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, L., Franceschet, C., Battistelli, E. S., de Bernardis, P., Bersanelli, M., Cacciotti, F., Capponi, A., Columbro, F., Conenna, G., Coppi, G., Coppolecchia, A., D'Alessandro, G., De Gasperis, G., De Petris, M., Gervasi, M., Isopi, G., Lamagna, L., Limonta, A., Marchitelli, E., Masi, S., Mennella, A., Montonati, F., Nati, F., Occhiuzzi, A., Paiella, A., Pettinari, G., Piacentini, F., Piccirillo, L., Pisano, G., Tucker, C., Zannoni, M.
• Other Authors: Zmuidzinas, Jonas, Gao, Jian-Rong
• Format: Article in Journal/Newspaper
• Language: English
• Published: SPIE 2024
• Subjects: Cosmic Microwave Background; feed-horn antenna; multi-mode propagation; spectral distortions; Antarc*; Antarctica
• Online Access: https://research.manchester.ac.uk/en/publications/dce3f59d-3eb8-408a-8c47-c32bea934cd0; https://doi.org/10.1117/12.3018730; http://www.scopus.com/inward/record.url?scp=85204736890&partnerID=8YFLogxK

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.