A waveguide-coupling device (14) for a hydraulic cylinder (10) has an outer conductor element (22) to which a microwave signal can be supplied, an inner conductor element (24) for coupling a waveguide mode into a liquid-filled inner chamber (16) of the hydraulic cylinder (10) and a dielectric insulating element (22) arranged between the inner conductor element (24) and the outer conductor element (20), wherein the inner conductor element (24) and the dielectric insulating element (22) as well as the dielectric insulating element (22) and the outer conducting element (20) are each connected to each other in a liquid-tight manner.