A large signal model is used to analyze the stability of a high gain helix TWT against the reflected signals caused by mismatches at its input and output terminations. The stability of a section of the tube in the presence of the reflected signals is determined by making a number of passes through the section for a very low drive power. A section is considered stable if the output power converges after 4 passes. If the output power goes on increasing or does not converge then the section is defined as unstable. Results are presented for a 60 W helix TWT which is designed both in a two-section and in a three-section configuration for a small-signal gain of more than 60 dB. It is shown that the two-section TWT can be made stable with 10 dB return loss at the input and output terminations, only if the tip loss profile at the sever can be designed to have a return loss of more than 30 dB. For the three-section tube, the return loss at the sever does not need to be greater than 20 dB for stable output performance.
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