The radio frequency (RF) antenna assembly has sets of antenna conductors that leave an opening between the sets. A radiotherapy beam path may pass through the opening so that the antenna conductors are at most minimally exposed to the radiation. Each set of antenna conductors has a surface conductor loop and a transverse conductor loop. The surface conductor loop is arranged on cylindrical surface and generates an RF field mostly in its axial range. The transverse conductor loop extends radially and generates an RF field in the axial range of the opening. In this way a homogeneous RF field within the RF antenna assembly.
