A fill level switch for determining a level limit of a medium in a container, has a sensor unit having an antenna unit, a process window for protecting the sensor unit and for connection to a container wall, an electronic unit and a sensor housing. The antenna unit has at least a first emitting element for emitting a first electromagnetic transmission signal, at least a feed line, at least a reference surface having a reference potential and at least a dielectric layer, wherein the dielectric layer is arranged between the reference surface and the emitting element. An aperture coupling is provided via which the emitting element is fed by the feed line during operation. The reference surface has an opening for passage of the electromagnetic transmission signal, and the first emitting element is arranged such that it has direct contact with the medium during operation.