A Circularly Symmetric Antenna Design With High Polarization Purity and Low Spillover

TL;DR

This paper presents the design and testing of two circularly symmetric antennas with high polarization purity and low spillover, optimized for all-sky polarization surveys at 5 GHz, achieving excellent beam and polarization performance.

Contribution

The paper introduces a novel antenna design with symmetric structure and optimized optics that significantly improve polarization purity and reduce spillover for radio surveys.

Findings

Achieved cross-polarization better than -50 dB.

Beam performance of 0.73 degrees FWHM.

Far-out sidelobes below -70 dB.

Abstract

We describe the development of two circularly symmetric antennas with high polarization purity and low spillover. Both were designed to be used in an all-sky polarization and intensity survey at 5 GHz (the C-Band All-Sky Survey, C-BASS). The survey requirements call for very low levels of cross-polar leakage and far-out sidelobes. Two different existing antennas, with 6.1-m and 7.6-m diameter primaries, were adapted by replacing the feed and secondary optics, resulting in identical beam performances of 0.73deg FWHM, cross-polarization better than -50 dB, and far-out sidelobes below -70 dB. The polarization purity was realized by using a symmetric low-loss dielectric foam support structure for the secondary mirror, avoiding the need for secondary support struts. Ground spill-over was largely reduced by using absorbing baffles around the primary and secondary mirrors, and by the use of a low-sidelobe profiled corrugated feedhorn. The 6.1-m antenna and receiver have been completed and test results show that the optics meet their design goals.