In the invention, a four-port junction (10) has a port (12) connected to an RF signal source (14). The four-port junction (10) also has an antenna port (16) coupled to a combined transmit/receive antenna (18). Two ports (20, 22) of the four-port junction (10) develop output power that is applied to an analogue-to-digital converter (24), that is in turn coupled to a central processor (26), that is in turn coupled to a user interface (28).