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This paper presents a methodology of exciting a pure circular TEmn modes using cascaded Y-type power dividers. The dividers partition the input signal into several parts, which are then coupled to a circular waveguide through apertures. The coupling apertures induce magnetic dipoles. With proper arrangement of the magnetic dipoles on the circumference of the circular waveguide, they then jointly excite the desired mode. The coupling strength and the mode purity are calculated using the reciprocity theorem and magnetic current sources. Three mode converters, i.e., TE21, TE01, and TE41, were designed, built, and tested at W-band. Back-to-back transmission measurements exhibit excellent agreement to the results of computer simulations when the conductor loss is taken into consideration. The measured transmissions are high and the bandwidths are broad. These Y-type converters are structurally simple, but the machining errors are critical. The factors affecting the performance will be discussed in detail.