Resonant loss for helix traveling wave tubes

This paper describes a scheme for suppressing backward wave oscillations (BWO) in high-power, PPM focused helix traveling wave tubes. The scheme consists of lossy, resonant meander lines deposited on each of the helix support rods and located between each helix turn By varying the length of the line, it can be made to resonate at the helix circuit\´s π-mode frequency. In this way the "resonator" provides loss in the vicinity of the π-mode, thereby inhibiting backward wave interaction. Test results have shown at least a four-fold increase in the BWO start current for a tube employing this resonant loss scheme.