A suitable alternative
to the use of corrugated horns would be the use of a smooth walled
horn which is optimised to retain the high performance of a corrugated
horn in terms of the figures of merit which are of interest for
CMB missions, at the expense of those which are less critical. For
example, a typical requirement for a CMB mission to measure
polarisation would be low cross-polar power, high beam symmetry
and low return loss in order to maximise the throughput of the
measured signal, so these performance metrics could be optimised