A UHF wide-band SAR antenna

This UHF wideband antenna was developed for DARPA´s synthetic aperture radar (SAR) applications. The small array could be mounted at the belly of an aircraft. The operating frequency covers a 4:1 bandwidth. The antenna including the feed and a contoured ground plane was designed to fit in a shallow radome (50 cm×100 cm×100 cm). The depth of the radome is only one quarter of a wavelength at the low end of the UHF band. The small volume imposes a severe constraint in the electrical design and the packaging of the antenna, which is required to produce an elevation (EL) beam pointed at 30 degrees depression angle from the horizon (60 degrees from the nadir). The 3 dB beam at mid-band is about 50 degrees in both EL and azimuth (AZ) planes. Since the beam is broad enough to cover a region from 10 to 60 degrees depression angle, no steering in the EL plane is required. Furthermore, the antenna must provide dual linear polarizations over a 4:1 bandwidth with VSWR less than 2:1 over 90% of the band. With the beam peak steered to 60 degrees from the broadside, it is equivalent to imposing a wide scan in the EL plane. To suppress grating lobes at the high end of the band, the element spacing was kept less than 25 cm. This spacing is about one-eighth of a wavelength at the low end of the band, which makes it difficult to match the input impedance of the antenna because the element is electrically small