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This paper presents a broadband and low-loss power combiner based on a transition between eight microstrip lines and a rectangular waveguide. A tray-type structure is utilized in this design and all the trays are stacked up together along the narrow wall of the rectangular waveguide to take advantage of its uniform electric field distribution. The combiner has a wide bandwidth of 5.8-8.2 GHz, which is only limited by the operating frequency range of the WR-137 rectangular waveguide. The measured output return loss is better than 22 dB over the whole frequency range. The measured insertion loss of a back-to-back configuration is less than 0.78 dB, which is equivalent to a combining efficiency of 91.4% for a single combiner. The measured isolations between input ports are better than 12 dB. With eight microstrip input ports where three-terminal solid-state power amplifier devices are integrated and with a rectangular waveguide output port, the proposed structure is very suitable for high power-combining applications.