The microwave signals are coupled from a transmission line embedded in a Printed Circuit Board PCB to a resonant cavity mounted on an external metalized surface of this PCB. The coupling mechanism implements an easy-to-fabricate mechanism leading to high-quality filtering owing to the fact that the end of the transmission line is provided with a metalized feeding pad located at the external layer of the PCB inside the resonant cavity. The resonant cavity is provided with a re-entrant inner stub orthogonal to the PCB and separated from the PCB by a capacitive gap. The metalized feeding pad is facing the inner stub in the area of the capacitive gap and is offset from the axial direction of this inner stub. The metalized feeding pad is further separated from the external metalized surface of the PCB by a surface capacitive gap.