The AAMID Consortium, working on the Mid-Frequency Aperture Array (MFAA), an Advanced Instrumentation Work Package, aims to demonstrate the feasibility, competitiveness and cost-effectiveness of MFAA technology for SKA2. The key advantage of AAs is the capability of realising a very large Field of View and sensitivity, which results in an unsurpassed survey speed. Furthermore, AAs are capable of generating multiple independent FoVs, enhancing the efficiency of the system, for calibration and for multiple concurrent observations.
The AAMID consortium, in collaboration with key scientists, published a white paper (https://arxiv.org/abs/1612.07917) that presents the main characteristics of a wide‐field MFAA precursor that we envisage to be built at the SKA site in South Africa. Known as MANTIS (the Mid‐Frequency Aperture Array Transient and Intensity‐Mapping System), this ambitious instrument will represent the next logical step towards the MFAA based SKA telescope. The goal is to use innovative aperture array technology at cm wavelengths, in order to demonstrate the feasibility of deploying huge collecting areas at modest construction and operational cost. Such a transformative step is required in order to continue the exponential progress in radio telescope performance, and to make the ambitious scale of the SKA Phase 2 a realistic near‐time proposition.
This white paper summarises the ideas that were discussed at the 2016 MFAA/MIDPREP workshop in Cape Town. Consultations with the science community at workshops in Stellenbosch (2014) and Cape Town (2016) indicated a strong interest in a wide‐field science demonstrator instrument with a collecting area of about 1500 – 2500 m2, operating in the 450 – 1450 MHz frequency range. Such an instrument could perform an outstanding science programme in its own right but would also support and provide leverage to various MeerKAT and SKA1‐mid science opportunities. An instrument of this size matches very well with the necessity to realise a sizable system to demonstrate competitiveness, feasibility and technology readiness of several key MFAA technologies and concepts, as required for the MFAA PDR. MANTIS will also provide a new reference for the costing of an MFAA based SKA, both the deployment costs (hardware and installation) and the operational costs (in particular power consumption and maintenance). Finally, MANTIS will present an opportunity to involve the South African science, engineering and industrial communities in this innovative technology in an early stage.
Progress on MFAA Front-End Design
The fully differential Front-End solution based on a Crossed Octagonal Ring Antenna array that is being developed at the University of Manchester has been measured in the anechoic chamber at Leonardo MW in Edinburgh. The radiation patterns of the centre element and 4×4 subarray with an analogue beamformer have been measured. The array (10 x 10 finite array) has been fabricated with 8×8 elements integrated with LNAs. Therefore, the combined gain of the antennas and the integrated LNAs can be measured.
As observed in Figure 2, the cross-polarisation component is significantly lower than the co-polar component. This demonstrates the distinguishing feature of this type of antenna. The intrinsic cross polarisation (IXR) of the C-ORA based on the measurements will be analysed. The preliminary assessment from the measured data is that IXR of the ORA array can meet the SKA polariation purity requirement as it has a high polarisation isolation, even at high scan angles.
An artist impression of the ORA array with a planar array structure is shown in Figure 3. It indicated the layered structure and the integrated LNAs in housing boxes can be fabricated with a defined order.
Station de Radioastronomie de Nançay prototyped a differential beamformer board based on time delays. 16-Channel beamformer boards with differential inputs (SATA standard) have been under fabrication. The total delay of 1.2ns can be achieved per channel with 64 steps. The measured data for one channel is shown in Figure 4.
The MFAA team in Cambridge has continued its work on the development of a sparse array solution for MFAA benefiting from the LFAA development work. The goal is to maximise the sensitivity across the frequency band and field of view. The small LPDA is being mechanically upgraded for ease of manufacturing and deployment and low cost production in collaboration with Cambridge Consultants Ltd. The new mechanically enhanced design does not need a radome as it is design to be environmentally protected. A first prototype model of this design using 3D printed support parts has been recently tested in the Karoo (see Figure 5). The team had members of Cambridge and the South African SKA Office and realised RFI measurements to inform the LNA and receiver design as well as impedance measurements. The Cambridge team is now accelerating the construction of a 128 antenna demonstrator at the Mullard Radio Astronomy Observatory at Lords Bridge, Cambridge to continue the validation of the sparse array LPDA technology for MFAA.
Furthermore, a new 2-stages single ended 50 Ohm LNA (see Figure 6) has been developed with 40 dB of gain up front and a noise temperature of 35 K.
At ASTRON, we are progressing on multiple fronts towards our full 2m2 MFAA Vivaldi tile prototype. We are busy with improving the prototypes we currently have, like the antennas and LNA modules. And we are working hard towards finalising the designs and creating prototypes for the missing components.
We have made multiple improvements on our LNA modules. These improvements impact the stability, noise temperature, manufacturability and ease of assembly. An image of the new modules can be found in Figure 7. The layout of the printed circuit board and the shape of the shielding can, have been improved to compartmentalise the different parts of the module to improve the stability. The outline of the module has been adapted to improve the ease of mounting the LNA module to the Vivaldi antenna element. Also improvements in the electric circuit have been made.
Antenna and noise modelling
We have created a computer cluster capable of performing distributed HFSS simulations, this enables us to perform full electromagnetic simulations of a 16-element passive array. A physical prototype of this array has been fabricated and measured in our anechoic chamber. The results of the simulation and measurement agree almost perfectly.
We have also measured the scattering and noise parameters of our LNA module. This, together with the data from the antenna, makes it possible to simulate the performance of the antenna and LNA module combined. This simulation provides us with a lot of insight in what is exactly going on in the system. In the simulation, it is possible to enable or disable certain effects to assess their influence on the system’s performance.
These simulations have also been verified with measurements, by measuring an active 16-element array. These measurements have been performed twice, once with passive combiners and a second time with our beamformer boards to combine the signals from the LNA modules. The results are being processed at this moment and will be presented in September at the ICEAA 2017 in Verona. A picture of both the active and passive arrays is shown in Figure 8.
MFAA 2m2 Vivaldi tile
For the complete tile, all monitoring and control has been sorted out. We know exactly which components we are going to use, and how the communication between these components will be implemented. Also, a concept for the communication between our Vivaldi tile and the back-end is being worked on.
When the tiles are placed in the field to create a station, it is important to know the influence of their placement on the performance of the complete system. A study on the configuration of the Vivaldi tiles, which was focused on disconnecting the tiles, is presented at the EuCAP 2017 in Paris. Two examples of possible station layout configurations are shown in Figure 9.
North-West University (South Africa), as part of a collaboration with ASTRON, constructed a first prototype tile using the NWU receivers and a digital beamformer (see Figure 10). The tile consist of
1) Four PCB antennas with integrated low-noise amplifiers:
2) A ground plane with a number of short connector cables to the PCBs and a number of calibration noise sources.
3) A low-cost digital beamformer consisting of commercial SDRs (software defined radios) for each element and two SBCs (single board computers) using PoE (power over ethernet).
4) Software for the digital beamformer has been developed and interferometry between the elements has been demonstrated for the galactic 21cm line.
Eight more tiles (64 elements) are currently being constructed.