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A new TE10-to-TE20-mode transformer has been developed on the basis of a fin-line array for waveguide spatial power-combiner applications. The fin-line array is designed such that the input TE10 mode is split in space into multiple segments, then subject to different signal paths, which effectively converts the TE10 mode into a TE20 mode, while eliminating the TE10 mode at the output. Our Ku-band (14-15 GHz) design indicates that the proposed mode converter makes it possible to obtain the TE20 mode with greater than 22-dB suppression of the TE10 mode over the band of interest. This converter was used in the design of a waveguide spatial power amplifier involving four monolithic microwave integrated circuit power chips and an output power of 31 dBm has been obtained with a combining efficiency of 80%. A measurement technique has been also developed to measure the electric-field profile inside the rectangular waveguide. A good agreement between measured and simulated results has been observed, showing an effective suppression of the TE10 mode, as well as a good TE20-mode formation judging from its amplitude and phase.