The computer simulation of helix travelling-wave tubes requires accurate information about the phase velocity, interaction impedance and attenuation of the helix slow wave structure at each frequency. This has received considerable attention, from the pioneering work of Pierce onwards, and it is possible to compute or measure the phase velocity with sufficient accuracy. The methods available for determining the interaction impedance, however, cannot yet give good enough results and there are appreciable differences between the figures given by different methods. The purpose of this paper is to review the assumptions and approximations which are made in each case with a view to finding ways of achieving agreement between them.
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