Design and Realization of a Dual-Polarized Offset Reflector Antenna with Digital Feed Array for Synthetic Aperture Radar

Abstract

The German Aerospace Center (DLR) is currently developing a ground-based Synthetic Aperture Radar (SAR) with digital beamforming (DBF) capabilities for scientific use. Primary focus of the project is on the reflector antenna and on the implementation of onboard DBF pre-processing concepts & data reduction in real-time. The system design is based on proposals for spaceborne SAR missions in L band, like Tandem L, NISAR, Rose L, or Sentinel1 NG. In order to reduce the size and mass of the demonstrator, the system is scaled to X band at a center frequency of 9.6 GHz. Due to the short time delay between transmission and reception of a radar pulse in a ground-based scenario, it is not possible to use the same frontend for transmit and receive, e.g. T/R-module fed reflector antenna. Thus, transmitter and receiver are separate in a monostatic configuration, while the reflector antenna is used for receive only. The receive antenna consists of four main parts: the reflector itself, the boom as connection between feed and reflector, the feed antenna array with 4x32 dual-polarized elements, and the 2x32 azimuth power combiners, which implement a fixed analog beamforming of the four azimuth channels. Finally, DBF is applied in elevation over the 32 dual-polarized channels. The whole antenna was designed and optimized via a hybrid simulation setup consisting of IE and FEM solver. The elliptically shaped offset reflector antenna has a diameter of approx. 1.7 m and a focal length of 1.8 m. It allows scanning in elevation via DBF within a range of 17.6°. While in a spaceborne scenario a deployable reflector will be used, in this ground-based hardware a solid type manufactured out of carbon fiber with conductive painting will be used. The reflector as well as the fiberglass boom are currently in production by the industry. A significant new contribution is the feed antenna array consisting of patch exited cup antennas (pending patent). This special antenna type consists of a resonator (cup) with two round plates within the cup. Two ring hybrids are arranged in two individual layers with half-wavelength separation (one for each polarization) at the backside of the antenna in order to feed both polarizations of the antenna symmetrically. Besides an element size of only 0.6*lambda a quite high isolation to neighboring elements and a cross-polar suppression within the mainlobe of better than 33 dB could be achieved. First measurements of the prototypes matched well to the simulations. The modular pluggable power combiners are based on microstrip technology. They combine each four azimuth channels with cascaded Wilkinson power dividers for an optimized sidelobe ratio of the reflector’s far field.