Antenna arrangement for emitting microwaves and measuring arrangement comprising at least one such antenna arrangement

Description

[0001] The present invention relates to a measuring arrangement with two antenna arrangements for emitting microwaves. It is known from the literature that the propagation time and attenuation of an electromagnetic wave in a medium can be used to determine the complex-valued permittivity of that material. The complex-valued permittivity can, in turn, be used to draw conclusions about media properties such as water content.

[0002] Therefore, measuring arrangements for determining dielectric properties such as the complex-valued permittivity of process media are known. Published patent application DE 30 38 725 A1 discloses a device for determining moisture content. Published patent applications DE 44 26 280 A1 and DE 101 64 107 C1 disclose devices for measuring the solid content of a gas stream. Publication US 2016 313 259 A1 discloses temperature compensation for determining the dielectric properties of a medium. WO 1991 005 243 A1 discloses a device for measuring the concentration of two substances. Published patent application DE 102017 131 269A1 discloses a device for determining the fat content of milk. The published patent application DE 10 2012 105 281 A1 discloses a level measuring device for determining the level of a process medium in a container using a transit time method.For this purpose, the level measuring device has exactly one means for determining the dielectric constant, which comprises a waveguide for a high-frequency measuring signal that is filled with a dielectric.

[0003] Patent GB 2 293 014 discloses a measuring arrangement with a waveguide antenna having a stainless steel body, in whose cavity a glass-ceramic filling is fused. Firstly, this requires complex manufacturing, and secondly, the antenna has a large volume because the permittivity of the glass-ceramic is comparatively low. The invention is therefore based on the object of providing a compact waveguide antenna with high pressure and temperature resistance that can be used over a wide frequency range. This object is achieved according to the invention by the measuring arrangement according to independent patent claim 1.

[0004] The measuring arrangement according to the invention comprises: Two antenna arrangements for radiating microwaves, each comprising: a dielectric waveguide body; and a carrier body; wherein the dielectric waveguide body has an electrically conductive surface at least along a circumferential surface, wherein the dielectric waveguide body has an electrically non-conductive radiating surface, wherein the dielectric waveguide body has a coupler receptacle; wherein the carrier body surrounds the dielectric waveguide body at least along the surface, wherein the dielectric waveguide body is fixed in the carrier body, wherein the carrier body has a radiating opening, wherein the radiating surface is aligned with the radiating opening; a measuring tube with two opposing antenna openings, to each of which one of the antenna arrangements is mounted, a measuring and operating circuit connected to each antenna arrangement by means of a signal line, and is characterized in that the carrier body comprises a material with a modulus of elasticity of not less than 50 GPa.

[0005] In a further development of the invention, the dielectric waveguide body has a permittivity of not less than 8 at 2 GHz, in particular not less than 9.5 at 2 GHz.

[0006] In a further development of the invention, the dielectric waveguide body comprises a ceramic material, in particular aluminum oxide, zirconium oxide or titanium dioxide.

[0007] In a further development of the invention, the dielectric waveguide body has a basic shape of a parallelepiped, in particular a cuboid.

[0008] In a further development of the invention, the electrically conductive surface of the dielectric waveguide body comprises a metallic coating, whereby a filled waveguide is formed.

[0009] In a further development of the invention, the metallic coating comprises an active brazing alloy.

[0010] With the above material selection and the above shape, a compact antenna can be realized, because the size of the antenna is determined by the cutoff frequency fc of the filled waveguide. For the commonly used fundamental mode TE10 of a rectangular waveguide with the rectangular sides a, b, a = cd / (2 fc ), where the propagation velocity cd in the dielectric is cd = c 0 / ε r 1 / 2< . The size of the antenna is therefore inversely proportional to the square root of the permittivity of the dielectric filler material. For a permittivity ε r,S-Class ≈ 5.2, such an antenna waveguide would have a minimum cross-sectional width of approximately 3.3 cm at a cutoff frequency fc = 2 GHz. However, if a dielectric with a permittivity ε r of at least 8 at 2 GHz is used, the minimum width is only 2.7 cm. This is advantageous in that the breakdown in the carrier body is then also limited to this dimension. Furthermore, the materials selected for the dielectric significantly increase transmission in an aqueous medium compared to antennas with a glass dielectric.

