Linear analysis of folded waveguide tube in terahertz band

Perturbed dispersion relation considering the ohmic losses of the waveguide wall in folded waveguide tube is derived and solved by using Matlab. Small signal gain increases with the increasing of ratio of the center frequency to the cutoff frequency, but the increase will be slower with further increasing of the ratio and the band width becomes narrower, as calculation results show. With the beam current density fixed, there exist optimal values of the width of the beam tunnel and the width of the cross section for the maximum gain. A quiescent waveguide mode propagating backward appears in the waveguide while taking the ohmic losses into consideration. With the increasing of the operating frequency, the gain decreases obviously while the intensity of the quiescent waveguide mode is enhanced. Therefore, for a folded waveguide tube operating at higher frequency in terahertz band, the period number of the folded waveguide increases correspondingly.