Radar beacons of the so called sweep frequency type are heretofore known which provide responding signals to a vessel whereas the frequency of the sweeping responding signal coincides with the frequency of the signal transmitted from the vessel. Such a technique has a certain delay of the responding signal of the magnitude 1-2 minutes which, however, is not considered to be fully satisfying by, among others, the shipping authorities. Another disadvantage of previously known radar beacons is that they under certain circumstances they also respond to signals originating from the side lobes of the vessel antenna. This implies ambiguity concerning the position of the beacon when this position is indicated on the indicator of the vessel. The useful range of the radar beacon will then be limited to the position of such vessels which are situated outside a critical area within which radar pulses originating from the side lobes are also so strong that the radar beacon respond.