Radar-based level measurement device for detecting the level of a low-permittivity medium located in a container

Abstract

The invention relates to a radar-based level measurement device (1) for detecting the level of a low-permittivity medium (3) located in a container (2), said measurement device comprising: a feed and receiving element (3), arranged outside the container (2), for generating and receiving electromagnetic radiation (5); a measuring window (6) in the wall of the container (2), which measuring window is permeable to electromagnetic radiation (5); and a waveguide (7) which is arranged in the container (2) and extends over a level measurement range, wherein the waveguide (7) can be flooded by the medium (3), wherein the feed and receiving element (4), the measuring window (6), and the waveguide (7) form a measurement path via which, during the measurement operation, electromagnetic radiation (5) generated by the feed and receiving element (4) propagates through the measuring window (6) and through the waveguide (7), and wherein electromagnetic radiation (9) reflected by a surface (8) of the medium in the waveguide (7) is guided back, through the waveguide (7) and through the measuring window (6), to the feed and receiving element (4).

Description

[0001] Radar level measuring device for detecting the level of a medium with low permittivity in a container

[0002] The invention relates to a radar level measuring device for detecting the level of a medium of low permittivity in a container.

[0003] Determining the level of a medium within a container using a radar level measuring device is a standard measurement task in process measurement technology, although the specific implementation of the radar level measuring device can vary greatly. Radar level measurement essentially involves two different configurations and associated techniques.

[0004] In both methods, a feed and receive element for generating and receiving electromagnetic radiation is usually located outside the container whose fill level is to be monitored. Consequently, the associated control and evaluation electronics are also located outside the container. The electromagnetic radiation generated and emitted by the feed and receive element enters the interior of the container through an opening in the container, is partially reflected by the surface of the medium in the container, the reflected electromagnetic radiation is received by the transmitting and receiving element, and evaluation electronics then directly or indirectly obtains time-of-flight information, which provides information about the distance to the medium surface and thus the fill level of the medium in the container.

[0005] The electromagnetic radiation guided through the opening in the container into the interior of the container is either emitted as a free-space wave or the electromagnetic radiation is guided into the interior of the container via a cable, with the cable length simultaneously defining the measuring distance. Radar level measuring devices that operate with free-space waves usually operate as frequency-modulated continuous wave radar (FMCW), whereas wired radar level measuring devices usually work with a direct time-of-flight evaluation of a pulsed radar signal. It is also known in the art to use waveguides for level measurement. In this case, the electromagnetic radiation generated by the feed and receive elements is coupled directly into the waveguide outside the container.The waveguide extends from the exterior of the vessel through an opening in the vessel wall into the interior of the vessel, and extends within the vessel over the measurement distance of interest. The feed and receive element and the waveguide, including the necessary attachments for mounting the radar level measuring device, form a single mechanical unit.

[0006] For radar distance measurements, it is essential that the surface of the medium of interest in the container reflects some of the electromagnetic radiation; more precisely, that the interface between the medium of interest and the container not filled with the medium generates a corresponding reflection. The reflectivity of the boundary layer depends on the refractive indices of the media involved, i.e., the refractive index of the medium in the container and the refractive index of the space not filled with the medium, which can be filled with air, a protective gas, or another medium. Air has a permittivity close to 1.

[0007] The refractive index of a medium is the square root of the product of the relative magnetic permeability and the relative permittivity of the medium. Since the relative magnetic permeability for non-ferromagnetic materials is a good approximation of 1, the refractive index depends significantly on the relative permittivity. If the medium has a refractive index (and thus a relative permittivity) close to 1, then the proportion of reflected electromagnetic radiation at an air-air interface is very small, which also results in the reflected electromagnetic signal being very weak, making it very difficult to evaluate the reflected signal from the medium. The problem described arises, for example, if the medium in the container is liquid hydrogen, which has a relative permittivity close to 1.

[0008] The object of the present invention is to provide a radar level measuring device with which the level of a medium with low permittivity in a container can also be reliably determined by radar measurement.

