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A bottom reflector for a radar-based level gauge, which measures a level of a liquid stored in a tank by means of transmitting a microwave signal towards the surface of the liquid from the top of the tank, receiving the microwave signal as reflected against the surface of the liquid, and calculating

A bottom reflector for a radar-based level gauge, which measures a level of a liquid stored in a tank by means of transmitting a microwave signal towards the surface of the liquid from the top of the tank, receiving the microwave signal as reflected against the surface of the liquid, and calculating the level of the liquid from the propagation time of the transmitted and reflected microwave signal, wherein the bottom reflector is adapted to be mounted close to the bottom of the tank and comprises a reflecting structure having a first reflection coefficient for the microwave signal when the level of the liquid is above the reflecting structure and a second reflection coefficient for the microwave signal when the level of the liquid is below the reflecting structure, wherein the first reflection coefficient is substantially lower than the second reflection coefficient.

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1. A bottom reflector for a radar-based level gauge, which measures a level of a liquid stored in a tank by means of transmitting a microwave signal towards the surface of the liquid from the top of the tank, receiving the microwave signal as reflected against the surface of the liquid, and calculat

1. A bottom reflector for a radar-based level gauge, which measures a level of a liquid stored in a tank by means of transmitting a microwave signal towards the surface of the liquid from the top of the tank, receiving the microwave signal as reflected against the surface of the liquid, and calculating the level of the liquid stores in the tank from the propagation time of the transmitted and reflected microwave signal, said bottom reflector being mountable at a predetermined height in said tank for reflecting the microwave signal when no liquid is present at said predetermined height in the tank, wherein said bottom reflector comprises a reflecting structure having a first reflection coefficient for said microwave signal when the level of the liquid is above the reflecting structure and a second reflection coefficient for said microwave signal when the level of the liquid is below the reflecting structure, said first reflection coefficient being substantially lower than said second reflection coefficient. 2. The bottom reflector of claim 1 wherein said first reflection coefficient of said reflecting structure for said microwave signal is lower than a reflection coefficient of the surface of said liquid for said microwave signal. 3. The bottom reflector of claim 2 wherein said first reflection coefficient of said reflecting structure for said microwave signal is substantially lower than the reflection coefficient of the surface of said liquid for said microwave signal. 4. The bottom reflector of claim 1 wherein said second reflection coefficient of said reflecting structure for said microwave signal is approximately of the same magnitude as, or slightly higher than, a reflection coefficient of the surface of said liquid for said microwave signal. 5. The bottom reflector of claim 1 wherein the liquid has a dielectric constant in the range 1.6-3. 6. The bottom reflector of claim 1 wherein the liquid is a liquid in a group comprising crude oil, a liquid petroleum gas, a liquid natural gas, other hydrocarbon liquids, and liquids at least partly transparent to said microwave signal. 7. The bottom reflector of claim 1 wherein said microwave signal has a specified polarization state; and said first and second reflection coefficients are given for the microwave signal in said specified polarization state. 8. The bottom reflector of claim 1 wherein said microwave signal is within a specified frequency range. 9. The bottom reflector of claim 1 wherein said first reflection coefficient is lower than 0.2 10. The bottom reflector of claim 1 wherein said first reflection coefficient is lower than 0.1. 11. The bottom reflector of claim 1 wherein said first reflection coefficient is lower than 0.05. 12. The bottom reflector of claim 1 wherein said second reflection coefficient is higher than 0.1. 13. The bottom reflector of claim 1 wherein said second reflection coefficient is higher than 0.2. 14. The bottom reflector of claim 1 wherein said second reflection coefficient is lower than 0.5. 15. The bottom reflector of claim 1 wherein said second reflection coefficient is lower than 0.4. 16. The bottom reflector of claim 1 wherein said second reflection coefficient is lower than 0.3. 17. The bottom reflector of claim 1 wherein said reflecting structure is adapted for use in manual level gauging by means of hand dipping. 18. The bottom reflector of claim 1 wherein said level gauge comprises a substantially vertical tube, in which the transmitted and reflected microwave signal is guided. 19. The bottom reflector claim 18 wherein said bottom reflector is mountable in a lower end portion of said substantially vertical tube. 20. The bottom reflector claim 18 wherein said microwave signal has a specified mode of propagation; and said first and second reflection coefficients are given for the microwave signal in said specified mode of propagation. 21. The bottom reflector of claim 18 wherein said reflecting structure is a cut-of grid. 22. The bottom reflector of claim 21 wherein said microwave signal propagates in H 11 mode, and said cut-off grid comprises a number of essentially parallel strips arranged essentially in parallel with an electric field of said microwave signal. 23. The bottom reflector of claim 21 wherein said microwave signal propagates in H 01 mode, and said cut-off grid comprises a number of concentrically arranged circular strips arranged in parallel with an electric field of said microwave signal. 24. The bottom reflector of claim 1 further comprising a deflecting structure arranged below said reflecting structure. 25. The bottom reflector of claim 1 further comprising an absorbing structure arranged below said reflecting structure. 26. The bottom reflector of claim 18 wherein said reflecting structure is a resonant structure. 27. The bottom reflector of claim 26 wherein said resonant structure includes dipoles. 