Conceptual example of a periodic SWS (modeled as a periodic MTL made of two TLs) of finite length interacting with an electron beam. A sequence of N cascaded unit cells forms a Fabry-Pérot cavity, where each unit cell composed of five segments of 2-TLs coupled among each other at the interfaces between adjacent segments through rotation matrices X . Resonance occurs when a mode travels a whole round trip with a total phase accumulation that is multiple of 2π. This structure can start oscillations when a system pole of the interacting SWS-beam system move to the instability region when increasing the electron beam current I 0 [30].