Tracking the sky with no moving parts
The analog beamformer receives dual polarization signals from all 16 crossed bowtie antennas 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.
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 48V DC to 5V DC for the low noise amplifiers (LNAs) and beamformer electronics. Figure 2 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 LNAs of the nearby tile are shown in Figure 4.
Figure 2: Under a beamformer's lid
Figure 3: Antenna signals are combined and sent to the receiver over the two thicker coaxial cables; one for X-polarised signals and one for Y-polarised signals
Figure 4: A beamformer in front of a long-baseline tile