Field of the Invention

The present invention relates to an antenna for a radar level gauge system. In particular, the present invention relates to a parabolic antenna arrangement suitable for use in a radar level gauge system and to a radar level gauge system comprising such an antenna.

Background of the Invention

Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby

electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe. The probe is generally arranged to extend vertically from the top towards the bottom of the tank.

An electromagnetic transmit signal is generated by a transceiver and propagated towards the surface of the product in the tank, and an

electromagnetic reflection signal resulting from reflection of the transmit signal at the surface is propagated back towards to the transceiver.

Based on a relation between the transmit signal and the reflection signal, the distance to the surface of the product can be determined.

For some applications, such as for non-contact radar level gauge systems using microwave signals in a relatively high frequency band, antenna types such as horn antennas and parabolic antennas can be used to achieve a narrow beam antenna solution.

All antennas can in principle be mechanically scaled for different wavelengths and get the same function in terms of radiation pattern, input mismatch and other relevant antenna parameters. Flowever, due to size, cost etc., pure scaling may not be preferable for some applications. For instance a horn antenna with a lens is a very common narrow-beam antenna solution above 50GHz but there may still be applications where a parabolic antenna may be more suitable.

Accordingly, it is desirable to further develop narrow beam antenna solutions comprising parabolic antennas.

Summary

In view of above-mentioned, it is an object of the present invention to provide an improved parabolic antenna arrangement for use in a radar level gauge system.

According to as first aspect of the invention, there is provided a narrow beam antenna for a radar level gauge system. The antenna comprises: a main reflector; a sub-reflector (310) arranged at a distance from and facing the main reflector; and a dielectric block filling a volume between the main reflector and the sub-reflector, and a volume defined by wave propagation from the main reflector in the direction of a plane of the sub-reflector, such that all wave propagation between the main reflector and the plane of the sub-reflector takes place entirely within the dielectric block.

The antenna is configured to be vertically arranged in a tank or container for measuring the level of a content of the container. In particular, the antenna is typically arranged to emit a signal vertically into a tank such that the signal reaches a surface of a product in the tank and is reflected back towards the antenna where it is received such that the distance from the antenna to the surface can be determined, thereby making it possible to determine the fill level in the tank. For a vertically arranged antenna, the main reflector can be seen as the top portion and the sub-reflector can be seen as the bottom portion of an antenna reflector arrangement.

The present invention is based on the realization that an antenna as described above comprising a dielectric block filling the normally empty space between the main reflector and the sub-reflector provides an antenna design made of very few parts and with exceptional mechanical stability, and where the focusing takes place entirely within the dielectric antenna body. The dielectric block will thus ensure that the reflector positions are stable, providing a mechanically robust antenna suitable for use also in difficult environments where the antenna may be moving or be subject to mechanical impact.

According to one embodiment of the invention, the antenna may further comprise an antenna housing having sidewalls enclosing the sides of the dielectric block which are not in the propagation direction of the antenna. The dielectric block may for example be cylindrical in which case the housing can have a tubular shape with one end of the tube being open and the other end forming a part of the housing. The housing is thus open in the direction where the emitted beam leaves the dielectric block. The housing, which may be made from a metallic or plastic material, thus provides additional protection for the antenna and may also facilitate installation of the antenna in a tank or the like.

According to one embodiment of the invention, the housing may comprise a first opening in the propagation direction of the antenna and a second opening providing a coupling to a transceiver module. For a cylindrical dielectric block, the housing can be seen as a tube which is open in one end for signal propagation, i.e. the first opening, and which has second opening in the opposite end for providing a connection to suitable drive circuitry of the antenna, such as a transceiver module.

According to one embodiment of the invention, the main reflector may be an integral part of the antenna housing. Thereby, even fewer parts are needed compared to if the main reflector would be a separate element arranged within the antenna housing.

According to one embodiment of the invention, the dielectric block may completely fill the antenna housing. Thereby, the entirety of the inside of the antenna housing is both mechanically and environmentally protected.

According to one embodiment of the invention, the antenna may further comprise a feed antenna in the form of a horn antenna or as an array antenna. A horn antenna will thereby act as the primary radiator for the antenna, and by forming the horn antenna as an integral part of the antenna housing and in the connection to external circuitry, no additional components or elements are required for providing the primary radiator. However, it should be noted that it is possible to use also other types of primary radiators, such as a radiating pattern made on a circuit board and suitably attached to the antenna housing. It is also possible to provide the feed antenna as an array antenna arranged on a circuit board located in the opening of the main reflector, or in the main reflector as such with an opening for connecting the array antenna to control circuitry.

