In al magnetic resonance system and a method for operating the magnetic resonance system to set RF transmit parameters for a radio-frequency antenna that is excitable in a number of different, linearly independent transmit modes, the transmit antenna is successively caused to be excited in the different transmit modes, and, for each transmit mode in which the radio-frequency antenna is excited, a measured value distribution representing the radio-frequency field produced by the antenna in a defined volume area is determined. The homogeneity of the measured value distribution is evaluated with respect to a defined evaluation criterion and, if the evaluation criterion is satisfied, an optimized radio-frequency field distribution is determined from a combination of the measured value distributions up to that time. If the evaluation criterion is not fulfilled, the radio-frequency antenna is excited with a further transmit mode, and the procedure is repeated.