A three-port vectorial combining arrangement (191, 192) having first and second input ports and an output port, the arrangement further comprising: a) first and second power dividers (201, 211) respectively connected to the first and second input ports, each configured to provide a defined sample of the input power at a first output and the remainder of the input power at a second output; b) phase detection circuitry (203, 213) configured to detect a phase difference between the first outputs, respectively, of the first and second power dividers (210, 211) and to output a control signal representative of a phase angle between RF signals applied to the first and second input ports; c) tuneable phase shifter circuitry (202, 212) connected to the second output of at least one of the first and second power dividers (201, 211), the phase shifter circuitry (202, 212) having a control port to receive the control signal output by the phase detection circuitry (203, 213) such that the phase shift introduced by the tuneable phase shifter circuitry (202, 212) is controlled by the control signal, the tuneable phase shifter circuitry (202, 212) having at least one output; d) a power combiner having first and second inputs respectively connected to the second outputs of the first and second power dividers (201, 211), at least one of the second outputs of the first and second power dividers (201, 211) being routed through the tuneable phase shifter circuitry (202, 212), and an output; e) a further tuneable phase shifter having an input connected to the output of the power combiner and a control port to receive the control signal from the phase detection circuitry, the further tuneable phase shifter being configured to output to the output port of the combining arrangement (191, 192) an RF signal having a phase substantially equal to an arithmetic mean of the phases of two RF signals fed to the respective first and second input ports of the combining arrangement (191, 192). a) first and second power dividers (201, 211) respectively connected to the first and second input ports, each configured to provide a defined sample of the input power at a first output and the remainder of the input power at a second output; b) phase detection circuitry (203, 213) configured to detect a phase difference between the first outputs, respectively, of the first and second power dividers (210, 211) and to output a control signal representative of a phase angle between RF signals applied to the first and second input ports; c) tuneable phase shifter circuitry (202, 212) connected to the second output of at least one of the first and second power dividers (201, 211), the phase shifter circuitry (202, 212) having a control port to receive the control signal output by the phase detection circuitry (203, 213) such that the phase shift introduced by the tuneable phase shifter circuitry (202, 212) is controlled by the control signal, the tuneable phase shifter circuitry (202, 212) having at least one output; d) a power combiner having first and second inputs respectively connected to the second outputs of the first and second power dividers (201, 211), at least one of the second outputs of the first and second power dividers (201, 211) being routed through the tuneable phase shifter circuitry (202, 212), and an output; e) a further tuneable phase shifter having an input connected to the output of the power combiner and a control port to receive the control signal from the phase detection circuitry, the further tuneable phase shifter being configured to output to the output port of the combining arrangement (191, 192) an RF signal having a phase substantially equal to an arithmetic mean of the phases of two RF signals fed to the respective first and second input ports of the combining arrangement (191, 192).