The effectiveness of positive phase velocity tapering to improve the performance of a broadband helix traveling-wave tube (TWT) has been investigated. A large-signal model of an existing 4.5-18 GHz two-section mini-TWT with uniform pitch has been validated against the experimental results, and used as a starting point to design and develop a number of TWTs with different taper lengths. Comparisons between the experimental results of uniform pitch TWT and positively tapered TWTs show significant improvements in output power, electronic efficiency, and second harmonic content throughout the operating frequency band for the positively tapered TWTs. Increases in electronic efficiency and output power up to 5.2% points and 50 W (1.7 dB), respectively, have been achieved.
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