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Mapping the LOFAR antennas

Submitter: Tammo Jan Dijkema
Description: Of course we know that we have the position coordinates of the LOFAR antennas right, to a tiny fraction of the wavelength. Otherwise, all kinds of funny effects would be visible in the images of the sky. But it is always nice to see it confirmed by other means. So, we converted the antenna coordinates from the International Terrestial Reference Frame (IRTF) into the WGS84 frame used by popular internet mapping systems, and overlaid the antennas on the map.

The sequence of images gives an overview of the current array of LOFAR stations (soon to be extended), and some zoom-ins of various parts. All LOFAR stations actually consist of one station (array) of Low Band Antennas (LBA, 10-80 MHz) and one or two stations of High Band Antennas (HBA, 120-240 MHz). The central superterp has a diameter of 300m, and contains six LBA and 12 HBA stations.

Here are some of the underlying considerations:

  • The individual antennas (dipole-pairs) of an LBA station have a random distribution, with a decreasing density towards the edge. This minimizes the primary beam sidelobes.
  • An HBA station consists of close-packed square units of 4x4 dipole-pairs. Note that all HBA stations are slightly rotated w.r.t. all others, again to minimize the far sidelobes of the primary beam.
  • NB: The individual dipole-pairs of all LOFAR stations have been de-rotated, so that they are all parallel to each other. This greatly simplifies wide-field polarization calibration.
  • The HBA stations in the 3km LOFAR core have been split in two, in order to increase the uv-coverage for short baselines, and thus the sensitivity for extended structures like the EoR. Obviously, the larger size of the more remote HBA stations has an impact on sensitivity and primary beam size. This has advantages and disadvantages.

    The existence and operation of LOFAR has a huge influence on the design of the Square Km Array (SKA).
  • Copyright: Background (c) Microsoft
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