According to one embodiment of the invention, the sub-reflector may be provided in the form of a metal coating on a surface portion of the dielectric block, or as a separate element arranged on a surface portion of the dielectric block, meaning that the sub-reflector may be formed at an outer surface of the dielectric block.

Moreover, the sub-reflector may advantageously be embedded within the dielectric block. An embedded sub-reflector may be provided as a separate electrically conductive element embedded in the dielectric block or as a metal coating formed on a portion of the dielectric block during manufacturing of the block.

According to one embodiment of the invention, the main reflector and the sub reflector are advantageously arranged and configured to form a Cassegrain antenna. In the parabolic Cassegrain antenna the feed antenna is mounted may advantageously be arranged at the first opening, or provided as a horn antenna as described above, at the surface of the main reflector, aimed at a smaller convex secondary sub-reflector located in front of the main reflector.

According to one embodiment of the invention, the main reflector and the sub reflector are advantageously arranged and configured to form a Gregorian antenna. A Gregorian antenna operates based on similar principles as the Cassegrain antenna with the difference that the sub-reflector facing the main reflector is concave.

It is also possible to form similar antenna arrangements with two reflectors having parabolic, hyperbolic and/or elliptical shapes. According to one embodiment of the invention, the dielectric block may be a solid block made from PTFE or PEEK, which are materials having both advantageous signal propagation properties as well as being resistant to wear and tear and exposure to chemicals. Moreover, PTFE is a hydrophobic material, thereby reducing contamination of the surface of the dielectric block. It is also possible to provide the surface of the dielectric block with a hydrophobic and/or oleophobic coating.

According to one embodiment of the invention, an end surface of the dielectric block facing in the emission direction may advantageously comprise a pattern comprising surface structures configured to reduce reflections at the interface between the dielectric block and the ambience outside of the antenna. The surface structures will thus be configured to act as a matching layer between the antenna, i.e. the dielectric block, and the tank atmosphere.

It is also possible to reduce reflections at the surface of the dielectric block by arranging an attenuation material on the surface.

According to one embodiment of the invention, a height of the surface structures is approximately equal to a quarter of the wavelength of the operating frequency of the antenna. Thereby, reflections at the surface will effectively be cancelled. The surface structures may for example have a square cross-section, and the length of a side of the square being

approximately equal to the wavelength of an operating frequency of the antenna. Moreover, the surface structures may be conical or pyramidal structures.

According to one embodiment of the invention, an end surface of the dielectric block facing in the emission direction may advantageously be curved, and the curved end surface may be either concave or convex. A curved surface will help to keep the antenna surface clean since

condensation droplets will accumulate either at the edges or at the center of the curved surface where they will subsequently drip off. The lens effect which may arise as a result of a curved surface can be compensated by suitably adjusting the shape of the reflectors. According to one embodiment of the invention, and end surface of the dielectric block facing in an emission direction may be tilted in relation to an emission direction of the antenna. A tilted end surface will help to keep the surface clean since condensation droplets and other fluids will run off the surface towards the lower edge of the dielectric block. The tilt is preferably selected so as to minimize reflections at the interface between the dielectric block and the tank atmosphere.

According to one embodiment of the invention, the main reflector and the sub-reflector may advantageously be tilted in relation to the tilted end surface of the dielectric block such that a vertical emission from the antenna direction is achieved. Accordingly, the tilt of the reflectors is adapted based on the tilt of the dielectric block so that a change in emission direction at the tilted surface is taken into account by means of the tilt of the reflectors. Moreover, the tilt of the reflectors and the tilt of the end surface of the dielectric block is preferably also selected to minimize reflections at the interface.

According to one embodiment of the invention, the antenna may further comprise a feed antenna extending into the dielectric block. The feed antenna may for example be a horn antenna extending into the dielectric block, in which case also the feed antenna is protected and integrated in the overall antenna structure.