[0009] The previously described and derived problem is solved in the radar level measuring device described above, comprising a feed and receiving element arranged outside the container for generating and receiving electromagnetic radiation, a measuring window in the wall of the container that is permeable to electromagnetic radiation, and a waveguide arranged in the container and extending over a level measuring range, wherein the waveguide can be flooded by the medium. The feed element, the measuring window, and the waveguide form a measuring path over which the electromagnetic radiation generated by the feed element propagates through the measuring window and the waveguide during measuring operation.Since the waveguide is floodable by the medium, the medium in the container can easily penetrate into the waveguide, so that electromagnetic radiation reflected from a medium surface in the waveguide is guided through the waveguide and through the measuring window back to the feed and receive element.

[0010] The described radar level measuring device operates with a waveguide in the container. The waveguide has the advantage that the energy of the electromagnetic radiation coupled into the waveguide is virtually completely retained over the measuring distance, thus the measuring system operates with negligible attenuation. The use of the waveguide also has the advantage that relatively high energies of the electromagnetic radiation can be used, so that even small reflection components lead to a relatively well-evaluated reflection signal.

[0011] The radar level measuring device is preferably constructed in several parts, so that the feed and receive elements, the measuring window, and the waveguide are mechanically decoupled from each other. This has the advantage that the container seal created by the measuring window is not stressed by possible movements and / or mechanical loads on the feed and receive elements and / or the waveguide, and therefore the sealing effect cannot be negatively affected, which is particularly important for highly volatile media – in the extreme case of hydrogen.

[0012] A first variant of the radar level measuring device is characterized by the waveguide being monomode with respect to the frequency of the electromagnetic radiation generated by the feed and receive elements. This means that only electromagnetic waves whose half wavelength corresponds to the maximum dimension of the waveguide propagate in the waveguide.

[0013] In an alternative embodiment of the radar level measuring device, the waveguide is designed to be overtired with respect to the frequency of the electromagnetic radiation generated by the feed and receive element, in particular as a circular waveguide or a rectangular waveguide. The dimensions of the waveguide are preferably a maximum of 3.5 times larger than in a corresponding monomode design, with the dimensions of the waveguide being particularly preferably a maximum of 2.5 times larger than in a corresponding monomode design of the waveguide. The waveguide is referred to as overtired because its dimensions are selected to be larger than would be necessary to guide the mode that is actually only excited. For example, only the fundamental mode is actually excited, but the waveguide has dimensions such that it could also transmit higher modes.This type of design offers several advantages related to the fact that the group velocity of an excited mode—relative to a given frequency—increases with waveguide dimensions (cross-sections, diameters, cross-section axes). At the same time, however, the frequency dependence of the group velocity decreases with increasing waveguide cross-sections. This has the effect that, with the proposed design, the measurement range is extended due to the higher group velocity while simultaneously reducing dispersion effects and thus reducing temporal "signal smearing." These relationships can be better explained using corresponding diagrams, which are described in the figure description.

[0014] In a further development of the radar level measuring device, care is taken to ensure that the feed and receive element deliberately excites only the fundamental mode, in particular at a frequency above the cut-off frequency of the fundamental mode, i.e. in a range of reduced dispersion and higher group velocity. Particular preference is given to working with a frequency as far above the cut-off frequency of the fundamental mode as possible, without deliberately exciting a higher mode; the advantages are identical to those described previously. A further advantage is that, for a given frequency of the electromagnetic radiation, the overfed waveguide has a larger cross-section, at which capillary effects of the medium in the waveguide play a lesser role. The reflection surface is then flatter, and consequently the usable reflection area is also larger than with a monomode waveguide design.

[0015] In a further development of the radar level measuring device, the end of the waveguide facing the measuring window is provided with an antenna for coupling electromagnetic radiation into and out of the waveguide. In particular, the antenna is a horn antenna.

[0016] A further preferred embodiment of the radar level measuring device is characterized in that the feed and receive element has an antenna for radiating the generated electromagnetic waves and for receiving the reflected electromagnetic waves, wherein the antenna is also preferably designed as a horn antenna. By using the antennas—on the waveguide or on the feed and receive element—the overall directivity of the components is improved, thus improving the overall signal-to-noise ratio of the measuring section.

[0017] In a further preferred embodiment, a beam-shaping element is arranged in the region of the measuring window—also to improve the directivity of the components and to improve the signal-to-noise ratio—for aligning and / or focusing the electromagnetic radiation between the feed and receive element and the waveguide. In a special embodiment, the beam-shaping element is mounted on the outside of the measuring window and / or on the inside of the measuring window, for example in the form of lens elements. In a further embodiment, the measuring window itself is designed as the beam-shaping element and can thus partially or completely realize the function of the beam-shaping element.

