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The paper presents a fullwave analysis of the coupling characteristics of various large apertures between two parallel or crossed rectangular waveguides. Based on the formally exact mode-matching method, the generalised scattering matrix of a three-port rectangular waveguide T-junction with an arbitrary waveguide as the side arm is initially derived. Then, two T-junctions are cascaded to form various single-aperture coupling structures and, in turn, various multiaperture couplers. By employing only one key-building block, the analysis procedure is extremely simple and computationally efficient. The method presented is versatile and can handle both broad-wall and narrow-wall, parallel and crossed, identical and dissimilar waveguide coupling structures. Three directional couplers are designed using rectangular, cross and circular apertures, respectively. Good agreement between measurements and mode-matching results is observed for all cases considered.