Analysis of some periodic structures of microwave tubes: part I: analysis of helical slow-wave structures of traveling-wave tubes

The interaction structures used in microwave tubes that continue to be important despite competitive incursion from solid-state devices can have the periodicity of both the axial and azimuthal types. The present Part I of the paper reviewed the analysis of helical slow-wave structures of traveling-wave tubes (TWTs), excluding and including the effects of axial space harmonics generated due to the axial periodicity of helix turns revealed by the sheath-helix and tape-helix models, respectively. The field and the equivalent circuit analyses yielded one and the same dispersion relation of the structure. The models for taking into account in the analysis the effect of azimuthal harmonics generated due to the azimuthal periodicity of discrete dielectric helix-support rods and metal vanes provided with the envelope of the structure were also presented. Rigor was added to the analysis by considering the effect of non-uniformity of radial propagation constant over the equivalent dielectric tube regions to model the discrete dielectric helix-support rods. The validation of the results with respect to the dispersion and interaction impedance characteristics of the structure against experimental measurements was also discussed. The non-resonant perturbation technique of measurement of these characteristics was also briefly outlined. The method to consider in the analysis the effect of the finite resistivity of the material of the helix and that of the attenuator coating of the helix-support dielectric rods was also outlined. The present analysis is capable of finding optimum structure parameters for flat dispersion characteristics of the structure without causing deterioration in its interaction impedance, that is without causing consequent reduction in the gain and efficiency of the TWT in which the structure is intended to be used. The problem of the stop-band produced due to the asymmetry in discrete helix-support rods, which can cause the band-edge oscillation in the device, was also analytically appreciated. The analysis of helical structure in the slow-wave regime, revisited in the present Part I, kept outside its purview the analysis of axially periodic disc-loaded circular waveguide of a gyro-TWT in the fast-wave regime, which will be taken up in Part II of the paper to follow.