[0018] In the radar level measuring device described here, the electromagnetic transmission signal emitted by the feed and receive element travels the entire measuring path and exits the waveguide at its opening, which is located at the end of the waveguide facing away from the measuring window. The waveguide opening then typically points toward a container wall. The electromagnetic radiation leaving the waveguide there is then reflected by the container wall—or possibly by other components within the container—and at least partially reenters the waveguide and is then guided back toward the transmit and receive element. This interference signal can, under certain circumstances, be considerably stronger than the electromagnetic radiation reflected by the medium surface, which actually represents the useful signal of interest; this is particularly the case for media with low permittivity, which are considered here.In a preferred embodiment of the radar level measuring device, a reflector is therefore arranged in the area of ​​an opening in the waveguide facing away from the measuring window, so that electromagnetic radiation emerging from the waveguide is not reflected back toward the opening of the waveguide facing away from the measuring window. The reflector can be arranged, for example, on the wall of the container, but it can also be attached to the waveguide itself.

[0019] Another measure to mitigate the problem of interference signals caused by reflection in a further development of the radar level measuring device is to arrange a polarizer in the area of ​​an opening in the waveguide facing away from the measuring window. This polarizer polarizes the electromagnetic radiation emerging from the waveguide upon reflection into reflected electromagnetic radiation to which the transmitting and receiving element is not sensitive. The polarizer could, for example, ensure a 90° phase shift of the electromagnetic radiation. The polarizer can also be arranged on the wall of the container or, alternatively, on the waveguide itself.

[0020] In a further development of the radar level measuring device, a coded reflector is arranged in the area of ​​the waveguide opening facing away from the measuring window, which generates reflected electromagnetic radiation with a corresponding coded signature. In this variant, the reflected interference signal is not suppressed (as is the case with a polarizer), but the interference signal can be calculated out of the received signal during signal processing due to its known signature, i.e. due to a known temporal signal curve. This solution therefore requires that an evaluation unit - e.g. based on a digital signal processor - carries out corresponding signal processing. In a further embodiment of the radar level device, the feed and receiving element and the measuring window orThe feed and receive element and a beam-shaping element facing the feed and receive element (arranged on the measurement window) are aligned in such a way that the electromagnetic radiation emitted by the transmit and receive element is not reflected back to the transmit and receive element, or only a small portion of it is reflected back to the transmit and receive element. This measure can also suppress interference signals and improve the signal-to-noise ratio.

[0021] In an alternative or additional measure for reducing reflections, the measuring window and / or the beam-shaping element are configured with an optical adaptation layer to reduce reflections. This is preferably a dielectric adaptation layer, with the permittivity of the material used for the adaptation layer being matched to the permittivity of the measuring window and / or the beam-shaping element to which the adaptation layer is applied.

[0022] In a further preferred embodiment of the radar level measuring device, a mode converter is arranged in the region of the waveguide opening facing the measuring window. This mode converter converts the electromagnetic radiation directed from the measuring window to the waveguide opening facing the measuring window into the desired mode. For example, the mode converter can convert a TE 11 wave into a TEO 1 wave and vice versa. The advantage of the mode conversion mentioned as an example is that the TEO 1 mode requires a larger waveguide, which has a positive effect on capillary effects (reduced capillary effects). Deposits on the waveguide are less disruptive because there is less field in the outer region for this mode. Additional openings in the waveguide wall are also possible.

[0023] In an alternative design of the radar level measuring device, a mode converter is arranged after the feed and receiving element, which converts the generated electromagnetic radiation into the desired mode outside the container.

[0024] In detail, there are numerous possibilities for designing and developing the radar level measuring device according to the invention. Reference is made, on the one hand, to the claims subordinate to the independent patent claim and, on the other hand, to the following description of exemplary embodiments in conjunction with the drawings. The drawings show:

[0025] Fig. 1 schematically shows a radar level measuring device for detecting the level of a medium of low permittivity in a container,

[0026] Fig. 2a-2c schematically show three variants of a radar level measuring device with different measures to combat the effect of interference reflection signals,

[0027] Fig. 3 schematically shows a radar level measuring device using an adaptation layer to reduce reflections,

[0028] Fig. 4 schematically shows a radar level measuring device using a mode converter and

[0029] Fig. 5a, 5b Diagrams explaining the frequency dependence of the group velocity of waveguide waves and the effect of larger waveguide dimensions - due to the overfatigue design of the waveguide - on the group velocity of the waveguide waves.

