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This paper concerns dielectric tube antenna design in the frequency range from 8.5 to 10.5 GHz for the use in the field of industrial radar level gauging for process automation industry. The application is based on monostatic radar configurations, which thus demands for highly directive endfire antennas made of chemically inert dielectrics, such as polytetrafluoroethylene (PTFE) or polypropylene (PP). Herein, the antennas consist of solid and tubular waveguides with cylindrical cross sections as well as transitions among these sections. In order to approach the promising theoretical directivity limits of thin-walled dielectric tubes at small aperture diameters, a fundamental HE11 mode-preserving waveguide transition forms the core element of the antenna. This yields compact dielectric antennas with directivity levels of around 20 dBi. By this means, a significant improvement in comparison to a circular metallic horn aperture of the same outer diameter as the radiating tube and a length reduction with regard to dielectric rod antennas is achieved. Finally, prototypes of the proposed antennas are manufactured and verified by measurements.