A simplified and improved approach for the one-dimensional nonlinear multisignal analysis of a complete helix TWT has been developed. The model takes into account both the loss and circuit variation along a tube and the inherent backward wave and reflections due to mismatch at the input and output couplers. The model is generalised to consider any number of simultaneous signals, including generation of higher-order harmonics and intermodulation components. It is fast, for interactive use, and takes less than two minutes on a Micro-Vax computer to determine the detailed characteristics of a typical tube for four-signal operation with frequencies separated by nearly 10%. The energy balance factor is satisfied within ±1%, even at saturation. The accuracy of the model was checked by comparing computed results with published and experimental results for different types of helix TWT.