Abstract: A waveguide junction for joining together two pairs of rectangular waveguide arms, each pair being connected to present a corner connection, to form a directional coupler in which the four arms are arranged to define a cross, the junction being composed of an element constituting a partially radiation-permeable obstacle oriented diagonally to all of the waveguide arms and two waveguide sections each disposed between a respective side of the obstacle and a respective pair of waveguide arms and dimensioned so as to match the impedance at the surfaces of the obstacle to the values required at the corner connections in a manner to permit the coupler to operate in a frequency range which permits propagation of the desired waveguide mode while spurious modes excited in the coupler are evanescent.