A small and foldable radio device antenna and a radio device which has an antenna according to an embodiment of the invention. The radiating element in the antenna is a conductor having an outline shaped substantially like a rectangle and defining a plane which is perpendicular to the ground plane situated on the circuit board of the radio device. The radiating element fits inside the foldable device in the perpendicular position. The element is connected to the radio device only by its feed point. Resonating frequencies of the element can be arranged by shaping the element, and by means of discrete components. The matching of the antenna is arranged by providing an appropriate distance between the radiating element and ground plane. In an operating situation, an antenna gain is achieved which is considerably higher than that of a PIFA of equal height.