Dual polarized subarray for spaceborne SAR at X-band

Following EADS CASA Espacio heritage on space borne SAR radiators (ASAR instrument at C-Band already flying on board ENVISAT platform and LARSAR developments in L-Band) the qualified technology is applied to the antenna of the SEOSAR/PAZ SAR instrument. It consists of 384 16-element subarrays working at X band (9.65 GHz with a bandwidth of 300 MHz) and performing in dual linear polarization. The elementary radiator for the sub-array is a dual linearly polarized circular patch excited by electromagnetic coupling by stripline technology. The ring aperture is excited by electromagnetic coupling through two orthogonal end stubs providing horizontal and vertical polarization. Compared to conventional slotted waveguide radiators, it offers attractive features such as low profile, lightweight, flexibility of BFN topologies (possibility of using series or parallel feeding), easiness of manufacturing (capability of maintain a series production) while providing good radiation efficiency and low losses. The beam forming network is full corporate and it is implemented in low-losses stripline technology. Tests on first subarray breadboards are presented, showing ohmic losses around 0.5 dB, good polarization purity (crosspolar better than -25 dB) and good bandwidth performances regarding radiation patterns and sidelobes.