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High power, broad bandwidth, high linearity and low noise are among the most important features in amplifier design. Broadband spatial power combining techniques address all these issues by combining the output power of a large quantity of microwave monolithic integrated circuit (MMIC) amplifiers in a broadband coaxial waveguide environment, while maintaining good linearity and improving phase noise of the MMIC amplifiers. A coaxial waveguide was used as the host of the combining circuits for broader bandwidth and better uniformity by equally distributing the input power to each element. A new compact coaxial combiner with much smaller size is investigated. Broadband slotline to microstrip line transitions are integrated for better compatibility with commercial MMIC amplifiers. Thermal simulations are performed and a new thermal management scheme is employed to improve the heat sinking in high power applications. A high power amplifier, using the compact combiner design, is built and demonstrated to have a bandwidth from 6 to 17 GHz with 44 W maximum output power. Linearity measurement has shown a high IP3 of 52 dBm.