Bandwidth limits of beamforming networks for focal-plane arrays

Current designs for radio telescopes have only one radio beam, or at most a few separated beams. To increase sensitivity, future telescopes will have both multiple overlapping beams and instantaneous bandwidths approaching an octave. This advancement may be enabled by wideband phased arrays with closely spaced elements, such as Vivaldi antennas. These arrays can be placed at the focal plane of reflector antennas and the outputs combined in a beamforming network (BFN) to synthesize simultaneously multiple beams on the sky. The spacing of beams is much closer than possible with arrays of conventional horn antennas and satisfies Nyquist sampling conditions. In a focal-plane array BFN, the weights for the individual elements are set to match the field distribution of the focal spot for a single beam at the design frequency. With a simple ideal BFN, these weights are frequency independent, making the beam pattern of the feed frequency dependent. As a consequence, at frequencies below the design frequency, the reflector is over-illuminated, increasing the spillover noise received, and conversely, at higher frequencies, the reflector is under-illuminated, resulting in a loss of efficiency. The paper examines how the sensitivity of a focal-plane array varies as a function of bandwidth and briefly outlines methods to mitigate this effect.