Dielectrically Loaded Quad-ridge Flared Horns for Ultra Wideband Reflector Feed Applications in Radio Astronomy

Doctoral thesis, 2022

Reflector-based radio telescopes are used as tools for observations in both radio astronomy and space geodesy. To observe the weak sources in space, highly sensitive receivers, fronted by optimized reflector feeds, are therefore needed. Wideband and ultra-wideband (UWB) systems enable large continuous frequency bandwidth and reduce the number of receivers that are needed to cover the radio spectrum. Therefore, they are attractive for existing and next generation of reflector arrays such as the Square Kilometre Array (SKA), Allen Telescope Array (ATA), Deep Synoptic Array (DSA), and the Next Generation Very Large Array (ngVLA). To achieve sensitive wideband and UWB performance with reflector feeds, a near-constant beamwidth and good impedance match are required over large frequency bands. The quad-ridge flared horn (QRFH) is a robust and compact UWB feed technology for this purpose, and is easily designed with single-ended excitation for 50-Ohm ports. The QRFH is dual-linear polarized and can typically achieve good performance up to 6:1 bandwidth with high band-average aperture efficiency and good impedance match. A drawback in existing state-of-the-art QRFH designs, is that they suffer from gradually narrowing beamwidth and increasing cross-polarization in the upper part of the frequency band. This is especially challenging for QRFHs that are designed to illuminate deep reflector geometries. The narrowing beamwidth leads to reduced aperture efficiency, and therefore also reduced sensitivity. To meet the demand for high sensitivity observations over large bandwidths, these challenges need to be addressed.
This thesis introduces and investigates low-loss, dielectric loading of the QRFH design to achieve ultra-wideband performance that reaches beyond decade bandwidth exemplified with 20:1 bandwidth in one single QRFH. The dielectric load is homogeneous, with a small and non-intrusive footprint and improves the beamwidth performance over the frequency band, while keeping the complexity low and the QRFH footprint compact. Keeping the QRFH robustness and compact footprint is favorable for practical receiver installation in real-world applications for radio observations. Three quad-ridge designs with dielectric loading are investigated, both for room temperature and cryogenic applications, and are shown to be highly suitable for wideband operation in existing and future reflector arrays.