Directional couplers are an important category for passive microwave circuits. They are
implemented in a variety of ways resulting in a range of capabilities and constraints. Directional couplers
are used to sample propagating microwave energy for the purpose of monitoring or measuring. Also
directional couplers are used to divide signals from a single channel into multiple channels in both small
signal and large signal applications.
The most general definition for a directional coupler describes it as a multi-port (usually 4 port)
matched, lossless, reciprocal circuit that has an isolated port which depends upon the incident signal port.
Four port directional couplers are usually represented schematically by two single line representations of
transmission lines with a crossed line between them to represent the coupling between the lines, as shown in
Figure 1. We will see that this coupling between lines can occur by providing an electrical connection
between the lines (branch line or Wilkinson's coupler) or by bringing two lines close together so that their
magnetic and electric fields interact (parallel line or Lange couplers).