Abstract: Designing an octave bandwidth helical traveling wave tube for phase and gain matched operation in the EHF region presents many obstacles, the dimensional controls must be imposed in the design. In this paper, a code is programmed based on the sheath model. In the sheath model the helix is approximated by a helically conducting cylindrical surface of negligible thickness. The code is used to determine the effect on dispersion and onaxis interaction impedance of several helical slowwave circuit parameter variations, including thickness and relative dielectric constant of the support rods, pitch width, height of the metal vane and helix average radius. The specific results can serve as a general guide for similar devices, and the computational techniques are readily applicable to other TW