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A low-loss ring-type channel diplexer consisting of two deformed ring cavities, three directional couplers, a TE01 mode semicircular waveguide for the through channels, and a TE10 mode rectangular waveguide for the dropped channel, has been developed as a channel-dropping filter for a millimeter-wave channel multiplexing network. The structure and experimental results of the diplexer are described, the design method is also discussed. Measurements show that insertion losses of the through and dropped (coupled) channels, and VSWR are less than 0.15 dB, 0.68 dB, and 1.12, respectively, for a diplexer centered at 81.91 GHz with 3-dB bandwidth of 800 MHz. Specifically, the through channel loss is reduced by half, as compared with a conventional rectangular waveguide diplexer, owing to the low insertion loss characteristics of the TE01mode semicircular waveguide. As a result, an overall loss of a channel multiplexer, in which several channel diplexers are connected in tandem, is remarkably decreased, particularly at higher frequencies covering W-120 GHz.