There is also provided a tank comprising an antenna according to any one of the preceding embodiments, wherein the dielectric block of antenna is arranged at an opening of the tank and configured to acts as a tank seal. The antenna canthus be provided as a single unit acting as both antenna and as tank seal, simplifying both mounting and maintenance of the antenna as well as eliminating the need for a dedicated tank seal.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention. Brief Description of the Drawings

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing an example embodiment of the invention, wherein:

Fig. 1A schematically illustrates a radar level gauge system comprising an antenna according to an embodiment of the invention;

Fig, 1 B is a block diagram schematically illustrating control circuitry of the radar level gauge system in Fig 1A;

Fig. 2 schematically illustrates an antenna for a radar level gauge system according to an embodiment of the invention;

Fig. 3 schematically illustrates an antenna for a radar level gauge system according to an embodiment of the invention;

Fig. 4 schematically illustrates an antenna for a radar level gauge system according to an embodiment of the invention;

Figs. 5A-C schematically illustrate details of a surface of an antenna for a radar level gauge system according to an embodiment of the invention

Figs. 6A-B schematically illustrate antennas for a radar level gauge system according to embodiments of the invention

Fig. 7 schematically illustrates an antenna for a radar level gauge system according to an embodiment of the invention;

Fig. 8 schematically illustrates an antenna for a radar level gauge system according to an embodiment of the invention; and

Fig. 9 schematically illustrates a tank comprising an antenna according to an embodiment of the invention.

Detailed Description of Example Embodiments

In the present detailed description, various embodiments of the radar level gauge system according to the present invention are mainly discussed with reference to a radar level gauge system for monitoring the level of a fluid product in a tank. It should be noted that this by no means limits the scope of the present invention, which equally well includes, for example, radar level gauge systems in a process management system or radar level gauge systems for monitoring the level of solid materials.

Fig. 1A schematically shows a tank level monitoring system 100 comprising an example embodiment of a radar level gauge system 102 wirelessly connected to a host system 104. In the illustrated example, the radar level gauge system 102 is battery powered. However, the described radar level gauge system 102 may equally well be loop-powered or powered with dedicated power lines.

The radar level gauge system 102 comprises a measurement electronics unit 106 arranged on an outside of the tank 108, an antenna 110 at least partly arranged on an inside the tank 108, and a feed-through 112 connecting the measurement electronics unit 106 with the antenna 110.

The radar level gauge system 102 is arranged on a tank 108

containing a product 114 to be gauged. To reduce the energy consumption of the radar level gauge system 102, at least parts of the radar level gauge system 102 may be operated intermittently and energy may be stored during inactive or idle periods to be used during active periods.

With reference to Fig. 1 B, the radar level gauge system 102 in Fig. 1A comprises a measurement unit (MU) 210, a wireless communication unit (WCU) 211 and a local energy storage unit for example in the form of a battery 212. The wireless communication unit 211 may advantageously be compliant with WirelessHART (IEC 62591 ). As is schematically indicated in Fig. 1 B, the MU 210 comprises a transceiver module 213 and a measurement processor 220. The transceiver module 213 is controllable by the

measurement processor 220 for generating, transmitting and receiving electromagnetic signals having frequencies defining a frequency bandwidth, such as 75G-80GHz. The measurement processor 220 is coupled to the transceiver 213 for determining the filling level in the tank 108 based on a relation between the transmit signal S and the reflection signal SR.

The measurement processor 220 may include a microprocessor, microcontroller, programmable digital signal processor or another

programmable device. The measurement processor 220 may also, or instead, include an application specific integrated circuit, a programmable gate array or programmable array logic, a programmable logic device, or a digital signal processor. Where the measurement processor 220 includes a programmable device such as the microprocessor, microcontroller or programmable digital signal processor mentioned above, the processor may further include computer executable code that controls operation of the programmable device.

Fig. 2 is a schematic illustration of an antenna 1 10 comprising a main reflector 306, a sub-reflector 310 arranged at a distance from and facing the main reflector 306; and a dielectric block 312 filling a volume between the main reflector 306 and the sub-reflector 310, and a volume defined by wave propagation from the main reflector 306 in the direction of a plane of the sub reflector 310, such that all wave propagation between the main reflector 306 and the plane of the sub-reflector 310 takes place entirely within the dielectric block 312. The plane of the sub-reflector can here be defined as a horizontal plane aligned with the lower edge of the sub-reflector 310. In Fig. 2, the plane is aligned with the bottom surface 313 of the dielectric block 312. A lower limit on the width, or diameter, of the dielectric block 312 is practically defined by the diameter of the main reflector 306. In practice, the volume to be filled by the dielectric block 312 can thus be seen as a substantially cylindrical volume limited in one end by the main reflector and in the other end by the sub reflector. The dielectric block 312 may also extend past the sub-reflector 310 as will be illustrated in the following.