[0030] 1 to 4 each show radar level measuring devices 1 and various aspects of their implementation. The radar level measuring devices 1 shown have in common that they are used to detect the level of a medium 3 of low permittivity located in a container 2. In the present case, the medium 3 is liquid hydrogen. The radar level measuring devices 1 have, outside the container 2, a feed and receive element 4 for generating and receiving electromagnetic radiation 5. Furthermore, they have a measuring window 6 arranged in the wall of the container 2 and permeable to electromagnetic radiation 5, as well as a waveguide 7 in the container 2 extending over a level measuring range, wherein the waveguide 7 can be flooded by the medium 3, i.e. the level of the medium 3 in the container 2 is identical inside and outside the waveguide 7.

[0031] The feed and receive element 4, the measuring window 6, and the waveguide 7 form a measuring path over which, during measuring operation, the electromagnetic radiation 5 generated by the feed and receive element 4 propagates through the measuring window 6 and through the waveguide 7. Electromagnetic radiation 9 reflected by a medium surface 8 of the medium 3 in the waveguide 7 is guided by the waveguide 7 through the measuring window 6 back to the feed and receive element 4. If—as in the present case—the medium 3 has a permittivity close to 1, the electrical properties relevant to the propagation of electromagnetic waves at the medium surface 8 do not change or change only very slightly, so that only a small portion of the electromagnetic radiation 5 emitted by the feed and receive element 4 is reflected as reflected electromagnetic radiation 9 from the medium surface 8.This means that the measurement signal of interest in the form of reflected electromagnetic radiation 9 is correspondingly weak and therefore difficult to evaluate.

[0032] The use of the waveguide 7 has the advantage of extremely low attenuation of the electromagnetic radiation 5 or the reflected electromagnetic radiation 9. In addition, the entire energy of the electromagnetic radiation 5 is concentrated and guided through a defined surface area 8 of the medium 3, so that a defined useful signal is also generated in the form of the reflected electromagnetic radiation 9.

[0033] The radar level measuring device 1 in Fig. 1 operates with a waveguide 7 which, with respect to the frequency of the electromagnetic radiation 5 generated by the feed and receive element 4, is monomode, in this case as a circular waveguide. In the monomode design, the characteristic dimension in the cross-section of the circular waveguide (i.e., in the case of a circular waveguide, the diameter) is selected such that only a single wave mode, in this case the fundamental mode, is capable of propagation. To obtain a better idea of ​​the behavior of waveguide waves in waveguides, reference is made to Fig. 5a. The group velocities of various modes (TE11, TM01, TEO 1 / TM 11, TM21) of the waveguide waves are plotted against the frequency of the waveguide waves, with the TE11 wave being the fundamental mode, and the following waveforms denoting successively higher wave modes.The family of curves represents the behavior of a waveguide with a constant relevant cross-sectional dimension (circular waveguide with a diameter of 2.6 mm). As the frequency of the electromagnetic radiation increases and thus as wavelengths decrease, increasingly higher wave modes become capable of propagation in the waveguide, and these modes are also capable of propagation together if excited. A characteristic of waveguide waves is that they exhibit distinct dispersion behavior, i.e., they have frequency-dependent group velocities. The frequency dependence of the group velocity is significantly more pronounced at low frequencies of a wave mode than at high frequencies. Even during monomode operation of the radar level measuring device 1, dispersion effects generally occur because the excitation of a wave mode occurs with a signal with a certain bandwidth and thus with frequencies in the corresponding frequency range.

