An antenna tuning apparatus for a multiport antenna array used for sending and/or receiving electromagnetic waves for radio communication comprises: 4 antenna ports, 4 user ports, 10 adjustable impedance devices each presenting a negative reactance and having a terminal coupled to one of the antenna ports, 4 windings each having a first terminal coupled to one of the antenna ports and a second terminal coupled to one of the user ports, and 10 adjustable impedance devices each presenting a negative reactance and having a terminal coupled to one of the user ports. All adjustable impedance devices are adjustable by electrical means. Any small variation in the impedance matrix of the antenna array, caused by a change in operating frequency or a change in the medium surrounding the antennas, can be compensated with a new adjustment of the adjustable impedance devices.