A parabolic aerial comprises a parabolic mirror (2), an exciter (3) for transmitting electromagnetic waves to the parabolic mirror, and a diffusion disc (1) for laterally removing interfering fraction of electromagnetic waves past the exciter. The parabolic mirror concentrates the electromagnetic waves. The diffusion disc is arranged in the center of parabolic mirror. The shape of diffusion disc is such that a beam of parallel radiation that impinges the diffusion disc along an axis from the exciter is diverted from the axis by a backscatter process at the diffusion disc. Independent claims are included for: (1) a fill level radar; and (2) fill level measurement.