Dielectric filling member with microwave absorbing element
A radar level gauge including a wave guiding structure and a cavity formed inside the wave guiding structure, and a dielectric filling member arranged at least partly within said cavity. The dielectric filling member includes a main body formed of a polymer material, at least one microwave absorbing element formed of a modified polymer material providing a microwave attenuation of at least 1 dB/centimeter at an operating frequency of the radar level gauge, the modified polymer material being obtained by modifying said polymer material with a filler material, wherein the at least one microwave absorbing element is integrally formed with said main body by sintering.
1. A radar level gauge, for determining a process variable of a product in a tank using electromagnetic measuring signals, comprising:
a signal propagation device adapted to direct a microwave transmit signal toward said product and return reflections thereof from a surface of said product,
said signal propagation device including a wave guiding structure and a cavity formed inside said wave guiding structure; and
a dielectric filling member arranged at least partly within said cavity, said dielectric filling member including:
a main body formed of a polymer material,
at least one microwave absorbing element formed of a modified polymer material providing a microwave attenuation of at least 1 dB/centimeter at an operating frequency of the radar level gauge, said modified polymer material being obtained by modifying said polymer material with a filler material,
wherein said at least one microwave absorbing element is integrally formed with said main body by sintering,
wherein said at least one microwave absorbing element is embedded in the main body, and serves to reduce resonance in microwave signals in the wave guiding structure.
2. The radar level gauge according to claim 1 , wherein said polymer material is a fluoropolymer, preferably PTFE.
3. The radar level gauge according to claim 1 , wherein said filler material is one of a conductive material such as carbon or metal, and a magnetic material such as ferrite.
4. The radar level gauge according to claim 1 , wherein said filler material constitutes less than 15% by weight of the modified polymer material, and preferably less than 10% by weight of the modified polymer material.
5. The radar level gauge according to claim 1 , wherein said filler material constitutes at least 0.5% by weight of the modified polymer material, and preferably at least 1% by weight of the modified polymer material.
6. The radar level gauge according to claim 1 , wherein said wave guiding structure includes a horn antenna having a waveguide section and a horn section, said cavity being formed inside said waveguide section and said horn section.
7. The radar level gauge according to claim 6 , wherein said main body has a substantially cylindrical portion filling said waveguide section, a substantially conical portion filling said horn section, and a convex end portion extending beyond an opening of the horn section.
8. The radar level gauge according to claim 7 , wherein said main body further includes an annular flange for providing sealing between a tank connection of the radar level gauge and a tank flange on which the radar level gauge is intended to be mounted.
9. The radar level gauge according to claim 7 , wherein said at least one microwave absorbing element is a pin embedded beneath an outer surface of said convex end portion.
10. The radar level gauge according to claim 1 , further comprising:
transceiver circuitry electrically connected to said signal propagation device and configured to generate and transmit an electromagnetic transmit signal S T and receive an electromagnetic return signal reflected from the tank,
processing circuitry configured to determine the distance between a reference position and the surface of the product based on a relationship between the transmit signal and the return signal, and
an interface configured to communicate measurement data.
11. A method for manufacturing a dielectric filling member for a wave guiding structure of a radar level gauge, including the steps:
forming a main body of a polymer material,
obtaining a modified polymer material by modifying said polymer material with a filler material, said modified polymer material providing a microwave attenuation of at least 1 dB/centimeter at an operating frequency of the radar level gauge,
forming at least one microwave absorbing element of said modified polymer material,
arranging said at least one microwave absorbing element in pressurized contact with said main body, and
heating said main body and said at least one microwave absorbing element to such an extent that said at least one microwave absorbing element is sintered with said main body,
wherein the step of arranging said at least one microwave absorbing element in pressurized contact with said main body includes embedding said at least one microwave absorbing element in said main body, in order to reduce resonance in microwave signals in the wave guiding structure.
12. The method according to claim 11 , wherein said polymer material is a fluoropolymer such as PTFE.
13. The method according to claim 11 , wherein said filler material is one of a conductive material such as carbon or metal, and a magnetic material such as ferrite.
14. The method according to claim 11 , wherein said filler material constitutes less than 15% by weight of the modified polymer material, and preferably less than 10% by weight of the modified polymer material.
15. The method according to claim 11 , wherein said filler material constitutes at least 0.5% by weight of the modified polymer material, and preferably at least 1% by weight of the modified polymer material.
16. The method according to claim 11 , wherein said main body is formed to have a substantially cylindrical portion for filling a waveguide section of wave guiding structure, a substantially conical portion for filling a horn section of said wave guiding structure, and a convex end portion intended to extend beyond an opening of the horn section.
17. The method according to claim 16 , wherein said main body is formed to have an annular flange for providing sealing between a tank connection of the radar level gauge and a tank flange on which the radar level gauge is intended to be mounted.
18. The method according to claim 16 , further comprising:
forming a hole in the main body extending from an outer surface of said convex end portion,
forming said at least one microwave absorbing element as a pin, having a shape adapted to fit tightly in said hole,
pressing said pin into said hole,
forming a plug of the polymer material, said plug having a shape to fit tightly in said hole,
pressing said plug into said hole to cover the pin,
so that, when said main body and said pin are integrated by sintering, said pin is completely concealed in said main body.
19. The method according to claim 18 , further comprising machining the dielectric filling member subsequent to the sintering step.
20. The method according to claim 18 , wherein the pin is one of substantially cylindrical and tapered.