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Simple closed-form formulas for the estimation of the π-mode stopband and the stopband attenuation in an azimuthally asymmetric helical slow-wave structure (SWS) are developed, following the coupled-mode analysis of multiple reflections. The formulas are simple and amenable to easy computation, and also allow the use of the dispersion characteristics of the structure obtainable from any standard electromagnetic modeling, thereby accruing the accuracy of 3-D electromagnetic analysis. The analysis is benchmarked against published results with close agreement. Based on the analysis, SWS configurations that would reduce the deleterious effects of asymmetry and also would improve TWT efficiency are suggested.