Helix travelling-wave tubes (TWTs) are extensively employed as final power amplifiers in satellites. These applications demand low spectral noise and high efficiency across a broad band. Research has been carried out to achieve these criteria in a helix TWT design. A deeper understanding has been gained on how the basic parameters of a TWT affect its non-linear performance. By selecting and controlling the parameters that are critical to the amplifier’s nonlinear performance the designs corresponding to the important and desired conditions have been identified. The main simulation tool for modelling the interaction processes in a generic helix TWT was a large-signal model (LSM). A helix slow-wave structure is normally tapered to maintain its phase relationship with the electron beam and to maximise its output RF power. By determining the sensitivity of the helix dimensions on the nonlinear performance at different regions along the tube, a non-uniform slow-wave structure design has been developed for a more linear performance. Since the conditions of high linearity and efficiency could not be achieved simultaneously, the best trade-off was attained. The performance across the frequency band of 10.7 to 12.75GHz was computed for the uniform and tapered helix designs. With the use of a simulated multi-stage collector with optimised electrodes, the overall TWT performance was determined. Further understanding has been gained on the fundamental processes in the tube that cause the generation of nonlinear transfer curves and spectral distortion. The modelling of RF beam current and helix voltage waveforms and their characteristics provided a unique insight. In addition, the formation and deceleration of the electron beam bunches have been shown for the various important conditions; revealing the desirable physical conditions within the beam.