A motor (1) for driving the antenna (3) is connected to a source of power (5) for providing current for driving the motor. The source of power is connected to the motor by a pair of conductors and a voltage is developed across the pair of conductors. The system includes an arrangement for measuring and controlling the speed of the motor and therefore the antenna which includes a current interruption facility (7) for interrupting the current during a current interruption period. The current interruption period is adjustable whereby to adjust the speed of the motor (1) and thereby the antenna (3). The voltage developed across the pair of conductors is measured during the current interruption period. The voltage measured during the current interruption period is proportional to the speed of the antenna.