A directional coupler is a general-purpose microwave/millimeter-wave component, an RF passive device commonly used in RF circuit design as a power distribution element with directionality.  It is a four-port element that is usually formed by combining two sections of transmission line called the straight-through line (primary line) and the coupled line (secondary line). Straight through the line and the coupling line through a certain coupling mechanism (such as gaps, holes, coupling line segments, etc.) to the straight through the line power part (or all) coupled to the coupling line, and requires the power in the coupling line is only transmitted to a certain output port, the other port is no power output. If the direction of wave propagation in the straight-through line becomes the opposite of the original direction, the coupled line of power output port and no power output port will also change, that is, the power of the coupling is directional, and is therefore called directional coupler.
Directional coupler's basic feature is that it will only be coupled to the specified direction of the signal, can also be transmitted in the line of RF power coupled to another line. The signal amplitudes at its two output ports can be equal or unequal.