A directional coupler is a four-port microwave junction with the properties described below. With reference to Fig.1, which is schematic illustration or a directional coupler, the ideal directional coupler has the property that a wave incident in part 1 couples power into ports 2 and 3 but not into part 4. Similarly, power incident in part 4 couples into ports 2 and 3 but not into part 1. Thus ports 1 and 4 are uncoupled. For wave incident in port 2 or 3, the power also uncoupled. In addition, all four ports are matched. That is, if three ports are terminated in matched loads, the fourth port appears terminated –coupling and directivity. Let be the incident power in port 1 and left
in a matched load, and an incident wave in this port suffers no reflection. Directional couplers are widely used in impedance bridges for microwave measurements and for power monitoring. Since these devices are required to operate over a band of frequencies, it is not possible to obtain ideal performance over the whole frequency band. The performance of a directional coupler is measured by two pameters 1f P be the pled power in the forward direction in port 3. The coupling in decibels is then given by