Abstract: A diplexer having receive and transmit sections connected to a common port comprises square-cross section cavities arranged in rows and columns. At least three adjacent cavities form a triangular group in which the non-adjacent cavities are cross-coupled. The cross coupling means comprises an inductive loop or a capacitative probe. Adjacent cavities in each section are also coupled, on folded paths, by irises. Cavity wall thickness is reduced where possible. The antenna port is diplexed to both sections by a T-branch connector having wire sections which are tapped into central resonators at a predetermined distance above electrical ground. Low pass filters with non-overlapping peak responses provide further isolation between the sections.