An electrical connecting element (1) comprises a dielectric substrate (2) on which a plurality of coplanar and substantially parallel conductor paths (3, 4, 5; 11, 12 13; 23, 24, 25, 26, 27) is arranged. At least one of these conductor paths constitutes a signal-carrying conductor path (3; 11; 23, 24), and at least one conductor path (4, 5; 12, 13; 25, 26, 27) on each side of the signal-carrying conductor path constitutes a ground plane, so that the three conductor paths together constitute a waveguide. The dielectric substrate (2) is formed by a flexible sheet. In a method of making such an electrical connecting element, a carrier member (31) is caused to rotate at a high rate, and then plastics material (32) in liquid form is placed on the rotating carrier member, so that the plastics material is slung into a thin sheet (33). Metallic conductor paths (37) are subsequently arranged on the plastic sheet thus produced, and the plastics sheet (33) is then removed from the carrier member (31).