In order to calibrate in amplitude and phase the individual transceiver elements (4) of an active antenna array for a mobile telecommunications network, each transceiver element including a transmit and a receive path (8, 10) coupled to an antenna element (12), each transceiver element includes a comparator (100) for comparing phase and amplitude of transmitted or received signals with reference signals in order to adjust the characteristics of the antenna beam. In order to provide an accurate means of reference signal distribution, a feed arrangement distributes the reference signals and includes a waveguide (50) of a predetermined length which is terminated at one end (52) in order to set up a standing wave system along its length, and a plurality of coupling points (56) at predetermined points along the length of the waveguide, which are each coupled to a comparator of a respective transceiver element.