Abstract: In the housing of a filter realized with helix resonators, it is known to locate a coupling aperture in the partition between the resonators. According to the invention, the aperture in the partition is made so that outside the housing a milling cutter (33) is positioned at the point of the wall (31) of the housing where the partition (32) joins the wall of the housing. The longitudinal axis of the cutter extends in the plane of the partition. Then the cutter (33) is forced into the housing to a desired depth (H), whereby the cutter cuts an aperture in the partition open end at the housing surface, the aperture having a width equal to the width of the cutter and a height equal to the distance (H) the cutter was pushed. In this way it is easy to make coupling apertures also in extremely small sized filters, which are lower than 5 mm.