Stopbands produced by asymmetrical support rod system in helix structures

Analysis of π-mode Stopband in an Asymmetric Millimeter-Wave Helical Slow-Wave Structure

“…Here, d is the radial gap between the helix and the metal envelope, w is the width of the helix tape and k is an effective dielectric loading parameter with its value assigned as 0.5 by Lien [4].…”

“…The analysis is simple, includes the effects of circuit loss, and applicable for broadband high frequency slow-wave structures as well. The analysis has been benchmarked against published results of Lien [4] and Onodera [5] and subsequently furthered for inferring some useful insights into the stopband phenomenon of a typical millimeter-wave helical slow-wave structure.…”

“…Both the approaches of Hinson [3] and Lien [4] consider the circuit as dispersion free and calculate the relative frequency of the stop-band with respect to the π-point frequency, and not the absolute value; the former ignores the effects of circuit loss whereas the latter considers it. A dispersion-free model may be suitable for narrow-band structures at low frequencies but would not predict required accuracy for broadband and/or high frequency structures in which the dielectric loading plays a significant role in controlling the dispersion behavior.…”

“…There have been three analytical methods in vogue for evaluation of the stopband characteristics in a helical slow-wave structure due to support rod asymmetry, from Hinson [3], Lien [4] and Onodera [5] that are computation-intensive and do not provide any direct parametric relationship one may expect in a closed form relation. Both the approaches of Hinson [3] and Lien [4] consider the circuit as dispersion free and calculate the relative frequency of the stop-band with respect to the π-point frequency, and not the absolute value; the former ignores the effects of circuit loss whereas the latter considers it.…”

“…2), play the principal role in the beam-wave interaction [1,2]. The degeneracy of these modes at the crossover frequency (w p ) leading to mode coupling and generation of a stopband due to azimuthal asymmetry of the support rods [3][4][5] has always been a concern for the TWT community, as it limits the capability of the device by causing band-edge oscillation and in-band power hole [2,3]. At the stopband region, the RF interaction efficiency becomes extremely high leading to bandedge oscillation as the energy velocity of the propagating mode approaches zero, and as a consequence, in-band power hole occurs at the sub-harmonic frequency of the band-edge oscillation due to power diversion to harmonic frequency.…”