The present invention relates to a method for adapting transmission parameters in a transmitter Tx in communication with at least one antenna 27; 38; 47; 57; 77, 88. The method comprises: transmitting a signal from the transmitter Tx measuring at least one reflection coefficient S11 of said signal for each antenna 27; 38; 47; 57; 77, 88, and adapting the transmission parameters based on the measured reflection coefficient S11. The antenna could be a transmit antenna and/or a receive antenna. In a preferred embodiment antenna calibration may be performed by measuring propagation time for each individual transmitter chain 481, 482; 781, 782, and compensate for differences in time delay between the individual transmitter chains 481, 482; 781, 782 to assure that the signal is transmitted simultaneously from all transmit antennas.