This paper reports numerical and experimental analyses of a stepwise modulated TEon mode converter based on the mode‐matching formulation. By systematic analyses, the dimensions of the TE02 to TE01 converter at 41 GHz are optimized so that a power conversion efficiency higher than 99.3% is theoretically obtained with a reflection coefficient less than 0.2%. The simple structure of the converter not only allows easy manufacturing with ordinary machining but it also enables the converter characteristics to behave well and be readily adjusted on the laboratory site. Based on the numerical analyses, a mode converter is designed and fabricated for use in the ECRH experiment on the GAMMA 10 tandem mirror. Preliminary tests show that the converter works well at a high power, above 100 kW. The transmittable power is limited by external nonaxisymmetric modes which can eventually be suppressed with efficient mode filters.