A microwave transmission line assembly operated in vacuum for satellite antennas and beamforming networks comprising a first ground plane and a conductor strip positioned a distance from the first ground plane. The conductor strip comprises a first strip portion and a second strip portion (6). The first strip portion is positioned at a first distance from the first ground plane and wherein the second strip portion is positioned at a second distance from the first ground plane. The first distance is smaller than the second distance, wherein the first distance is chosen to avoid multipaction.