[0034] The radar level measuring devices 1 shown in Figs. 2 to 4, on the other hand, operate with a waveguide 7 which is overmoderated with respect to the frequency of the electromagnetic radiation 5 generated by the feed and receive element 4. This means that the dimensions of the waveguide are selected to be larger than necessary for guiding the actually only excited mode. In the exemplary embodiments, the dimension of the waveguide 7 relevant for the wave mode is 2.5 times larger than in a corresponding monomode version of the waveguide 7. This design of the waveguide 7 has proven to be very advantageous because it represents a good compromise between achieving a high group velocity with a low frequency dependence of the group velocity (i.e. with low dispersion) and, at the same time, structurally limiting the propagation capability of only a few higher wave modes.The fact that in an overmoderated waveguide 7 also other wave modes can be transmitted has already been explained with reference to Fig. 5a, as well as the fact that a wave mode reaches higher group velocities with lower dispersion with increasing frequency.

[0035] Fig. 5b shows how the group velocity changes in a frequency range for an excited wave mode when the dimensions of the waveguide 7 are changed, i.e., when it is increasingly overmoderated (from 2.6 mm to 5.0 mm, to 10 mm, and finally to 20 mm). The effect here is that with increasing overmoderation, the group velocity increases, namely, approaches the speed of light, while simultaneously the dispersion decreases: the curves are significantly flatter.

[0036] If it could be ensured during operation of the radar level measuring device 1 that only the fundamental mode is excited and propagated, there would be no limit to the fatigue. However, practice shows that, despite targeted excitation of the fundamental mode by the feed and receive element 4, higher wave modes also develop. This is particularly true in the event of disturbances in the measuring path, such as a moving or tilted reflection surface through the medium 3 in the waveguide 7. In this case, higher wave modes are also excited, so that energy from the targeted excited fundamental mode is transferred to the unintentionally generated higher wave mode. This leads to a weakening of the reflection signal in the fundamental mode and therefore to a more difficult detection and evaluation of the reflection signal in the fundamental mode by the feed and receive element 4.The moderate overfatigue of waveguide 7 with a geometry factor of 2.5 compared to the single-mode case has proven to be a good restriction of the propagating higher modes (acceptable energy loss to higher modes), while simultaneously increasing the group velocity of the waveguide waves in the fundamental mode and reducing the dispersion of the waveguide waves in the fundamental mode. This results in an increase in the measurement range with good detectability of the reflection signal.

[0037] In the radar level measuring devices 1 shown in Figs. 2 to 4, the feed and receive element 4 deliberately excites only the fundamental mode at a frequency above the cutoff frequency of the fundamental mode, i.e., in a range of reduced dispersion and higher group velocity. Special care is taken here to operate at a frequency as far above the cutoff frequency of the fundamental mode as possible without deliberately exciting a higher mode.

[0038] In all radar level measuring devices 1, the end 10 of the waveguide 7 facing the measuring window 6 has an antenna 11 for coupling and decoupling electromagnetic radiation 5 into and out of the waveguide 7. This allows a better directivity of the arrangement to be achieved, which also leads to an improvement in the signal-to-noise ratio.

[0039] The same applies to an antenna 12, which is provided on the feed and receive element 4 and serves to radiate the generated electromagnetic radiation 5 and to receive the reflected electromagnetic radiation 9. In the illustrated embodiments, both antennas 11, 12 are designed as horn antennas.

[0040] The radar level measuring devices 1 shown in Figs. 2a, 2b, and 2c have a beam-forming element 13 arranged in the region of the measuring window 6, which serves to align and / or focus the electromagnetic radiation 5 between the feed and receive element 4 and the waveguide 7. The beam-forming element 13 consists of two lens-shaped elements, one of which is mounted on the outside of the measuring window 6, and the other beam-forming element 13 is mounted as a lens-shaped element on the inside of the measuring window 6. This measure also leads to an improvement in the signal-to-noise ratio of the measuring section.

[0041] The radar level measuring devices 1 shown in Figs. 2a, 2b, and 2c demonstrate various measures to suppress undesirable effects of reflected electromagnetic radiation that does not originate from the medium surface 8, i.e., which constitutes interference signals. This concerns the influence of electromagnetic radiation 16 emerging from the waveguide 7, which is reflected, for example, by the wall of the container 2 and, assuming it is reflected back into the waveguide 7, returns to the feed and receive element 4. There, it must be distinguished from a useful signal. The radar level measuring device 1 according to Fig. 2a shows a reflector 15 arranged in the region of an opening 14 of the waveguide 7 facing away from the measuring window 6.This reflector 15 is arranged such that electromagnetic radiation 16 emerging from the waveguide 7 is not reflected back toward the opening 14 of the waveguide 7 facing away from the measuring window 6. In the present case, the reflector 15 is arranged on the wall of the container 2. In other embodiments—not shown here—the reflector 15 is attached to the waveguide 7.

