A radio communications system includes a communication radio and an antenna module positioned external to the communication radio. An antenna cable connects the radio and the antenna module. The radio superposes a power voltage for operating the antenna module on a signal line of the antenna cable and transmits to the antenna module through the antenna cable a control signal that controls a gain of a transmission amplifier and duplexers while the transmission signal is not transmitted therethrough. In such manner, a control of the antenna module is enabled without affecting the transmission signal.