[0011] In a further development of the invention, the coupler receptacle comprises a bore through which a pin-shaped coupler, in particular a coaxial coupler, extends, which is terminated with a capacitively acting element.

[0012] In a further development of the invention, the capacitively acting element comprises a capacitor, for example in the form of a circuit board piece with an integrated capacitor, a coaxial capacitor or an open line piece.

[0013] In a further development of the invention, the carrier body has electrically conductive surfaces which surround the dielectric waveguide body.

[0014] In a further development of the invention, the carrier body comprises a metallic material, in particular steel.

[0015] In a further development of the invention, the carrier body has a waveguide chamber for the dielectric waveguide body, wherein the waveguide chamber has cross sections which run parallel to the radiating surface and which are filled to at least 90%, in particular 95%, of the cross-sectional area by the dielectric waveguide body.

[0016] In a further development of the invention, the dielectric waveguide body is fixed in the waveguide chamber with a polymer.

[0017] In a further development of the invention, the carrier body has a process connection for connecting the antenna arrangement to an antenna opening of a pipeline or a container in order to position the radiation opening in the region of the antenna opening.

[0018] In a further development of the invention, the material has a modulus of elasticity of not less than 150 GPa.

[0019] The invention will now be explained in more detail with reference to the embodiment shown in the drawings.

[0020] It shows: Fig. 1 : a spatial representation of an embodiment of an antenna arrangement according to the invention; Fig. 2 : a cross section through an embodiment of a measuring arrangement according to the invention; and Fig. 3 : a diagram of the proportion of reflected power at the interface between the antenna and an aqueous medium.

[0021] The Fig. 1 The illustrated embodiment of an antenna arrangement 100 according to the invention comprises a dielectric waveguide body 110, which comprises a ceramic material with high permittivity, for example aluminum oxide, zirconium dioxide, or titanium dioxide. The waveguide body 110 has a substantially cuboidal basic shape with slightly rounded edges. A continuous coupling bore 112 extends through the waveguide body 110 parallel to its largest principal axis of inertia. A lateral surface of the waveguide body 110, which surrounds its minimum principal axis of inertia, and a rear end face 114 of the waveguide body 110, whose surface normal runs in the direction of the minimum principal axis of inertia of the waveguide body, has a metallic coating 116, which is prepared using an active solder, wherein a second end face facing away from the first end face 114, which serves as a radiating surface 118, is free of the metallic coating.Furthermore, a circumferential end section of the lateral surface adjacent to the radiating surface is free of the metallic coating 116.

[0022] The antenna arrangement 100 further comprises a metallic carrier body 120, which is made in particular of stainless steel and has a first end section with a cylindrical basic shape. An end face of the first end section has a radiation opening, from which a waveguide chamber 122 extends into the carrier body 120, in which the waveguide body 110 is arranged. The waveguide chamber 122 has walls that run essentially parallel to the metallized surfaces of the waveguide body 110. The end section of the lateral surface of the waveguide body 110, which is free of metallization, is arranged near the end face of the first end section of the carrier body 120. The carrier body 120 has, in a rear end face facing away from the first end face, a filling opening 124 that communicates with the waveguide chamber 122.An adhesive is pressed into the waveguide chamber 122 through the filling opening in order to fill a circumferential gap volume between the surfaces of the waveguide body 110 and the walls of the waveguide chamber 122, whereby the gap volume, including that through the non-metallized end section of the lateral surface of the waveguide body 110 up to the radiating surface 118, is sealed. This reliably prevents the penetration of a medium from the first end face of the carrier body 120 into the waveguide chamber 122, in particular to avoid corrosion of the metallic coating 116 of the lateral surface. Should the adhesive in the gap near the first end face become damaged, corrosion will not occur because the end section of the waveguide body 110 is not metallized.

