The invention relates to the technical field of radio communications, and in particular to an antenna device for a radio base station, and a method for precoding data in a Multiple-Input Multiple-Output (MIMO) system. Embodiments of the invention disclose a secondary precoder 24 in series with a multiple-input multiple-output precoder 22. The secondary precoder 24 has a plurality of inputs 34, 36 and a plurality of outputs 38, 40. The second plurality of inputs being in communication with each plurality of outputs 38, 40 such that one or more signals input to a respective one or more of the second plurality of inputs 34, 36 provides a substantially equal power of signals at the second plurality of outputs 38, 40.