which is the ratio of the powers delivered to the coupled port and the isolated port. The ideal coupler is characterized
solely by the coupling factor, as the isolation and directivity are infinite. The ideal coupler is also lossless and matched at all ports. Directional couplers can be made in many different forms. We will first discuss waveguide couplers, followed by hybrid junctions. A hybrid junction is a special case of a directional coupler, where the coupling factor is 3 dB (equal split), and the phase relation between the output ports is either 90◦ (quadrature hybrid), or 180◦ (magic-T or rat-race hybrid). Then we will discuss the implementation of directional couplers in coupled transmission line form.