A junction between transmission lines, either waveguides or coaxial lines. An ideal junction has no direct coupling between arms 1 and 4 or between arms 2 and 3 (see diagrams). If arm 1 is excited power flows in arm 4 as a result of reflections in arms 2 and 3.

The two main types of hybrid junction are the ring junction, in which all 4 arms are coplanar (Fig. a), and the T-junction, in which arm 4 is perpendicular to the plane containing the other three arms (Fig. b). If the decoupled arms (1 and 2) are independently matched in impedance and the other two arms (3 and 4) are terminated in their characteristic impedances, Z₀, then all four arms are matched at their inputs.

(a) Waveguide ring junction (E-bend)

(b) Waveguide T-junction