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 which must be imposed in the design and fabrication of parallel-operating are difficult. Recent advance in the state of computer modeling offer the possibility to evaluate the effect that slow-wave structure parameter and operating parameter variations, such as manufacturing tolerances, this will enable manufacturers to determine the cost effectiveness of controlling the dimensions of the component parts of the TWT. The computer code MAFIA is used to determine the effect on dispersion and on-axis interaction impedance of several helical slow-wave circuit parameter variations. In this paper, computer code Phase-Coincidence TWT CAD