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

Abstract

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

Measuring radiotelescope phased array feed noise and sensitivity

“…A key factor in our PAF testing is generating the frequency dependent weights to cover the ASKAP bandwidth (2.6:1). We achieved this initially by storing the raw voltages from each port [15] and later by storing the covariance matrix of the port signals. The former required less digital-signalprocessing infrastructure but is limited in bandwidth by the Fig.…”

“…The Y factor is then the ratio of the total beamformed power with the hot load and the cold load. The aperture array system temperature is given by [15]…”

“…The uncertainties in this method are explored in detail in [15] and the work is continuing as the technique is refined and the measurement system upgraded.…”

“…From the point of view of our work in testing fully assembled systems, it is more convenient to refer these quantities to before the antenna losses [14]. We refer to them as sky referenced [15] and apply a hat notation. They are related to the standard terms through the beam radiation efficiency [13], [15], for example T sys = η radTsys and η aperture = η rad η ant .…”

“…The work conducted on PAFs at three sites will be discussed: a two dish interferometer at the CSIRO Radiophysics Laboratory in Sydney [14], [17], [18], a number of arrays tested at the Parkes Observatory, both in a 12 m dish [19] and on the ground in 'aperture array' mode [15] and in the Boolardy Engineering Test Array (BETA) [20] at the Murchison Radio-astronomy Observatory (MRO). BETA consists of six PAF equipped dishes and is the first deployment phase of ASKAP.…”