The invention relates to a fastening device (3), comprising a base plate (4) for fixation of an antenna rod (5) to a body panel (2), a cover (6) fixed to the base plate (4) from above and a substantially square formed clamping element (10) located below the base plate (4) and connected to the base plate (4) by means of e.g. a screw (13). The invention is achieved by that a first guiding element (15) is arranged as an end stop intended to abut an edge (8a) of a key grip (8) arranged in the mounting hole (7). The first guiding element (15) and the key grip (8) are arranged to automatically orientate the fastening device (3) in a correct rotational/-angular position in the body panel (2). As the first guiding element (15) is arranged inside the circumference of the mounting hole (7,9) makes it possible to install the antenna assembly (1) also in a basic circular hole (9) having no "key grip" (8).