The Broadband Instrument for Global HydrOgen ReioNisation Signal (BIGHORNS) is a total power radiometer developed to identify the signal from the Epoch of Reionisation in the all-sky averaged radio spectrum at low frequencies (70-300 MHz).
The Epoch of Reionisation (EoR) refers to an era of the universe where the previously neutral intergalactic medium was ionized by the first luminous sources. The observation of the 21cm transition line of neutral hydrogen is incredibly difficult for this period, but its detection is considered to be a vital goal for radio astronomy. To this end detection is being attempted with a range of systems from single antenna such as BIGHORNS to large interferometer arrays such as the MWA.
The Aperture Array Verification System (AAVS) is the testbed for the SKA consortium, to evaluate equipment and confirm the performance of antennas before the rollout of the full SKA_LOW.
SKA_LOW will consist of 500 ‘field nodes’ (equivalent to MWA tiles), each containing 256 antennas sitting on a mesh ground-plane about 40m in diameter. The log-periodic style antennas under consideration are called SKALA. The SKALA are arranged in a pseudo-random pattern on the field node. The antennas currently deployed on a field node at the MRO are the second revision; SKALA-2. There are a few singular SKALA-4 antennas also under test on site. The SKALA antennas are best tuned to a frequency range of 50-350MHz, similar to MWA dipoles. The main diﬀerence between the MWA and SKA_LOW is the immediate transfer of analog signal to fiber (RFoF) in the SKALA antennas. This reduces signal loss, but introduces additional antenna complexity and cost.
Power is distributed to the antennas via a large box that sits in the center of the station, called the Antenna Power Interface Unit (APIU). The cables between the APIU and the SKALAs are a hybrid copper and fibre, to both deliver power to the antennas and transfer RFoF signal. Once the data reaches the control building, it enters Tile Processing Modules (TPMs) which are similar in function to MWA receivers. The first field node on site to be fully populated with antennas is called AAVS1.1. See the webpages on EDA-2 and the SKA Bridging Phase to discover how additional field nodes are being used.
The SKA Bridging Phase, AAVS1.5
With the successful completion of the SKA_LOW's element-level Critical Design Review (CDR) in 2018, the project now has sufficient momentum to carry it through the all-important transition from design to delivery.
This transition is taking place with the bridging phase, an on-site roll-out of 256 SKALA-4.1 antennas in a field node placement similar to AAVS1. This new field node is denoted AAVS1.5, and incorporates technology that is more easily compared to the MWA/EDA than to AAVS1.
These SKALA-4.1 antennas no longer use a hybrid copper/fibre cable for data transportation and power delivery, but instead a simple coaxial cable like those in the MWA. Data from a group of 16 antennas (much like an MWA tile or EDA cluster) is then combined in a SMART box ('Small Modular Aggregation & RFoF Trunk'); this box contains front-end modules that convert the coaxial cable RF signal to fibre, suitable for transport. The SMART boxes sit on the field node with the SKALA antennas and have the same physical chassis as an MWA beamformer. These SMART boxes have also been deployed for the same purpose in EDA-2.
Signal aggregation no longer happens in a central APIU either; a large shielded container now sits on the north side of the node for easy access. This container, called the Field Node Distribution Hub (FNDH), has a unit to deliver power to the node and one that combines and sends the 16 SMART box signals to the control building. One more piece of receiving equipment in the correlator room converts these signals back to copper, and then they can be passed into Tile Processing Modules (TPMs) where data computational tasks begin.
AAVS1.5 from above, populated with 48 antennas.
SKALA-4.1 antennas on the AAVS1.5 station. Image credit: ICRAR/Curtin
As the MWA antennas have already been thoroughly characterised, the EDA helps us understand how a larger group of randomly spread antennas can be expected to perform in the given layout.
The EDA is located near the core of the MWA. The MWA amplifiers were improved for use in the EDA, so that the frequency range of the EDA is 50MHz- 300MHz.
The EDA was conceived, constructed and commissioned in less than a year.
The first version of the EDA, also called EDA-1, did not incorporate conversion of signal straight to fibre like AAVS; instead it uses 16 regular MWA beamformers on the ground-plane to transfer the signal data to a piece of equipment that acts like a receiver, called the Kaelus Beamformer.
The next stage, EDA-2, has been constructed one of the empty AAVS field node locations. Like EDA-1, it has 256 new MWA-style antennas with modified amplifiers, all situated on the ground-plane mesh. However, instead of using MWA beamformers, EDA-2 has 16SMART boxes('Small Modular Aggregation & RFoF Trunk'). Each SMART box contains front-end modules that convert the coaxial cable RF signal to fibre, suitable for sending to the control building. This happens via a Field Node Distribution Hub (FNDH); a large shielded container on the edge of the ground plane that delivers power to the SMART boxes (and thus, to each antenna) and aggregates their data signals into a combined fibre for easy transport.
This new system is designed to closely replicate theSKA Bridging Phase project, AAVS1.5, with MWA antennas instead of SKALA-4.1s, and will be an excellent comparison tool as well as a powerful telescope in its own right.
SMART Technology - Converting signals for transport
Once AAVS1 was completed, it became clear that there were many disadvantages to having a signal aggregation unit (the APIU) in the middle of the antenna field, and that their hybrid power and signal cables were needlessly complex. However, the early signal conversion to fibre remained a necessary part of the SKA-LOW system, so a new component was required to bridge this gap in functionality. Engineers from INAF and CIRA worked in collaboration to design what is known as the SMART boxes.
These 'Small Modular Aggregation and RFoF Trunk' devices convert coaxial cable RF signal to RF over fibre. With the SMART box, RF signal from each individual antenna is directly transported via fibre optic cable to a Tile Processing Module (TPM) inside the MRO control building, located approximately 5 km away from the field node where it resides. The main component of a SMART box is the front-end modules, handling the copper-fibre conversion, but they also supply each antenna amplifier with 5V power. The principal components associated with the SMART box are shown in the schematic in Figure 3.
SMART boxes have been deployed on both AAVS1.5 and EDA2.
Figure 1: Inside a SMART box, showing the power transformer, the front-end modules for coax-to-fibre conversion, and the output fibre fed through a waveguide
Figure 2: Cabling up a SMART box on EDA-2
Figure 3: A top-level diagram of the SMART box system