The concept of a microwave transmission line suspended in air, and a methodology for electromagnetic and thermal optimisation of these structures in X-band (8-12 GHz) are reported. A reliable printed circuit process allows us to obtain compact, low loss and reproducible stripline structures. The measured stripline circuits demonstrate losses from only 4.2 to 5.2 dB/m at 9 GHz. The use of periodic ceramic contacts increases the power capability up to 50 W. The final structure is low loss and compact with high power handling in order to be used in power combination in the X-band.Introduction: The trend for the modern solid state power amplifier (SSPA) application is to generate higher power, more compact systems and a competitive cost. To reach such goals, a specific investigation on power splitters and combiners is essential [1,2]. Indeed, these devices are crucial for the improvement of transmission/reception performances of antennas. Consequently, reduction of their losses at strategic locations such as the upstream and the downstream front-end is critical. Power distribution and combination solutions of the full dielectric stripline present high losses, typically from 10 to 26 dB/m at 9.5 GHz [3], losses which are essentially due to the dielectric material. To increase the electrical performances, an air gap is generally introduced in the stripline structure. However, this solution increases the structural dimensions (effective permittivity decreases), which becomes a constraint for compact SSPA systems, and reduces the power capability, as the thermal transfer is then less efficient. Our aim is to use electromagnetic and thermal optimisation methods to reach the best trade-off between losses and compactness, with high power capacity to enable the next generation of stripline power combiners.