A method for communication includes, in a transmitter that transmits one or more data streams to respective target receivers and one or more jamming streams for preventing the data streams from being decoded by eavesdropping receivers, holding a definition of at least first and second transmission modes having respective, different first and second levels of security in preventing the data streams from being decoded by the eavesdropping receivers. One of the first and second transmission modes is selected for transmitting a data stream to a target receiver, by evaluating a selection criterion. The data stream and the jamming streams are transmitted via an antenna array using the selected transmission mode.