[0042] In the radar level measuring device 1 according to Fig. 2b, a polarizer 17 is arranged in the region of an opening 14 of the waveguide 7 facing away from the measuring window 6. This polarizer 17 polarizes the electromagnetic radiation 16 emerging from the waveguide 7 upon reflection into reflected electromagnetic radiation to which the feed and receive element 4 is not sensitive. Here, too, the polarizer 17 is arranged on the wall of the container 2. However, it is equally conceivable for the polarizer 17 to be attached to the waveguide 7. The polarizer 17 shown in Fig. 2b rotates the phase of the electromagnetic radiation 16 by 90°, so that the polarized electromagnetic radiation that returns to the feed and receive element 4 can practically no longer exert any effect there as interference signals, since the feed and receive element 4 is not sensitive to such polarized electromagnetic radiation.

[0043] The radar level measuring device 1 shown in Fig. 2c has a coded reflector 18 in the area of ​​the opening 14 of the waveguide 7 facing away from the measuring window 6. This reflector generates reflected electromagnetic radiation with a correspondingly coded signature. This known coded signature can then be extracted from the overall received signal by an evaluation unit through signal processing.

[0044] Fig. 3 shows that the measurement window 6 is equipped with a matching layer 19 to reduce reflections. In this case, this is a dielectric matching layer 19. The permittivity of the matching layer 19 is to be selected depending on the permittivity of the measurement window 6 and the permittivity of the medium—in this case, air—between the matching layer 19 and the feed / receiver element 4 or the waveguide 7 (usually the square root of the product of the two aforementioned permittivities).

[0045] Finally, Fig. 4 shows a radar level measuring device 1 with a mode converter 20 arranged in the region of the opening 10 of the waveguide 7 facing the measuring window 6. The mode converter 20 converts the electromagnetic radiation 5 directed from the measuring window 6 to the opening 10 of the waveguide 7 facing the measuring window 6 into the desired mode. In this case, the mode converter 20 converts a TE 11 wave into a TE 11 wave. The advantage is that, compared to a monomode waveguide, the waveguide needs to be larger when transmitting the TE 11 mode, thus minimizing possible capillary effects. In addition, the TE 11 mode has lower losses than the TE 11 mode. Furthermore, the field distribution reduces the influence of deposits at the edge and allows the introduction of additional openings.

[0046] Reference symbol

[0047] Radar level measuring device

[0048] container

[0049] medium

[0050] Feeding and receiving element electromagnetic radiation

[0051] Measuring window

[0052] waveguide

[0053] Medium surface reflected electromagnetic radiation the end of the waveguide facing the measuring window

[0054] Waveguide antenna

[0055] Antenna of the feed and receive element Beam forming element End / opening of the waveguide facing away from the measuring window Reflector Electromagnetic radiation emerging from the waveguide Polarizer Coded reflector

[0056] Adaptation layer

[0057] Mode converter

Claims

Patent claims 1. Radar level measuring device (1) for detecting the level of a medium (3) of low permittivity located in a container (2), with a feed and receiving element (3) arranged outside the container (2) for generating and receiving electromagnetic radiation (5), with a measuring window (6) in the wall of the container (2) that is permeable to electromagnetic radiation (5), with a waveguide (7) arranged in the container (2) and extending over a level measuring area, wherein the waveguide (7) can be flooded by the medium (3), wherein the feed and receiving element (4), the measuring window (6) and the waveguide (7) form a measuring section,via which, during measuring operation, electromagnetic radiation (5) generated by the feed and receive element (4) propagates through the measuring window (6) and through the waveguide (7), and wherein electromagnetic radiation (9) reflected by a medium surface (8) in the waveguide (7) is guided through the waveguide (7) and through the measuring window (6) back to the feed and receive element (4).

2. Radar level measuring device (1) according to claim 1, characterized in that the waveguide (7) is designed to be monomode with respect to the frequency of the electromagnetic radiation (5) generated by the feed and receive element (4), in particular as a circular waveguide or as a rectangular waveguide.

