This paper describes a novel broad band Spatial Power Combiner (SPC) Ampli- fier based on Square Coaxial (Squarax) Transmission Line (TL), able to tolerate the Thermo- Mechanical effects due to the transistor heating. In the proposed SPC, Fin Line to microstrip transitions (FLuS) are inserted into the Squarax TL, in order to connect many Monolithic Mi- crowave Integrated Circuit (MMIC) Solid State Power Amplifier (SSPA) while maintaining an easy integration. The proposed structure has some advantages over the traditional SPC’s. The Squarax SPC geometry allows the feeding of a higher number of MMIC than in a Waveguide SPC, and its straight profile allows to connect simple and low-cost square or rectangular heat-sink devices with high thermal dissipation attitude. Conversely, coaxial SPC needs apposite circular heat-sinkers that requires expensive custom production. Squarax structure ensures high power outputs and small sizes, together with theoretical DC frequency cut-off. In this work, a 16 cards Squarax SPC in the operative bandwidth 4–18 GHz has been designed, able to account for 32 MMIC SSPA’s. The design Electromagnetic and Thermo-structural simulation of the proposed device are shown.