Australia’s SKA tests antenna layout for Low telescope

SKA-Low is a radio telescope under construction in Western Australia. It will operate across a wide frequency range, from 50 to 350 MHz, and will be the most sensitive telescope of its kind ever built. The completed instrument will link 512 individual stations spread across an area roughly 75 km wide. Each station's antennas work together like a single large dish, so the way those antennas are arranged directly shapes what the telescope can detect.

An earlier prototype station, AAVS2, arranged its antennas in a scattered, pseudo-random pattern to avoid unwanted signal artifacts. But antennas placed too close together can interfere with one another, a problem known as mutual coupling. This interference grows stronger below about 150 MHz, squarely within SKA-Low's operating range.

To fix this, researchers within the Observatory proposed a new arrangement inspired by the spiral pattern seen in sunflower heads, known as the Vogel layout. Spacing the antennas this way reduces interference between neighbors while preserving the station's collecting power. The SKA Observatory built a new prototype station, AAVS3, using this layout to test it under real sky conditions.

As reported in the Journal of Astronomical Telescopes, Instruments, and Systems, researchers led by Dr. Shin’ichiro Asayama of the SKA Observatory describe how they built and tested AAVS3, and what the results mean for the telescope's final design. The team combined computer simulations with real observations to check the station’s sensitivity and calibration accuracy. AAVS3 uses 256 dual-polarized antennas and was tested both as a signal-combining beamformer and as an imaging array. The researchers pointed it at the Sun, bright cosmic radio sources, the Galactic Plane, and several pulsars.

They also built two ways to calibrate the station, essentially the process of correcting the raw data so it accurately reflects the sky. The simpler method used the Sun as a known reference point. The second, more thorough approach combined the Sun with a sky-brightness model and detailed antenna response profiles, and unlike the Sun-only method, it also worked at night.

Simulations run while AAVS3 was being built predicted a loss of sensitivity directly overhead at around 125 MHz. The team traced this to repeated antenna spacings, about 2.4 meters apart, that occur throughout the sunflower pattern and create interference patterns that reduce sensitivity. Older prototype AAVS2, with its scattered layout, showed no such problem.

Real sky observations confirmed the prediction. When the Galactic Plane passed directly overhead, images clearly showed the same dip in sensitivity at 125 MHz. This confirmed the problem was a built-in consequence of the antenna geometry, not a flaw in the equipment or method.

The team separately checked the station’s sensitivity at 230 MHz. At night, actual measurements closely matched predictions when using the more thorough calibration method. Daytime measurements showed larger gaps from predictions, most likely because unusually high solar activity during the observations made the Sun's brightness harder to predict accurately.

Because of the sensitivity loss at 125 MHz, engineers ruled out the original sunflower layout for the finished telescope. Further simulations led them to a modified version, called “Perturbed Vogel,” which keeps antennas spaced to avoid interference while removing the repeating pattern that caused the problem. Testing this new layout falls outside the current study.

The AAVS3 results give SKA-Low’s teams practical guidance for building and calibrating the actual stations in the construction phase. Combined with lessons from AAVS2, this work has directly shaped the observatory's testing process and supported a key construction milestone, known as AA0.5, marking the first stations to come online.