I. Introduction
For current advanced and next generation space-borne SAR missions, high gain, accurate beam shaping, and rapid beam steering are mandatory requirements for the antenna. Among several candidates including reflector antenna, passive phased array and active phased array, only the last one can meet all these requirements. However, in order to compensate an extremely high pathloss introduced by a large space-earth distance, usually in order of about 1000 km, the transmitting RF power of space SAR antenna must be on a very high level, and probably much exceeds the power generated by the solar array on board the spacecraft. Taking Cosmo-Skymed system [1] which was launched in 2007 and still operating now for example, the peak power demand for imaging (spotlight mode) is 17.3 kW (the majority goes to SAR antenna, especially the transmitting chain) while the solar array could only generate 4.5 kW (BOL) to 3.5 kW (EOL) DC power. The power gap is made up by a 336 Ah Li-ion battery which weighs up to 136 kg. With this power configuration, the max imaging duration for spotlight mode can only reach 10 seconds. Considering these facts, the reduction of power consumption would be significantly beneficial to the development and operating of space SAR systems. New materials, such as GaN [2], [3], can help making the active devices more power efficient. Nevertheless, further improvements are still highly needed.