Directional couplers are instruments generally used for separating incident and reflected signals so that power meters can measure each of them separately.
In Figure 3.27, the longitudinal section of a directional coupler for waveguides is sketched.
It is made up by two waveguides properly coupled through two holes.
The upper guide is the primary waveguide and connects the power source and load; the lower guide is the secondary waveguide and is connected to the power meter.
To explain the working of directional couplers, incident and reflected waves have been sketched separately in Figure 3.27(a) and 3.27(b).