Development of 400 W CW Ka-Band Communications Helix-TWT and 1 kW Peak Power Ka-Band Radar Helix-TWT

This paper summarizes the performance of L-3 ETI´s latest two high power Ka-band helix-TWTs under development for communications and radar applications. The 8926H, a Ka-band communications helix-TWT, currently demonstrates 350 W to 380 W CW output power and 50% minimum overall efficiency with a three-stage collector operating over 27.5 GHz to 30 GHz at 14.5 kV cathode voltage and 108 mA cathode current. 400 W of saturated output power was obtained at the low end of the commercial communications band when the unit was operated at 14.7 kV cathode voltage and 111 mA cathode current. 400 W operation was limited to below 29.5 GHz due to high interception current at the upper end of the band. The device was thermally stressed up to 100 degC baseplate temperature and demonstrated stable operation without circuit failures. The 8926H was derived from the 275 W production Ka-band communications helix-TWT, 8921HP-2, and utilizes the improved thermal/mechanical designs used in the previously reported model 8925H Q-band amplifier (Chong, 2005). The frequency response of the 8926H EMI was shown. With a cathode voltage of 14.5 kV, the output power ranges from 350 W to 380 W across the commercial communications band. With the higher voltage of 14.7 kV, the output power increases to approximately 400 W. The 8909H is a 1 kW peak power gridded helix-TWT designed to operate in the 33.4 to 36.0 GHz radar band. With 18.8 kV of cathode voltage and 350 mA of cathode current, the 8909H generates 1 kW peak power across the full band, and has demonstrated up to 10 % duty at 2 kHz repetition rate. The 8909H´s high operating voltage, required to satisfy the peak power requirement, presents a unique challenge for suppressing backward-wave oscillation (BWO). The circuit design of the 8909H employs L-3 ETI´s proprietary BWO suppression technology to ensure stability, and the 8909H EM3 has shown full stability from backward-wave oscillation and drive-induced oscillation at all drive conditions i- - n test. The 8909H EM3´s performance is shown. It produces 1 kW peak output power across the radar band. The helix interception current at DC is 21 mA and at saturation is 31 mA, which correspond to 6% and 9% of the beam current, respectively. The development programs for both the 8926H and the 8909H have continue into the next year under internal development funding, with a primary objective of improving focusing