7.

1 schematically shows the principle of the present invention. The electric field is symbolized by the letter E. The corresponding magnetic field is symbolized by the letter H. The letter I symbolizes the length of the waveguide used in this embodiment. The signs "+" and "-" symbolize half-waves of the same name of the electric field E. According to the invention - as already explained above - the openings in the shielding means according to the invention are arranged at the locations of the shielding means which are adjacent to that in FIG + designated amplitudes of the same name of the electric field E.

FIG. 2 shows an embodiment of the invention with a line resonator 2 arranged in a meandering shape in a schematic plan view. The schematically illustrated line resonator 2 is arranged in a plane parallel to the paper plane in FIG. 2. The line resonator 2 is located in a housing 4. The housing 4 has a wall 4a facing the viewer in FIG. 2. The wall 4a is interrupted for the purpose of illustration within the dashed line 6 in order to be able to represent the line resonator 2 arranged in a meandering manner.

The line resonator 2 is a dielectric waveguide, which is short-circuited at its ends 8 in order to produce the line resonator 2.

Furthermore, the housing has 4 openings 10. The openings 1 0 are circular. The diameter of the openings 1 0 is small compared to that through the Line resonator 2 generated resonance wavelength. In the area of the openings 10, the small arrow that extends radially from the center of the openings 10 symbolizes the stray electrical field coupled out from the housing resonator 2 at these points from the housing wall 4a in the direction of the viewer of FIG. 2. This stray electrical field is emitted by pins 1 2 arranged perpendicular to the plane of the drawing on the line resonator 2 at the center of the openings 10. The pins 1 2 thus serve as decoupling antennas for decoupling the stray electrical field from the line resonator 2 to the outside of the housing 4.

The arrangement of the openings 1 0 and the pins 1 2 is chosen so that the openings 1 0 and the pins 1 2 are located at the same amplitude at the same place as the electrical field generated by the line resonator 2. 1, the pins 1 2 are located at the amplitude maxima of the electric field E denoted by the sign "+" in FIG. 1.

3 shows the embodiment of FIG. 2 in a side view. The same parts are labeled with the same reference numerals. Again, the arrows from the pins 1 2 on the line resonator 2, which arc in the openings 10 in the housing wall 4a, symbolize outward and again on the outside 4b of the housing wall 4a, the stray field coupled out by the line resonator 2 by means of the pins 1 2.

In addition, the lower wall 4c of the housing 4 serving as shielding means is shown in FIG. 3. The lower wall 4c runs parallel to the upper wall 4a of the housing 4. The lower wall 4c has an opening 14 in the middle. In the opening 1 4 there is a coupling antenna 1 6 with its longitudinal axis perpendicular to the plane of the lower wall 4c. The field is coupled into the housing 4 or the line resonator 2 from the coupling antenna 1 6.

If a substance is to be examined for its dielectric properties, this substance is directed past the stray field probe 1, which is supplied with the electrical field via the coupling antenna 1 6, directly past the outside 4b of the wall 4a of the housing 4. If, for example, loose tobacco is to be examined for its dielectric properties in order, for example, to determine the moisture of the tobacco, this is shown on the first side 4b of the wall 4a by the stray field of the symbolized in FIG. 3 by the arrow above the wall 4a Line resonator 2 passed through. This can happen, for example, in that the stray field resonator 1 is part of an almost vertical, but slightly opposite, vertical wall, along which the loose tobacco glides. This resonance of the loose tobacco through the stray field emitted by the pins 1 2 changes the resonance curve of the line resonator 2. This change in the resonance frequency and the damping of the resonance frequency can be determined with the aid of the methods and test setups described in German patent applications DE 1 97 05 260 A1 and DE 1 97 34 978 A1. Subsequently, the average moisture content of the loose tobacco gliding past the outside 4b of the wall 4a can be determined.

The housing 4 of the stray field probe preferably consists of a material with a low coefficient of thermal expansion. This material preferably contains an alloy that is approximately 64% iron and approximately 36% nickel. The housing 4 can have a temperature control arrangement, not shown, which keeps the working temperature of the housing 4 at least approximately constant. This temperature control arrangement of the housing 4 can have a sensor (not shown) for the temperature of the housing 4, which sensor controls a transistor (not shown) in such a way that the heat loss from the transistor keeps the temperature of the housing 4 at least approximately constant, preferably above the ambient temperature. The walls 4a, 4c and also the side walls of the housing 4 are further preferably at least partially coated on their inner sides with a corrosion-resistant metal or at least partially consist of such a metal. This coating is preferably made of an electrically highly conductive metal. The outer surfaces 4b, 4d and also the lateral outer surfaces of the housing 4 can also be coated with a corrosion-resistant metal. This coating metal preferably contains gold. In addition, the surface 4a of the housing 4 can preferably additionally be at least partially coated with a plastic of the polyaryl ether ketone (PEAK) group, in particular with a plastic made of polyether ether ketone (PEEK). A further layer (not shown) made of such a plastic can also be attached to the outside 4b of the outside wall 4a parallel to the first wall 4a.

