A waveguide junction with four ports (see diagram). It consists of a primary waveguide 1–2 and a secondary waveguide 3–4. If all ports are terminated in their characteristic impedances (see transmission line), almost all the power entering port 1 will be transmitted to port 2. There is no power transmitted between ports 1 and 3 or between ports 2 and 4 because there is no coupling between these pairs of ports. However, there may be coupling between ports 2 and 3 and between ports 1 and 4 depending on the structure of the particular coupler.

Usually, directional couplers are designed so that the coupling between ports 1 and 4 and between ports 2 and 3 is as small as possible. In this case the device is often used to measure the power transmitted along a waveguide by connecting ports 1 and 2 in series in the waveguide. In this case a small portion of the power is received at port 4 and, using a calibration curve, the power flowing in the main waveguide can be calculated. The coupling between ports 1 and 4 (and 2 and 3) is required to be small so as not to affect the main waveguide, although typical good values are only 30 to 35 decibels below the coupling between ports 1 and 2.

Directional coupler