The feed antenna 314 is here illustrated as a horn antenna 314 acting as a primary radiator providing a narrow lobe signal which is primarily reflected by the sub-reflector 310 towards the main reflector 306. For an 80GFIz antenna, a suitable outer diameter of the main reflector 306 and of the dielectric block 312 may be 50mm and the diameter of the horn antenna 314 and the sub-reflector 310 may then be in the range of 10-12mm. In

applications where it is possible to arrange the antenna circuitry closer to the reflectors, an array antenna on a circuit board protected by e.g. a lens may also be used. The described antenna 110 may thus be configured to operate in the 75-80GHz range and also in the 60-65GHz range.

Fig. 3 is a schematic illustration of an antenna 110 for a radar level gauge system 102 according to an embodiment of the invention where the antenna 110 further comprises an antenna housing 300 comprising side walls 302 and a first opening 304 in an emission direction of the antenna 110. The antenna housing 300 is substantially tubular. The parabolic main reflector 306 here comprises an opening 308 providing a feed through coupling 309 to a transceiver module 213. The second opening 308 may be coupled to a horn antenna acting 314 as a feed antenna 314 for the parabolic antenna 110.

The main reflector 306 is here illustrated as an integrated portion of the antenna housing 300, i.e. acting as the roof of the antenna housing 300 such that the side walls 302 together with the main reflector 306 define the inner volume of the antenna housing 310. Thereby, the opening 308 in the main reflector 306 can be seen as a second opening 308 of the antenna housing 300.

Moreover, the antenna housing 300 can be made from two separate parts where a top part 316 comprising the main reflector 306 is attached to a cylindrical bottom part 318 defining the sidewalls 302 of the antenna housing 300. The main reflector 306 may be a metallized area of the top part 316 or it can be provided as a separate metallic reflector element 306 attached to the roof of the housing 300.

The antenna 110 further comprises a parabolic sub-reflector 310 arranged at a distance from and facing the main reflector 306. The sub reflector 310 illustrated in Fig. 3 is a convex reflector, thereby forming an antenna having a geometry referred to as a Cassegrain antenna. It is however equally possible to use a concave sub-reflector to form an antenna in the form of a Gregorian antenna (not shown). Moreover, the skilled person readily realizes that the described antenna functionality may be achieved with reflectors having different shapes, such as a parabolic or paraboloid main reflector and a hyperbolic or elliptical sub-reflector, all of which are within the scope of the present invention. Furthermore, the antenna 1 10 comprises a dielectric block 312 filling the antenna housing 300 at least between the main reflector 306 and the sub reflector 310 such that all wave propagation in the antenna housing 300 takes place within the dielectric block 312. The dielectric block 312 illustrated in Fig. 3 fills the entire volume of the antenna housing 300 but does not protrude outside of the antenna housing 300. Moreover, the dielectric block 312 may be formed separately to be inserted into the housing, or it may be formed directly in the housing 300 for example by molding. The dielectric block 312 may for example be made from PTFE (Polytetrafluoroethylene), PEEK

(Polyether ether ketone) or other suitable plastic dielectric materials.

Fig. 3 further illustrates that the top part 316 and the bottom part 318 are configured to form a cavity or recess 320 at the interface between the top and 316 bottom part 320 such that a corresponding flange or protruding portion 322 of the dielectric block 312 can be fixed in the recess 320 when the top and 316 bottom part 320 are mechanically connected. The dielectric block 312 can thereby be securely fixed in the antenna housing 300.

In Fig. 3, the sub-reflector 310 is formed as a metallic element on the outer surface of the dielectric block 312, and the sub-reflector 310 may either be provided as a separate metallic element attached to the surface of the dielectric block 312 or it may be formed by depositing a metal coating layer onto the surface of the dielectric block 312. As outlined above, the described antenna 1 10 is both mechanically robust as well as insensitive to

contaminants. Moreover, the feed antenna 314 is also filled by a dielectric material and consequently the transceiver module 213 and associated circuitry is well protected from the tank atmosphere. A further advantageous property of the described antenna is that there are no gaps or voids where matter may be deposited, which is particularly important in applications such as food processing.

Fig. 4 schematically illustrates an embodiment of the antenna 1 10 where the sub-reflector 310 is located within the dielectric block 312. The sub reflector 310 may be formed or arranged as an isolated electric element during manufacturing of the dielectric block, or it may be arranged on a dielectric plunger 402 as illustrated in Fig. 4. Furthermore, it is also possible for the feed antenna 314 to extend into the antenna housing 302.