4 shows a further embodiment of a stray field probe 1. Here too, the same parts are identified by the same reference symbols. In the stray field probe 1 shown in FIG. 4, the line resonator 2 is arranged in a cross shape. Here too, the line resonator 2 is through a dielectric waveguide formed, which is short-circuited at its ends 8. Pins 1 2 are also provided in the stray field probe 1 with a cross-shaped line resonator 2 for coupling the resonant electric field of the line resonator 2 to the substance to be measured.

This also applies to the line resonator 2 arranged in a cross shape in the stray field probe

1 according to FIG. 4, the pins 1 2 for decoupling the stray field into the outside space are arranged perpendicular to the line resonator 2 or to the plane of the drawing in FIG. 4.

FIG. 5 shows schematically in an exploded view the structure of the stray field probe 1 from FIG. 4. Here too, the same parts are identified with the same reference numerals. 5 shows the housing 4. The line resonator 2 is embedded in the housing 4 in a cross-shaped recess 18. In the perspective view of the line resonator 2 according to FIG. 5, the pins 1 2, which are perpendicular to the line resonator 2, can be clearly seen. The housing 4 of the stray field probe 1 shown in FIGS. 4 and 5 is flat to be used in the above-mentioned bulkheads for loose tobacco. A cover 20 is placed on the housing 4. The cover 20 is made of an electrically conductive material. The cover 20 has openings 10. The center of the circular openings 1 0 lies exactly above the pins 1 2 in the extension of the longitudinal axis of the pins 1 2. Further above in FIG. 5 is shown an end plate 22. This end plate 22 consists of the above-mentioned non-conductive, non-metallic material PEEK . This end plate 22 serves to protect the recess 1 8 in the housing 4, the line resonator 2 with the pins 1 2 and also the cover plate 20 against external influences.

FIG. 6 shows a schematic side view of the stray field probe 1 shown in FIGS. 4 and 5. In this drawing, the same parts are designated by the same reference numerals. The end plate 22 is not shown in this illustration. In addition to the physical objects, the electric field E is also indicated schematically in FIG. 6 by dashed lines. Fig. 6 that the maxima of the electric field E, which is resonant through the line resonator

2 is formed, exactly at the positions of the pins 1 2 for decoupling the stray field also shown by arrows. Since the maxima of the same name are concerned (see FIG. 1), only the stray field represented by the arrows is coupled to the outside through the openings 10 in the cover 20 according to the arrows. A far field, ie a radiation loss arises from the incoming reasons not explained.

For coupling, a coupling antenna 1 6 is guided into the housing 4 through an opening 24. The Einkoppelantelle 1 6 lies exactly below the pin 1 2 arranged in the middle (see Fig. 5). A coupling-out antenna 26 is provided for coupling out through an opening 28 provided on the edge of the housing 4 in the lower wall 4c of the housing 4.

7 schematically shows a stray field probe 1 that uses a cavity resonator 30. 7 symbolizes an H303 resonator. This means that three half-waves in the x- and z-direction and no half-wave in the y-direction can be propagated in the resonator (see the coordinate cross x, y, z, in FIG. 7). In the case of the cavity resonator 30 in FIG. 7, the coupling or decoupling takes place via coupling antennas 16 and 26 respectively. In FIG. 7, the ratio of the distance b 'of the coupling antenna 26 to the side wall 32 of the cavity resonator 30 and the total width b of the cavity resonator 30 28/100.

The cavity resonator 30 of FIG. 7 also has openings 10 in its cover 20. Under each opening is a plexiglass cylinder, not shown, which holds an antenna, not shown. The desired stray field can thus form between the antenna (not shown) and the cover 20. 7, only maxima of the same name of the electrical resonant field are provided with an opening 10 to the outside.

8 shows the electrical field distribution in a three-dimensional diagram. FIG. 8 shows the maxima of the electric field indicated in FIG. 7 by the sign "+".

In addition to the resonance H303 shown in FIG. 8, there are other resonances in the cavity resonator 30. All resonances with even numbers (e.g. H203, H204, H404, ...) can be easily suppressed because they all have a zero point of the electric field E at the center and the coupling with the coupling antenna 1 6 takes place at this point.

