Apparatus comprises a radio channel emulator of a radio system and a combiner. The combiner receives antenna data defining a characteristic radiation pattern of each predetermined antenna element of a predetermined antenna array; forms modified antenna data associated with a predetermined number of modified antenna elements by weighting the antenna data with a predetermined coefficient data; forms modified radio channel data by combining a predetermined radio channel data and the modified antenna data; and feed the modified radio channel data to the radio channel emulator; and the emulator receives at least one signal each of which being associated with a modified antenna element and perform emulation with the at least one signal and the modified radio channel data.