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; a dielectric filling member arranged in said signal propagation device to prevent entry of tank content into the signal propagation device; and a sealing arrangement for preventing tank content from escaping into the outside environment; said dielectric filling member including: a main body formed of a polymer material, wherein said signal propagation device includes a hollow wave guiding structure (115; 116) and a cavity being formed inside said wave guiding structure, and wherein said dielectric filling member is arranged at least partly within said cavity, wherein said hollow wave guiding structure includes a horn antenna having a waveguide section and a horn section, said cavity being formed inside said waveguide section and/or said horn section, wherein said main body has a center portion filling said waveguide section, and an annular collar portion extending radially from said center portion, said annular collar portion forming part of said sealing arrangement, characterized in that said dielectric filling member further includes at least one structurally reinforced element formed of a modified polymer material providing, said modified polymer material being obtained by modifying said polymer material with a filler material, wherein said at least one structurally reinforced element is integrally formed with said main body by sintering, and forms part of said sealing arrangement, wherein said structurally reinforced element forms at least part of said annular collar portion, and is located radially outside a microwave zone of said hollow wave guiding structure. a signal propagation device adapted to direct a microwave transmit signal toward said product and return reflections thereof from a surface of said product; a dielectric filling member arranged in said signal propagation device to prevent entry of tank content into the signal propagation device; and a sealing arrangement for preventing tank content from escaping into the outside environment; said dielectric filling member including: a main body formed of a polymer material, wherein said signal propagation device includes a hollow wave guiding structure (115; 116) and a cavity being formed inside said wave guiding structure, and wherein said dielectric filling member is arranged at least partly within said cavity, wherein said hollow wave guiding structure includes a horn antenna having a waveguide section and a horn section, said cavity being formed inside said waveguide section and/or said horn section, wherein said main body has a center portion filling said waveguide section, and an annular collar portion extending radially from said center portion, said annular collar portion forming part of said sealing arrangement, characterized in that said dielectric filling member further includes at least one structurally reinforced element formed of a modified polymer material providing, said modified polymer material being obtained by modifying said polymer material with a filler material, wherein said at least one structurally reinforced element is integrally formed with said main body by sintering, and forms part of said sealing arrangement, wherein said structurally reinforced element forms at least part of said annular collar portion, and is located radially outside a microwave zone of said hollow wave guiding structure.