[0023] The carrier body 120 further includes a coaxial coupler bore 126, which extends from an outer surface of the carrier body 120 into the waveguide chamber 122, in alignment with the coupling bore 112 of the waveguide body 110. Furthermore, the carrier body 120 includes a capacitor chamber bore 128, which, also in alignment with the coupling bore 112 of the waveguide body 110, extends from a surface opposite the coaxial coupler bore 126 into the waveguide chamber 122.

[0024] A perforated metallic contact screw 142 is screwed into the coaxial coupler bore 126, pressing its end face against the metallic coating 116, thus establishing a defined galvanic contact between the metallic coating 116 and the carrier body 120. A coaxial coupler 130, which has an outer conductor 132 and an inner conductor 134, is guided through the bore of the contact screw 142 and the coupling bore 112 of the waveguide body 110, with the inner conductor 134 extending into the capacitor chamber bore 128. The outer conductor 132 is also in galvanic contact with the metallic coating 116 and the carrier body 120 via the contact screw 142.

[0025] In order to enable effective capacitive coupling of a microwave into the waveguide body 110 in a frequency range of, for example, approximately 2 GHz to 8 GHz, the coaxial coupler 130 must also be terminated with a suitable capacitance, particularly because the waveguide body has a very high capacitance due to the selected materials. For this purpose, the inner conductor 134 of the coaxial coupler is connected to a capacitor 134, which is designed in particular as a circuit board capacitor and is arranged in the capacitor chamber bore 128. The capacitor 134 is axially clamped in the capacitor chamber bore 128 as follows. An annular disk-shaped pressure piece 114 rests against the outer surface of the waveguide body 110 with an annular axial projection. The capacitor 134 is supported on the pressure piece 114.On the side of the capacitor 134 facing away from the pressure piece 114, an insulating disk 148 is arranged, which is axially clamped against the capacitor 134 by a disc spring stack 148 and a clamping screw 144 screwed into the capacitor chamber bore 128. The disc spring stack 146 ensures that the clamping forces fluctuate only slightly despite the different thermal expansion coefficients of the components. The pressure piece 114 and the contact screw 142 also each press an O-ring 166, 164 against the outer surface of the waveguide body 110, thereby sealing the capacitor chamber bore 128 and the coaxial coupler bore 126 from the gap between the waveguide body 110 and the walls of the waveguide chamber 122.

[0026] The carrier body 120 is arranged with the cylindrical end section in an antenna opening 212 of a tube wall 210 of a measuring tube 200, wherein a sealing ring 162 is axially clamped between the tube wall 210 and the carrier body 120. The antenna arrangement 100 is covered with an optional protective housing 240, in which (in Fig. 2 shown) signal lines 410 are routed between an operating circuit and the coaxial coupler.

[0027] The measuring arrangement 300 according to the invention comprises two antenna arrangements 100 and a measuring tube 200 with two opposing antenna openings 212, in each of which one of the antenna arrangements 100 is mounted. The measuring arrangement further comprises an operating and measuring circuit 400, which is connected to the two antenna arrangements 100 via two coaxial lines 410. The antenna arrangements and coaxial signal lines are covered by a metallic protective housing 240, which firstly provides additional EMC protection and secondly prevents contamination and mechanical influences on the antenna arrangements and signal lines. The operating and measuring circuit is configured to radiate signal sequences of different frequencies in the frequency range of, for example, 2 to 8 GHz via one of the antenna arrangements into a medium located in the measuring tube 200 and to receive them with the other antenna arrangement. Based on the propagation time orBy attenuating the signal sequences, the complex permittivity and from this other media properties, such as solid content, can be determined.

