The ears that listen to the sky

Each MWA antenna is a small phased array, or "tile", operating over 70-300 MHz. The essential features of each tile are:

  • 16 dual-linear-polarization, wide-beamwidth antenna elements over a conductive ground screen
  • elements arranged in a planar, 4x4 grid with 1.10-meter spacing corresponding to half a wavelength at 136 MHz
  • low-noise amplifiers (LNAs) integral to each antenna element
  • an analog RF beamformer that combines the 16 signals of each polarization with appropriate delays to form a beam 15-50° wide (FWHM), depending on frequency

Figure 1: The 'Southern Hexagon' tiles, part of the MWA's compact configuration

Each of the 16 antenna elements is a pair of orthogonally crossed, vertical bowties with a span of 74 cm. This design yields a broad element pattern centered on the zenith. The mirror effect of the ground screen causes the pattern to roll off rapidly at elevations below 30°, with consequent enhanced rejection of terrestrial RF interference.

A balanced pair of HEMT amplifiers in the LNA/balun situated between each pair of bowtie arms amplifies the incoming signals while adding less noise than is received from the coldest regions of our Galaxy. The LNA for each element is housed in a protective, UV-resistant hub, to which the vertices of the bowtie arms are also attached. Figure 2 is a top view of the hub, with its cover removed to show the LNA board inside.

The elements are held ~10 cm above the ground screen by dielectric "feet" that clip to the ground screen mesh. The mesh, which has a 5x5-cm square grid pattern, is made of 3.15-mm-diameter galvanized wire. Three mesh panels, each measuring 2x5 meters, are overlapped slightly to form a 5x5-meter ground screen.

Figure 2: The amplifier (LNA) within each antenna

Figure 3: Constructing antennas for Phase II MWA

Figure 4: The North side of each antenna, determined by the X-Y configuration of the amplifier chip, is marked in pen before placement