This paper deals with the design of X-band T/R modules to be employed on a future generation Synthetic Aperture Radar (SAR) for space application. Since the final High Power Amplifier (HPA) stage of the transmitting chain directly affects the overall module performances (mainly efficiency and maximum output power), HPA topic has been deeply investigated. In particular, an HBT process has been extensively explored, because of its attractive features; however potential advantages can be really achieved only if thermal management is consistently addressed. In SAR applications the RF carrier phase control is of great concern and major causes of module output phase shifts have been identified: temperature variations of active devices, non-linear modulating phenomena (AM/PM conversion) and supply voltage dropping off. The designed HPA will successfully handle all of them without disregard efficiency. Expected improvements will be shown for the future generation T/R module employing the designed PA and future architecture solutions.