  We have measured the aperture-array noise temperature of the first Mk. II
phased array feed that CSIRO has built for the Australian Square Kilometre
Array Pathfinder telescope. As an aperture array, the Mk. II phased array feed
achieves a beam equivalent noise temperature less than 40 K from 0.78 GHz to
1.7 GHz and less than 50 K from 0.7 GHz to 1.8 GHz for a boresight beam
directed at the zenith. We believe these are the lowest reported noise
temperatures over these frequency ranges for ambient-temperature phased arrays.
The measured noise temperature includes receiver electronics noise, ohmic
losses in the array, and stray radiation from sidelobes illuminating the sky
and ground away from the desired field of view. This phased array feed was
designed for the Australian Square Kilometre Array Pathfinder to demonstrate
fast astronomical surveys with a wide field of view for the Square Kilometre
Array.
