Electromagnetic analysis of asymmetry of support rods of a helix slow-wave structure that causes the stop-band in the structure dispersion characteristics is reviewed. The asymmetry caused the lowering of the RF phase velocity andcutoff frequency and the increasing of the stop-bandwidth. The results obtained by the electromagnetic analysis agreed to those predicted by the simulation code HFSS (high frequency structure simulation) within 1.5%. The present study helps in the design of a of a wideband helix-traveling-wave tube.