A portable free-standing vertical RF antenna structure for use in an UHF timing system comprises a base structure for supporting the vertical antenna structure on a substantially horizontal surface. At least a first antenna unit comprising a first planar RF antenna and at least a second antenna unit comprising a second planar RF antenna, the second antenna unit being located above the first antenna unit, are coupled to the base structure and form a vertical RF antenna structure. The first and second antenna unit is positioned such that the inclination angle between a main axis of the radiation field of the first RF antenna and the horizontal surface is selected between 10 and 50 degrees. The inclination angle between a main axis of the radiation field of the second RF antenna and the horizontal surface is selected between −20 and 20 degrees.