We thus designed a sever with a linearly tapered conductivity to reduce the amplitude of the reflected backward wave interaction. Figure 5 (a) illustrates the 3D model (top) of the designed sever comprised of a 2 mm long tapered conductance area at the center of the interaction circuit. According to S-parameter simulation, the insertion loss through the sever steeply falls off with decrease of the sever conductivity, while the return loss
remains to be ~ - 10 dB, which thus suppresses oscillation of higher order mode backward waves.