The invention relates to a method for mounting a radiating antenna element used especially in the manufacture of small-sized radio devices and a radio device with a radiator mounted by the method. A thin thermoplastic plate coated with a metal foil is used in the manufacture of the antenna. A radiator pattern is formed in the metal foil. The antenna component obtained is placed (401) on the surface of some plastic part of the radio device, preferably on the inner surface of the thermoplastic cover of the radio device. The plastic layer of the antenna component becomes positioned against said plastic part, and the component is fastened (402, 403) by fusing together the mating plastic materials, which are against each other. In the complete antenna, the radiator is electrically connected to the other parts of the radio device by means of contacts. The radiator of the antenna of the radio device becomes fastened to the radio device very firmly, which has a stabilizing effect on the electric properties of the antenna. In addition, the antenna can be formed using relatively cheap raw materials.