28. The bottom reflector of claim 26 wherein said resonant structure comprises stacked dipoles. 29. The bottom reflector of claim 26 wherein said resonant structure comprises duel crossed dipoles. 30. The bottom reflector of claim 26 wherein said microwave signal propagates in H 11 mode, and the resonant structure comprises a single dipole element arranged at a central axis of said substantially vertical tube. 31. The bottom reflector of claim 26 wherein said microwave signal propagates in H 01 mode, and the resonant structure comprises multiple dipole elements arranged circumferentially spaced from a central axis within said substantially vertical tube. 32. The bottom reflector of claim 26 wherein said microwave signal propagates in H 01 modes, and the resonant structure comprises a ring having an axis of symmetry which coincides with a central axis of said substantially vertical tube. 33. The bottom reflector of claim 18 wherein said reflecting structure is a dielectric reflector. 34. The bottom reflector of claim 33 wherein said dielectric reflector is a perforated plate. 35. The bottom reflector of claim 33 wherein said dielectric reflector comprises an essentially horizontally arranged plate provided with small dielectric members on its bottom side. 36. The bottom reflector of claim 18 wherein said reflecting structure is a polarization turning structure. 37. The bottom reflector of claim 36 wherein said polarization turning structure is a twist reflector structure including ridges. 38. The bottom reflector of claim 36 wherein said polarization turning structure includes a reflector and multiple differently directed dipoles. 39. The bottom reflector of claim 18 wherein said reflecting structure is a microwave guiding structure, which is arranged within said substantially vertical tube and through which the transmitted and reflected microwave signal is guided away from the interior of said substantially vertical tube. 40. The bottom reflector of claim 39 wherein said microwave guiding structure is a funnel. 41. The bottom reflector of claim 1 wherein said level gauge is adapted for free space propagation of the microwave signal. 42. The bottom reflector of claim 41 wherein said reflecting structure is a cut-off grid, a resonant structure, a dielectric reflector, or a polarization turning structure. 43. The bottom reflector of claim 41 wherein said reflecting structure covers an area, which is at least an essential portion of an area of π*[square root(hλ)] 2 /4 below said radar-based level gauge, where h is the distance from said radar-based level gauge to said bottom reflector and λ is the wavelength of said microwave signal. 44. The bottom reflector of claim 41 wherein the area of said bottom reflector is smaller than the area of said microwave signal at the height of the bottom reflector; and the phase shift of the microwave signal as transmitted through the bottom reflector when the level of the liquid is above said bottom reflector, is controlled so that the portions of the microwave signal as reflected from the bottom of the tank outside said bottom reflector and portions of the microwave signal as reflected from the bottom of the tank after having passed through said bottom reflector will cancel out each other. 45. The bottom reflector of claim 1 wherein said bottom reflector is mountable close to the bottom of said tank. 46. A radar-based level gauge system comprising the bottom reflector of claim 1. 47. A bottom reflector for a radar-based level gauge, which measures a level of a liquid stored in a tank by means of transmitting a microwave signal towards the surface of the liquid from the top of the tank, receiving the microwave signal as reflected against the surface of the liquid, and calculating the level of the liquid stored in the tank from the propagation time of the transmitted and reflected microwave signal, said bottom reflector being mountable close to the bottom of said tank for reflecting the microwave signal when no liquid is present in the tank, wherein said bottom reflector comprises a reflecting structure having a first reflection coefficient for said microwave signal when the level of the liquid is above the reflecting structure and a second reflection coefficient for said microwave signal when the level of the liquid is below the reflecting structure, said first reflection coefficient being lower than a reflection coefficient of the surface of said liquid for said microwave signal. 48. The bottom reflector of claim 47 wherein said second reflection coefficient of said reflecting structure for said microwave signal is approximately of the same magnitude as, or slightly higher than, the reflection coefficient of the surface of said liquid for said microwave signal. 49. The bottom reflector of claim 47 wherein the liquid has a dielectric constant in the range 1.6-3. 50. The bottom of claim 47 wherein the liquid is a liquid in a group comprising crude oil, a liquid petroleum gas, a liquid natural gas, other hydrocarbon liquids, and liquids at least partly transparent to said microwave signal. 51. A bottom reflector for a radar-based level gauge, which measures a level of a liquid stored in a tank by means of transmitting microwaves towards the surface of the liquid from the top of the tank, receiving the microwaves as reflected against the surface of the liquid, and calculating the level of the liquid stored in the tank from the propagation time of the transmitted and reflected microwaves, said bottom reflector being mountable close to the bottom of said tank for reflecting the microwaves when no liquid is present in the tank, wherein said bottom reflector comprises a reflecting structure which rotates the polarization of the reflected microwaves differently when the level of the liquid is above the reflecting structure as compared to when the level of the liquid is below the reflecting structure. 52. The bottom reflector of claim 51 wherein said reflecting structure rotates the polarization of the reflected microwaves more when the level of the liquid is above the reflecting structure than when the level of the liquid is below the reflecting structure. 53. The bottom reflector of claim 51 wherein said reflecting structure is a twist reflector structure including ridges. 54. The bottom reflector of claim 51 wherein said reflecting structure includes a reflector and multiple differently directed dipoles.