Antenna / Beamformer
Each MWA antenna is a small phased array, or "tile", operating over 80-300 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 shows one of the antenna tiles at the MWA site in Western Australia.
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 two LNAs for each element are 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 two vertically oriented LNA boards 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.
The analog beamformer receives dual polarization signals from all 16 crossed bowties in a tile, and applies independent delays to each signal in a manner appropriate to form a tile beam in a particular direction on the sky. True delay steering is employed, rather than phase steering, in order to point the beam properly over the full operating frequency range. The delay is generated passively in coplanar waveguide transmission lines laid out on a printed circuit board. Delay sections of different lengths can be switched in or out of each signal path as required to steer the tile beam in the desired direction. The delayed signals are combined, amplified, and sent over coaxial cable to the node receiver for digitization.
Figure 3 is a simplified block diagram of the entire signal path from a bowtie through the beamformer for a single bowtie polarization signal.
The beamformer is housed in a chassis, inside which are two delay line boards (one per polarization), a digital interface board, and DC/DC converters to convert incoming 48 vdc to 5 vdc for the LNAs and beamformer electronics. Figure 4 shows the two large delay line boards and the digital interface board in a beamformer with its top cover removed. There is no active cooling or fan in the chassis. The chassis sits slightly elevated above the ground less than 1 meter from the edge of the tile. A beamformer and the 32 cables that connect it to the 32 LNAs of the nearby tile are shown in Figure 5.