Method and arrangement for determining at least one phase affecting error related to transmission from at least one pair of correlated antennas. The method involves transmitting (1002) reference signals from the correlated antennas in the at least one pair, in a set of frequency bands, wherein a number of controlled phase differences are introduced between reference signals transmitted from the antennas. The method further involves receiving (1004), in response to the transmitted reference signals, for a number of the controlled phase differences, one or more indications of a selected precoder matrix. Further, the method involves identifying (1006) changes of selected precoder matrix over the number of controlled phase differences, over the set of frequency bands, wherein the identifying is based on the received one or more indications; and determining (1008) at least one relation between the identified changes. Further, at least one phase affecting: error is determined based on said at least one relation. The method and arrangement enable adjustment of transmission from the at least one pair of correlated antennas, such that said at least one phase affecting error is reduced.