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One of the most popular microwave bands now is the W-band. It is attractive by the atmospheric transparency window, which makes it promising for a number of scientific and technical applications including radiolocation, communication, detection of hidden objects and the creation of medical devices. The output power level in both continuous and pulsed regimes for the aforementioned applications should be in the order of several kilowatts and above. This makes the gyrotron one of the most high-potential sources of electromagnetic radiation in this range.The alternate design of an electrodynamic system based on the use of a diaphragm in a wave transition is proposed. This system has been theoretically and numerically analyzed for a W-band gyrotron at the third harmonic of the cyclotron frequency. The optimal parameters of the diaphragm and its position have been selected. With that parameters, the highest value of the starting current of the competing mode is reached while maintaining the starting current of the operating mode. It allows to increase the output radiation power by more than 7 times. An estimate of the accuracy of the manufacturing of the diaphragm has been made. It holds out a hope of scaling this selection method into higher frequency ranges. Further development of this system is primarily tied with a more complex optimization of the system particularly the variation of the length of the cylindrical part of the cavity, which will allow reducing the share of ohmic losses while maintaining high efficiency.The paper is based on the research results, which was conducted in the framework of projects of the Russian Science Foundation No. 17-79-10422. Keywords: Gyrotron, W-band, Mode selection