Abstract: The anode cavity of a power magnetron is connected through a coupling slot to a reactively terminated waveguide structure containing one or more selectively magnetizable ferrite elements acting as adjustable differential phase shifters for two microwave components circulating in opposite directions through the structure or traversing a guide portion in the same direction but with opposite circular polarization. In the first instance the structure comprises two parallel rectangular-section waveguides with a common lateral wall having a proximal and a remote opening serving as directional couplers therebetween; in the second instance the structure is a circular-section waveguide with a main portion occupied by the ferrite element and transition portions separating that main portion from the coupling aperture and from the reactive termination.