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Aperture Array Polarization performance

Submitter: B. Fiorelli, M. Arts
Description: The Square Kilometre Array (SKA) is required to accurately measure the polarization state of the observed sources.
However, the evaluation of this accuracy for aperture arrays is not straightforward, as the performance at system level has to be taken into account, along with polarimeter calibration corrections.

The ASTRON R&D Antenna Group, now merged into the Radio Group, has been studying the polarimetric performance of antennas in the context of SKA AA-low and AA-mid. Some of the results were presented at the International Conference on Electromagnetics in Advanced Applications. ICEAA is an annual conference which is held in its home town Turin in Italy in odd years. In even years it is held somewhere outside Europe (2010: Sydney, 2012: Cape Town, 2014: Palm Beach, Aruba). This year there were around 50 sessions and two of them related to radio astronomy. In these sessions there were two papers from ASTRON that dealt with polarization.

The first paper discusses the polarization properties of arrays of Vivaldi antennas (like EMBRACE). One result is shown in first picture. It shows, for the case of a 144-element dual polarized array, the part of the scan volume (for scan angles up to 45 degrees) that fulfils the IXR-requirement. The blue line is for a minimum IXR of 25 dB, the red line for a minimum IXR of 20 dB. One can see that between 0.4 and 1.2 GHz the IXR is greater than 20 dB for all scan angles up to 45 degrees.

The second paper presents a study of the two families of misalignments errors expected to affect the array polarization performance of a low frequency irregular aperture array. The object of study is a low frequency Aperture Array Verification System (AAVS) for SKA named AAVS0.5. It is located at the Murchinson Radioastronomy Observatory among the Murchison Widefield Array tiles. A result is reported in the second plot, where the array with randomized antenna misalignment is compared with the original configuration. Misalignment errors are shown to produce negligible degradation of the IXR (intrinsic cross-polarization ratio) within a large range of tolerance angles.

Even people who are not concerned with antennas are inspired by the beautiful shape of the Vivaldi antenna. The third picture shows the building opposite the conference centre. If you look carefully, you will recognize the shape of a differentially fed Vivaldi antenna in one of the windows.
Copyright: B. Fiorelli, M. Arts
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