A microwave-frequency filtering structure includes two dielectric layers separated by a conducting layer that is etched in the pattern of a filter, the upper and lower exterior faces of the stack being covered over the larger part of their surface by a conducting plane constituting ground planes of the structure, which are interlinked by a metallization of the periphery of the structure; two identical devices, an input and an output transition device, each allowing the passage from a microstrip mode to a stripline mode and vice versa, configured so that the geometry of the transition device is optimized to minimize the standing wave ratios at the ports of the filter, and to minimize the excitation and the coupling of the TE10 mode, two conducting pillars perpendicular to the plane of the structure and situated close to its principal axis, without being coupled with the filter, and linking the upper and lower ground planes.