Measuring Noise Temperatures of Phased-Array Antennas for Astronomy at CSIRO

摘要

We describe the development of a noise-temperature testing capability forphased-array antennas operating in receive mode from 0.7 GHz to 1.8 GHz.Sampled voltages from each array port were recorded digitally as thezenith-pointing array under test was presented with three scenes: (1) a largemicrowave absorber at ambient temperature, (2) the unobstructed radio sky, and(3) broadband noise transmitted from a reference antenna centred over andpointed at the array under test. The recorded voltages were processed insoftware to calculate the beam equivalent noise temperature for a maximumsignal-to-noise ratio beam steered at the zenith. We introduced thereference-antenna measurement to make noise measurements with reproducible,well-defined beams directed at the zenith and thereby at the centre of theabsorber target. We applied a detailed model of cosmic and atmosphericcontributions to the radio sky emission that we used as a noise-temperaturereference. We also present a comprehensive analysis of measurement uncertaintyincluding random and systematic effects. The key systematic effect was due touncertainty in the beamformed antenna pattern and how efficiently itilluminates the absorber load. We achieved a combined uncertainty as low as 4 Kfor a 40 K measurement of beam equivalent noise temperature. The measurementand analysis techniques described in this paper were pursued to supportnoise-performance verification of prototype phased-array feeds for theAustralian Square Kilometre Array Pathfinder telescope.