3. Radar level measuring device (1) according to claim 1, characterized in that the waveguide (7) is designed to be overmoderated with respect to the frequency of the electromagnetic radiation (5) generated by the feed and receive element (4), in particular as a circular waveguide or as a rectangular waveguide, wherein preferably the dimensions of the waveguide (7) are at most 3.5 times larger than in a corresponding monomode design, wherein particularly preferably the dimensions of the waveguide (7) are at most 2.5 times larger than in a corresponding monomode design of the waveguide (7).

4. Radar level measuring device (1) according to claim 3, characterized in that the feed and receiving element (4) specifically excites only the fundamental mode, in particular with a frequency above the cut-off Frequency of the fundamental mode, i.e. in a range of reduced dispersion and higher group velocity, particularly preferably with a frequency as far above the cut-off frequency of the fundamental mode as possible, without deliberately exciting a higher mode.

5. Radar level measuring device (1) according to one of claims 1 to 4, characterized in that the end (10) of the waveguide (7) facing the measuring window (6) has an antenna (11) for the purpose of coupling and decoupling electromagnetic radiation into and out of the waveguide (7).

6. Radar level measuring device (1) according to one of claims 1 to 5, characterized in that the feeding and receiving element (4) has an antenna (12) for radiating the generated electromagnetic radiation (5) and for receiving the reflected electromagnetic radiation (5), in particular wherein the antenna (12) is a horn antenna.

7. Radar level measuring device (1) according to one of claims 1 to 6, characterized in that a beam shaping element (13) is arranged in the region of the measuring window (6) for aligning and / or bundling the electromagnetic radiation (5) between the feed and receiving element (4) and the waveguide (7).

8. Radar level measuring device (1) according to claim 7, characterized in that the beam-forming element (13) is placed on the outside of the measuring window (6) and / or the beam-forming element (13) is placed on the inside of the measuring window (6) and / or wherein the measuring window (6) itself is designed as at least a part of the beam-forming element (13).

9. Radar level measuring device (1) according to one of claims 1 to 8, characterized in that a reflector (15) is arranged in the region of an opening (14) of the waveguide (7) facing away from the measuring window (6), so that electromagnetic radiation (16) emerging from the waveguide (7) is not reflected back in the direction of the opening (14) of the waveguide (7) facing away from the measuring window (6), in particular wherein the reflector (15) is arranged on the wall of the container (2).

10. Radar level measuring device (1) according to one of claims 1 to 9, characterized in that in the area of ​​a measuring window (6) facing opening (14) of the waveguide (7) a polarizer (17) is arranged, which polarizes the electromagnetic radiation (16) emerging from the waveguide (7) upon reflection into such reflected electromagnetic radiation to which the feed and receiving element (4) is not sensitive, in particular wherein the polarizer (17) is arranged on the wall of the container (2).

11. Radar level measuring device (1) according to one of claims 1 to 10, characterized in that a coded reflector (18) is arranged in the region of an opening (14) of the waveguide (7) facing away from the measuring window (6), which generates reflected electromagnetic radiation with a correspondingly coded signature.

12. Radar level measuring device (1) according to one of claims 1 to 11, characterized in that the feed and receive element (4) and the measuring window (6) or the feed and receive element (4) and a beam shaping element (13) facing the feed and receive element (4) are aligned with one another in such a way that the electromagnetic radiation (5) emitted by the feed and receive element (4) is not reflected back to the feed and receive element (4) or is reflected back to a small extent only.

13. Radar level measuring device (1) according to one of claims 1 to 12, characterized in that the measuring window (6) and / or the beam shaping element (13) is equipped with an adaptation layer (19) to reduce reflections.

14. Radar level measuring device (1) according to one of claims 1 to 13, characterized in that a mode converter (20) is arranged in the region of the opening (10) of the waveguide (7) facing the measuring window (6), which converts the electromagnetic radiation (5) directed from the measuring window (6) onto the opening (10) of the waveguide (7) facing the measuring window (6) into the desired mode.

15. Radar level measuring device (1) according to one of claims 1 to 13, characterized in that a mode converter (20) is arranged after the feed and receiving element (4), which converts the generated electromagnetic radiation (5) into the desired mode already outside the container (2).

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