The unidirectional device is an irreversible attenuator. That is, the attenuation for the forward wave is small, and the attenuation for the reverse wave is large. The unidirectional device is placed in the waveguide line, and the electromagnetic field only passes in the direction indicated by the arrow of the unidirectional device housing. It mainly plays a role of isolation, can improve the standing wave ratio, and ensure that the components and units can work stably without affecting each other. Unidirectional devices, like circulators, both use the interaction of the magnetic field of electromagnetic waves with ferrite elements and a constant magnetic field to work. A ferrite sheet is placed perpendicular to the broad side of the waveguide, and its surface is coated with a layer of resistive film. A permanent magnet is used on the outside of the waveguide to generate a constant magnetic field perpendicular to the broad side of the waveguide. If the constant magnetic field changes direction, the direction of passage is also opposite.