Digital Beamforming Focal Plane Arrays for Radio Astronomy and Space-Borne Passive Remote Sensing

Doktorsavhandling, 2017

Dense Phased Array Feeds (PAFs) for reflector antennas have numerous advantages over traditional cluster feeds of horns in a one-horn-per-beam configuration, especially in RF-imaging applications which require multiple simultaneously formed and closely overlapping beams. However, the accurate analysis and design of such PAF systems represents a challenging problem, both from an EM-modeling and beamforming optimization point of view. The current work addresses some of these challenges and consists of two main parts. In the first part the mutual interaction effects that exist between a PAF consisting of many densely packed antenna elements and an electrically large reflector antenna are investigated. For that purpose the iterative CBFM-PO method has been developed. This method not only allows one to tackle this problem in a time-efficient and accurate manner, but also provides physical insight into the feed-reflector coupling mechanism and allows to quantify its effect on the antenna impedance and radiation characteristics. Numerous numerical examples of large reflector antennas with various representative feeds (e.g. a single dipole feed and complex PAFs of hundreds of elements) are also presented and some of them are validated experimentally. In order to analyze electrically large feeds efficiently, a domain-decomposition approach to Krylov subspace iteration, where macro basis functions (or characteristic basis functions) on each subdomain are naturally constructed from the different segments of the generating vectors, is also proposed. The second part of the thesis is devoted to the optimization of PAF beamformers and covers two application examples: (i) microwave satellite radiometers for accurate ocean surveillance; and (ii) radio telescopes for wide field-of-view sky surveys. Based on the initial requirements for future antenna systems, which are currently being formulated for these applications, we propose various figures-of-merits and describe the corresponding optimal beamforming algorithms that have been developed. Studies into these numerical examples demonstrate how optimal beamforming strategies can help to greatly improve the antenna system characteristics (e.g. beam efficiency, side-lobe level and sensitivity in the presence of the noise) as well as to reduce the complexity of the beam calibration models and overall phased array feed design.