A device for determining or monitoring the filling level of a filling material stored in a container in a process, comprising a signal generating unit, a coupling/decoupling unit which couples the high-frequency measurement signals to an antenna unit or decouples said high-frequency measurement signals from the antenna unit. The antenna unit has a wave guide and an antenna element which widens in the direction of the filling material. The antenna unit emits high-frequency measurement signals in the direction of the surface and receives the echo signals reflected by the surface of the filling material. A control/evaluation unit determines the filling level of the filling material in the container from the transit time of the measurement signals. At least one process separation device is thereby provided in or on the antenna unit, which process separation device protects the coupling/decoupling unit from influences of the process, wherein a first process separation device is produced at least partially from a dielectric material with a specified porosity, and wherein the first process separation device is configured to be at least approximately transparent to the high-frequency measurement signals.