Q-band polarizers for the LSPE-Strip correlation radiometric instrument

Abstract

This paper reports the design, manufacturing and testing of the cluster of polarizers developed for the LSPE-Strip correlation receiver array working in the Q band (39–48 GHz). Since the LSPE experiment targets the measurement of the very faint B-mode component of the Cosmic Microwave Background, the electromagnetic design of the polarizers was conceived in view of minimizing the measurement uncertainties introduced by the polarizers in the LSPE-Strip dual-circular-polarization correlation receiving chain. To this end, the main figures-of-merit of the LSPE-Strip polarizers were derived in terms of the Mueller sub-matrices relating the relevant input and output Stokes parameters. As a result, a dual-ridge layout, in which stepped-ridge discontinuities are interleaved with grooved cavities, was selected. The heights of both the stepped discontinuities and the grooved cavities were considered as design degrees of freedom in order to minimize the differential phase-error between the two principal polarizations of the polarizer w.r.t. the ideal 90-deg value. The latter condition is the one required for converting the incoming circular polarizations into two linearly polarized ones. The polarizer design allows for a complete manufacturing route (mechanical layout, machining process and assembling) exhibiting high accuracy (< 10 μm) for all the units. As a consequence, the measured performance of the whole polarizer cluster is in tight agreement with predictions. Notably, the in-band mean value of the spurious conversion from the total intensity I to the two linearly polarized Stokes parameters Q and U introduced by the polarizers is smaller than -28 dB (mean value of fifty-one polarizers) with a standard deviation less than 1 dB.

LSPE-Strip on-sky calibration strategy using bright celestial sources

“…The basic parameters of the Strip instrument are listed in table 1. Detailed information on the instrumental set-up of Strip can be found in [25][26][27] and in the forthcoming papers of this series.…”

The LSPE-Strip feed horn array

“…Each array element in the focal plane [16] includes a corrugated feed horn, followed by a dual-circular polarization chain (a groove polarizer [17,18] coupled to an orthomode transducer [19] in the Q-band; and a septum polarizer in the W-band), feeding the polarimetric module based on High Electron Mobility Transistor low noise amplifiers. The signal is then amplified and digitized in the warm electronics unit on board the telescope structure.…”