[0028] For the performance of the measuring setup, it is advantageous if reflection at the radiating surface 118 of the waveguide body is as low as possible. This can be influenced by the material of the waveguide body 110. Fig 3 shows the calculated reflection fraction at the radiating surface for different waveguide body materials. Water was assumed as the medium, and the width of the waveguide body was set at 30 mm. If the waveguide body is made of S-glass with an assumed permittivity of 5.1, wave propagation up to approximately 2.2 GHz is not possible, as these frequencies are below the cutoff frequency. A waveguide body made of Al2O3 is already significantly more suitable than the state-of-the-art waveguide bodies, although reflections in the range of s 11 ≈ -4 dB occur. A ZrO2 filling achieves consistently good transfer of the electromagnetic power into the water. It is thus demonstrated that the ceramic waveguide bodies used according to the invention can achieve efficient radiation into highly water-containing media with a given antenna geometry.

Claims

1. A measurement arrangement comprising: - two antenna arrangements (100) for radiating microwaves, each comprising: - a dielectric waveguide body (110); and - a carrier body (120); wherein the dielectric waveguide body (110) an electrically conductive surface at least along a circumferential sheath surface, wherein the dielectric waveguide body (110) has an electrically non-conductive radiating surface (118), wherein the dielectric waveguide body (110) has a coupler receptacle (112); wherein the carrier body (120) surrounds the dielectric waveguide body (110) at least along the lateral surface, wherein the dielectric waveguide body (110) is fixed in the carrier body (120), wherein the carrier body (120) has a radiating aperture, the radiating surface (118) being aligned with the radiating aperture; - a measuring tube with two opposite antenna openings (212) on each of which one of the antenna arrangements (100) is mounted, - a measuring and operating circuit which is connected to each antenna arrangement (100) by means of a signal line, characterized in that the carrier body (120) comprises a material having a modulus of elasticity of not less than 50 GPa.

2. Measuring arrangement according to claim 1, wherein the dielectric waveguide body has a permittivity of not less than 8 at 2 GHz, in particular not less than 9.5 at 2 GHz.

3. Measuring arrangement according to claim 1, wherein the dielectric waveguide body (110) comprises a ceramic material, in particular aluminum oxide, zirconium dioxide or titanium dioxide.

4. Measuring arrangement according to claim 1 or 2, wherein the dielectric waveguide body (110) has a basic shape of a parallelepiped, in particular a cuboid.

5. Measuring arrangement according to any one of the preceding claims, wherein the electrically conductive surface of the dielectric waveguide body (110) comprises a metallic coating (116).

6. Measuring arrangement according to claim 5, wherein the metallic coating (116) comprises an active solder or active brazing alloy.

7. Measuring arrangement according to one of the preceding claims, wherein the coupler receptacle comprises a bore through which extends a pin-shaped coupler, in particular coaxial coupler (130), which is terminated with a capacitively acting element.

8. Measuring arrangement according to claim 7, wherein the capacitively acting element comprises a capacitor, for example in the form of a piece of circuit board with integrated capacitor, a coaxial capacitance or an open conductor piece.

9. Measuring arrangement according to one of the preceding claims, wherein the carrier body (120) has electrically conductive surfaces which surround the dielectric waveguide body (110).

10. Measuring arrangement according to one of the preceding claims, wherein the carrier body (120) comprises a metallic material, in particular steel.

11. Measuring arrangement according to one of the preceding claims, wherein the carrier body (120) has a waveguide chamber (122) for the dielectric waveguide body (110), wherein the waveguide chamber (122) has cross-sections which run parallel to the radiating surface (118) and which are filled to at least 90%, in particular 95%, of the cross-sectional area by the dielectric waveguide body (110).

12. Measuring arrangement according to one of the preceding claims, wherein the dielectric waveguide body (110) is fixed with a polymer in the waveguide chamber.

13. Measuring arrangement according to any one of the preceding claims, wherein the support body (120) comprises a process connection for connecting the antenna arrangement (100) to an antenna opening (212) of a pipeline or a container in order to position the radiation opening in the region of the antenna opening (212).

14. Measuring arrangement according to any one of the preceding claims, wherein the material has a modulus of elasticity of not less than 150 GPa.