A combining arrangement comprising: a variable attenuator power combiner having at least four ports; a first match-dependent oscillator connected to input power at a first frequency to a first input port of the power combiner; a second match-dependent oscillator connected to input power at a second frequency to a second input port of the power combiner, wherein a match-dependent oscillator is an oscillator having an output which is dependent on how well its load is matched to it; and a mismatch connected to a third port of the power combiner; the power combiner being operative to combine power from the first and second oscillators and when the first and second frequencies are different, to apply a first fraction of the combined power to the mismatch, the mismatch reflecting at least some of the first fraction to the first and second oscillators to phase and frequency lock their outputs, and a fourth output port of the power combiner being connected to receive a second fraction of the combined power, the power combiner attenuation being variable to selectively adjust the proportion of the combined power split between the third port and fourth output port from 0% to 100% of the total combined power for any power values at the first input port and second input port by tuning the centre frequency of the power combiner, wherein the power combiner comprises: a first 3 dB hybrid coupler arranged to receive, at its inputs, power from the first input port and the second input port of the power combiner respectively; a second 3 dB hybrid coupler arranged to output power, at its outputs, to the third port and fourth output port of the power combiner respectively; waveguide sections arranged to transmit power from a first output of the first 3 dB hybrid coupler to a first input of the second 3 dB hybrid coupler and to transmit power from a second output of the first 3 dB hybrid coupler to a second input of the second 3 dB hybrid coupler; a third 3 dB hybrid coupler arranged in the transmission path between the second output of the first 3 dB hybrid coupler and the second input of the second 3 dB hybrid coupler; and a ganged sliding short circuit arranged between the second output of the first 3 dB hybrid coupler and the second input of the second 3 dB hybrid coupler thereby providing frequency adjustability of the second 3 dB hybrid coupler and hence distribution of power at the outputs of the 3 dB hybrid coupler. a first 3 dB hybrid coupler arranged to receive, at its inputs, power from the first input port and the second input port of the power combiner respectively; a second 3 dB hybrid coupler arranged to output power, at its outputs, to the third port and fourth output port of the power combiner respectively; waveguide sections arranged to transmit power from a first output of the first 3 dB hybrid coupler to a first input of the second 3 dB hybrid coupler and to transmit power from a second output of the first 3 dB hybrid coupler to a second input of the second 3 dB hybrid coupler; a third 3 dB hybrid coupler arranged in the transmission path between the second output of the first 3 dB hybrid coupler and the second input of the second 3 dB hybrid coupler; and a ganged sliding short circuit arranged between the second output of the first 3 dB hybrid coupler and the second input of the second 3 dB hybrid coupler thereby providing frequency adjustability of the second 3 dB hybrid coupler and hence distribution of power at the outputs of the 3 dB hybrid coupler.