Measuring the CMB spectral distortions with COSMO: the multi-mode antenna system

• Published in: Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XII
• Main Authors: Manzan, Elenia, Albano, Lorenzo, Franceschet, Cristian, Battistelli, Elia S., De Bernardis, Paolo, Bersanelli, Marco, Cacciotti, Federico, Capponi, Alessandro, Columbro, Fabio, Conenna, Giulia, Coppi, Gabriele, Coppolecchia, Alessandro, D'Alessandro, Giuseppe, De Gasperis, Giancarlo, De Petris, Marco, Gervasi, Massimo, Isopi, Giovanni, Lamagna, Luca, Limonta, Andrea, Marchitelli, Elisabetta, Masi, Silvia, Mennella, Aniello, Montonati, Filippo, Nati, Federico, Occhiuzzi, Andrea, Paiella, Alessandro, Pettinari, Giorgio, Piacentini, Francesco, Piccirillo, Lucio, Pisano, Giampaolo, Tucker, Carole, Zannoni, Mario
• Format: Conference Object
• Language: English
• Published: 2024
• Subjects: Cosmic Microwave Background; spectral distortions; multi-mode propagation; feed-horn antenna; Antarc*; Antarctica
• Online Access: https://hdl.handle.net/11573/1721044; https://doi.org/10.1117/12.3018730

In this work, we present the design and manufacturing of the two multi-mode antenna arrays of the COSMO experiment and the preliminary beam pattern measurements of their fundamental mode compared with simulations. COSMO is a cryogenic Martin-Puplett Fourier Transform Spectrometer that aims at measuring the isotropic y-type spectral distortion of the Cosmic Microwave Background from Antarctica, by performing differential measurements between the sky and an internal, cryogenic reference blackbody. To reduce the atmospheric contribution, a spinning wedge mirror performs fast sky-dips at varying elevations while fast, low-noise Kinetic Inductance detectors scan the interferogram. Two arrays of antennas couple the radiation to the detectors. Each array consists of nine smooth-walled multi-mode feed-horns, operating in the 120−180 GHz and 210−300 GHz range, respectively. The multi-mode propagation helps increase the instrumental sensitivity without employing large focal planes with hundreds of detectors. The two arrays have a step-linear and a linear profile, respectively, and are obtained by superimposing aluminum plates made with CNC milling. The simulated multi-mode beam pattern has a { 20° − 26° FWHM for the low-frequency array and {16° FWHM for the high-frequency one. The side lobes are below −15 dB. To characterize the antenna response, we measured the beam pattern of the fundamental mode using a Vector Network Analyzer, in far-field conditions inside an anechoic chamber at room temperature. We completed the measurements of the low-frequency array and found a good agreement with the simulations. We also identified a few non-idealities that we attribute to the measuring setup and will further investigate. A comprehensive multi-mode measurement will be feasible at cryogenic temperature once the full receiver is integrated.