However, a more disturbing resonance is superimposed on H 1 03 with H 301. This resonance has a zero point of the electric field E at approximately 28% of the total width on the axis of symmetry. For this reason, according to FIG. 7, the coupling antenna 26 is 28% (b ') of the total width b of the cavity resonator 30 intended. This value can be determined experimentally for each cavity resonator 30. 9 shows the three-dimensional field distribution of the electric field E at resonance H 1 03 with H301.

Claims

Expectations

1 . Scattering field probe for measuring the dielectric properties of substances, in particular loose tobacco, with generating means (2, 30) for generating an electric field (E), with shielding means (4, 4a, 4c, 20) for shielding the generated electrical

Field (E), the shielding means (4, 4a, 4c, 20) being designed such that the electrical field (E) is at least partially outside the shielding means (4,

4a, 4c, 20), characterized in that the shielding means (4, 4a, 4c, 20) for decoupling the electrical field have at least two openings (1 0).

2. stray field probe according to claim 1, wherein the openings (1 0) are arranged such that the electric field (E) can be coupled out at locations of the same amplitude.

3. stray field probe according to one of the preceding claims, wherein the generating means (2, 30) are designed such that the electric field (E) is in resonance.

4. stray field probe according to one of the preceding claims, wherein the generating means (2, 30) are designed such that the electric field (E) is a high-frequency alternating field.

5. stray field probe according to one of the preceding claims, wherein the openings (1 0) are arranged such that the electric field (E) can be coupled out at its maxima.

6. stray field probe according to claim 5, wherein the openings (1 0) are formed such that the electric field (E) can be coupled out to the maxima of the electric field (E), which have the same sign.

7. stray field probe according to claim 1, wherein the shielding means (4, 4a, 4c, 20) are designed as a housing (4) for the generating means (2, 30).

8. stray field probe according to any one of the preceding claims, wherein the openings (1 0) have diameters which are small compared to the wavelength of the electric field (E).

9. stray field probe according to one of the preceding claims, wherein the generating means (2, 30) are designed as waveguides (2).

1 0. stray field probe according to claim 9, wherein the waveguide is designed by a short circuit at its two ends (8) as a line resonator (2).

1 1. Scattering field probe according to one of claims 9 or 1 0, wherein the length of the waveguide (2) is approximately n x Λ / 2, where N> 3, preferably> 1 0.

1 2. stray field probe according to one of claims 9 to 1 1, wherein the waveguide (2) is designed as a meander.

1 3. stray field probe according to one of claims 1 1 or 1 2, wherein n is odd, and wherein in the middle of the waveguide (2), the electric field (E) is coupled into the waveguide (2).

1 4. stray field probe according to one of claims 1 0 to 1 2, wherein the waveguide (2) is designed as a cross.

1 5. stray field probe according to claim 1 4, wherein each leg of the waveguide cross has a length which corresponds to 5 x λ / 2.

1 6. stray field probe according to one of claims 1 0 to 1 5, wherein the waveguide (2) is high-resistance.

1 7. stray field probe according to one of claims 1 0 to 1 6, wherein the waveguide (2) at the points for decoupling the electric field (E) perpendicular to its longitudinal axis pins (1 2).

1 8. stray field probe according to claims 8, 1 7 and according to any one of the preceding claims, wherein the pins (1 2) bridge a distance between the wave conductor (2) provided in the interior of the housing (4) and the housing outer wall (4a).

1 9. stray field probe according to one of claims 1 1 to 1 8, wherein the measurement coupling necessary to complete the measuring circuit is opposite the stray field coupling for producing the electrical field (E) lying outside the shielding means (4, 4a, 4c, 20).

20. stray field probe according to claim 1 9, wherein the measurement coupling (26) is arranged at symmetrically arranged points.

21. Stray field probe according to one of claims 1 to 9, wherein the generating means (2, 30) are designed as a cavity resonator (30).

22. Stray field probe according to claim 21, wherein the coupling of the electric field (E) takes place in the middle of the cavity resonator (30).

23. Scattering field probe according to one of claims 21 or 22, wherein antennas (1 2) are provided in the interior of the shielding means (4, 4a, 4c, 20) at the points for decoupling the electric field (E) into the outer space, which the Connect the cavity resonator (30) inside the shielding means (4, 4a, 4c, 20) to the outside space to couple out the electric field (E) into the outside space.

24. Scatter field probe according to claim 23, wherein the antennas (1 2) are designed as cylinders made of a dielectric, preferably as cylinders made of plexiglass.

25. A method for measuring the dielectric properties of substances, comprising the steps: a resonant electric field (E) is generated within shielding means (4,

4a, 4c, 20); through openings (1 0) in the shielding means (4, 4a, 4c, 20), a part of the resonant electric field (E) is coupled as a stray field into places in which the max investigative substance located; the change in the resonant frequency of the resonant electric field

(E) is measured.