  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.
