On the Bandwidth Gap Between the Array-Feed and Cluster-Feed Regimes for Broadband Multifeed Systems

For reflector antennas with broadband multiple feed systems, under optimal beamforming conditions, the feed operates at low frequencies as a phased array feed (PAF), with multiple elements contributing to a synthesized illumination pattern, whereas at high frequencies, each element forms a single independent beam. There is a frequency range between these two regimes where it is difficult to achieve high aperture efficiency, which limits the ultimate performance of broadband multifeed systems. We show that this bandwidth gap is largely independent of the formed beam angle, number of array elements, array layout, and element radiation patterns. We use simulation and modeling to study the origin and nature of this physical effect, develop an upper bound on the sensitivity bandwidth of a high-efficiency array feed, and consider several possible methods for improving the efficiency of multifeed systems in the bandwidth gap.