A waveguide coupler comprising: a housing (110, 740) having a first outer waveguide branch, a second outer waveguide branch, and an inner waveguide branch; first (102, 702) and second (104, 704) input ports at a first end of the housing in communication with the first outer and second outer waveguide branches respectively; an output port (101, 701) at a second end of the housing in communication with the inner waveguide branch; a first wall separating the first outer waveguide branch and the inner waveguide branch, the first wall having a first iris; a second wall separating the second outer waveguide branch and the inner waveguide branch, the second wall having a second iris; characterised in that the waveguide coupler is a six port coupler and comprises: a first tapered section in the first outer waveguide branch; a second tapered section in the second outer waveguide branch; and a third input port (103, 703) at the first end of the housing in communication with the inner waveguide branch; isolated ports (105, 106) at the second end of the housing in communication with the first outer and the second outer waveguide branches respectively, two internal waveguide loads (750a, 750b) at the isolated ports, the first iris having a first slot width; the second iris having a second slot width; the second outer waveguide branch defining the isolated port having a first width; and the output port having a second width, the second width being different than the first width. a housing (110, 740) having a first outer waveguide branch, a second outer waveguide branch, and an inner waveguide branch; first (102, 702) and second (104, 704) input ports at a first end of the housing in communication with the first outer and second outer waveguide branches respectively; an output port (101, 701) at a second end of the housing in communication with the inner waveguide branch; a first wall separating the first outer waveguide branch and the inner waveguide branch, the first wall having a first iris; a second wall separating the second outer waveguide branch and the inner waveguide branch, the second wall having a second iris; characterised in that the waveguide coupler is a six port coupler and comprises: a first tapered section in the first outer waveguide branch; a second tapered section in the second outer waveguide branch; and a third input port (103, 703) at the first end of the housing in communication with the inner waveguide branch; isolated ports (105, 106) at the second end of the housing in communication with the first outer and the second outer waveguide branches respectively, two internal waveguide loads (750a, 750b) at the isolated ports, the first iris having a first slot width; the second iris having a second slot width; the second outer waveguide branch defining the isolated port having a first width; and the output port having a second width, the second width being different than the first width. a first tapered section in the first outer waveguide branch; a second tapered section in the second outer waveguide branch; and a third input port (103, 703) at the first end of the housing in communication with the inner waveguide branch; isolated ports (105, 106) at the second end of the housing in communication with the first outer and the second outer waveguide branches respectively, two internal waveguide loads (750a, 750b) at the isolated ports, the first iris having a first slot width; the second iris having a second slot width; the second outer waveguide branch defining the isolated port having a first width; and the output port having a second width, the second width being different than the first width.