A mobile-radio base station for a telecommunications system includes vector modulator means for independently controlling the phase and/or amplitude of a plurality of component signals representative of the signal to be transmitted or received, such that when these signals pass through a plurality of antenna elements, a beam is formed in a direction according to the phase relationship of the component signals. The invention also includes interface means allowing other base stations to be coupled to the same antenna, with each base station having independent control of its beam direction. Provision is includes for phase compensation of the signals to correct for errors introduced by unequal and variable component signal path lengths between the base station and the antenna. The vector modulator means is arranged to operate at low power levels, where it can operate more efficiently.