Figs. 5A-C illustrates an antenna 110 where an end surface 502 of the dielectric block 312 facing in the emission direction comprises a pattern made from surface structures 504 configured to reduce reflections at the interface between the dielectric block 312 and the ambience outside of the antenna 110, typically the tank atmosphere. Fig. 5A is a side view of the antenna 110 comprising rectangular surface structures 504 and 5A is a perspective view illustrating square surface structures 504 are arranged in a checkerboard pattern where a height of the surface structures 504 are approximately equal to a quarter of the wavelength of the antenna frequency (l/4) and where a surface area is approximately lcl, and preferably slightly less than lcl to avoid grating lobes. The described surface structure 504 can be formed by removing material from the surface 502 to form the checkerboard pattern. It is in principle also possible to form the surface structures 504 by depositing material.

In Fig. 5C, the surface structures 506 have a conical shape with a height of approximately l/4, where the conical shape may be advantageous to prevent contamination of the antenna surface 502 since contaminant will more easily form droplets leaving the conical structures 506. The described surface comprising surface structures 504, 506 is thereby adapted to form a matching layer between the dielectric block 312 of the antenna 110 and the surrounding atmosphere. In an example embodiment, for a dielectric block 312 made from PTFE and at a frequency of 80GFIz, the height of the surface structures 504, 506 should be approximately 0.7mm to achieve a reflection free surface.

Figs. 6A-B schematically illustrate antennas 110 where the end surface 502 of the dielectric block 312 facing in the emission direction is curved. In Fig. 2 the surface 502 is concave and in Fig. 6B the surface is convex, thereby reducing the amount of fluid contaminants adhering to the surface 502. As illustrated in Figs. 6A-B, droplets 602 moves towards the edge of the dielectric block 312 for a concave surface and towards the center of the dielectric block 312 for a convex surface.

Fig. 7 illustrates an antenna where the second opening 308 is offset from the center of the housing 300 and from the center of the main reflector 306. Moreover the feed antenna 314 is tilted from a central axis 700 of the antenna, and the main reflector 306 and the sub reflector 210 are

correspondingly tilted. In order to achieve transmission from the antenna in a vertical direction, i.e. along the central axis 700 of the antenna, the end surface 502 of the dielectric block 312 is inclined so that a wave reaching the interface from within the dielectric block 312 is refracted towards the normal such that it is transmitted in a vertical direction, i.e. parallel to the central axis 700. A tilted end surface 502 of the antenna 110 may be advantageous in that it may reduce the occurrence of fluid contaminants on the surface. Moreover, in some applications it may be preferable to have the feed antenna 314 and the second opening 308 offset from the center of the antenna which allows various arrangements of the transceiver module 213 and associated circuitry.

In the antenna 110 illustrated by Fig. 8, the second opening 308 is centrally located in the antenna 110 and in the main reflector 306, but the main reflector 306 and the sub-reflector 310 are both tilted in relation to a central axis 700 of the antenna. Flereby, a tilted end surface 502 can be incorporated in the antenna 110 without having to change the central location of the second opening 308 and the feed antenna 314. The same principles apply for the refraction of the wave as for the antenna of Fig. 7, where the wave is refracted towards the normal to achieve vertical emission. Preferably, the tilt of the end surface 502 in Figs. 7 and 8 is selected so as to minimize reflections at the interface between the dielectric block 312 and the tank atmosphere.

In Figs. 7-8, the dielectric block 312 extends outside of the antenna housing 300 due to the inclined lower surface 502 of the block. It would also be possible to adapt the antenna housing 302 so that the sidewalls 302 reach all the way down to the end of the dielectric block 312 on all sides, thereby fully encircling the dielectric block 312. As is schematically indicated in Fig. 9, the antenna arrangement 110 is attached to a mounting structure 900 of the tank 108. A mounting flange 902 of the antenna 110 is arranged on a receiving flange 904 of the mounting structure 900, and these flanges 902, 904 are pressed against each other, for example using fasteners 906 as is indicated in Fig. 9. The antenna 110 in itself and the dielectric block will thereby act as a seal for the tank 108 such that no specific separate seal arrangement is required.

Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. Also, it should be noted that parts of the antenna may be omitted,

interchanged or arranged in various ways, the antenna yet being able to perform the functionality of the present invention.

Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.