Experiment to Detect the Global EoR Signature (EDGES)
The Experiment to Detect the Global EoR Signature (EDGES) is a collaboration between ASU and MIT, with funding from the U.S. NSF and site support from the Australian CSIRO. The project aims to detect the global (all-sky) 21 cm signal through observations with individual dipole antennas. EDGES is located at the Murchison Radio-astronomy Observatory (MRO) in Western Australia (-26.7148, 116.6044), which is the same site used by Australian SKA Precursor (ASKAP), the Murchison Widefield Array (MWA), and the future SKA Low-Frequency Aperture Array (LFAA). This location was chosen due to its radio-quiet conditions below 200 MHz. The experiment consists of two instruments: 1) a high-band instrument sensitive to 100-200 MHz, and 2) a low-band instrument sensitive to 50-100 MHz. The instruments are nearly identical except for their antennas and ground planes, which are scaled copies.
Sky radiation is collected by a wideband dipole-like “blade” antenna consisting of two rectangular metal panels mounted horizontally above a metal ground plane. We have replaced our earlier “fourpoint” antenna because the blade provides nearly ideal achromatic beams (Mozdzen et al. 2016). A receiver is installed underneath the ground plane, directly below the antenna panels. A balun is based on the design of Roberts (1957) is used to guide radiation from the antenna panels to the receiver. The ground plane rests directly on the physical ground and consists of a 5x5 meter solid metal assembly (10x10 meter mesh for the low-band). This ground plane provides a stable high-conductivity electric boundary to the antenna under varying environmental conditions and is critical for achieving the expected smooth wideband response of the antenna.
Physically, the receiver consists of a metal box that houses the front-end RF electronics, switches, and associated circuitry. Key components include the first-stage low-noise amplifier (LNA), noise references for stability calibration, a second amplification stage, out-of-band noise for linearity, and electronics for remote measurement of the antenna reflection coefficient. The receiver is operated at 25°C at all times in order to maintain stable noise and reflection characteristics of the LNA. Temperature stability to better than 0.1°C is achieved using a PID feedback loop designed around a thermal controller that reads out a thermistor mounted inside the receiver. Based on the reading, the controller acts on a hot/cold plate attached to the bottom of the receiver which compensates for temperature drifts.
From the output of the receiver, the RF signal is taken along a 100-m coaxial cable into an EMI-shielded equipment rack where it is amplified and filtered by back-end electronics. The signal is then sent to a data acquisition system with 400-MS/s ADC that samples with 14-bit amplitude resolution and 6.1 kHz frequency resolution. Finally the data are recorded to disk.
During operations, the LNA input switches successively between the antenna and an internal comparison position (switch SW2 in Figure 4). When in the comparison position, the LNA receives two noise levels labeled internal ambient and internal “hot”. In the ambient state the noise comes from the output of a 30-dB attenuator. In the “hot” state the active noise-diode connected to the input of the attenuator is turned on for additional noise. The system measures for 10 seconds in each state, yielding a 33% duty cycle for the antenna. The system is assumed to be stable on timescales within this cycle. Data are recorded continuously throughout the night as the sky drifts above the zenith antenna pointing.
We have utilized two antennas for EDGES, both are horizontal planar dipole-like antennas placed over a ground plane. From beginning of the project through 2015, we used the "fourpoint" dipole design of Suh et al. (2004). The fourpoint antenna uses four diamond-shaped panels arranged to form a tulip-shaped planar structure. One pair of opposing panels is electrically active, while the other pair serves as a parasitic capacitance via a vertical rim along the panel's perimeter, to enhance both the beam's symmetry and the antenna's impedance match to the receiver. A Roberts transmission-line balun (Roberts 1957) is used to transition from the panels to the receiver. Discrete tuning capacitors located at roughly the middle of the Roberts balun and near the edges of the panels, along with a capacitive top plate above the central region of the antenna, improve the impedance match of the antenna to the receiver. EDGES has deployed this style of antenna at the Murchison Radio-astronomy Observatory (MRO) in Western Australia for several observing seasons. A version of this style without the Roberts balun provided the data used to set a lower limit on the duration of the reionization epoch (Bowman & Rogers 2010).
More recently, we have adopted the "blade" antenna design. This antenna was deployed in the field for the first time in July 2015. The blade-shaped antenna is simpler than the fourpoint design, because it uses two rectangular panels (no rim on the perimeter), a top capacitor, the Roberts Balun, and a small shield at the bottom. There are no inter-panel tuning capacitors nor a balun tuning capacitor. The beam has less variation with frequency than the fourpoint design, yielding less contamination in spectral measurements (Mozdzen et al. 2016). The current high-band ground plane consists of a 5 x 5 meter solid aluminum plate underneath the antenna, with four wire mesh extensions (each 2 m x 5 m) which form a plus-shaped ground plane.
Understanding the origin, formation, and evolution of the first galaxies is one of the major questions of astrophysics. Identified by the Astro2010 decadal review as a key science priority, the period of “Cosmic Dawn" encompasses the formation of the first stars, galaxies, and black holes that are responsible for First Light in the Universe. Read more about how EDGES will help open a new view on the early Universe.
Photos of the experiment
MIT memo series: http://www.haystack.mit.edu/ast/arrays/edges
ASU memo series: http://loco.lab.asu.edu/memos
Results from EDGES High-band. I. Constraints on Phenomenological Models for the Global 21 cm Signal, The Astrophysical Journal, Volume 847, Issue 1, article id. 64, 12 pp., (2017)
Improved measurement of the spectral index of the diffuse radio background between 90 and 190 MHz, Monthly Notices of the Royal Astronomical Society, vol. 464, issue 4, pp. 4995-5002 (2017)
Calibration of the EDGES High-band Receiver to Observe the Global 21 cm Signature from the Epoch of Reionization, The Astrophysical Journal, Volume 835, Issue 1, article id. 49, 13 pp. (2017)
One-Port Direct/Reverse Method for Characterizing VNA Calibration Standards, IEEE Transactions on Microwave Theory and Techniques, Volume PP, Issue 99, p.1-9 (2016)
Limits on foreground subtraction from chromatic beam effects in global redshifted 21 cm measurements, Monthly Notices of the Royal Astronomical Society, Volume 455, Issue 4, p.3890-3900 (2016)
Radiometric Measurements of Electron Temperature and Opacity of Ionospheric Perturbations, Radio Science, Volume 50, Issue 2, pp. 130-137 (2015)
Absolute calibration of a wideband antenna and spectrometer for accurate sky noise temperature measurements, Radio Science, Volume 47, RS0K06, 9 PP. (2012),
A lower limit of dz>0.06 for the duration of the reionization epoch. Nature, Volume 468, Issue 7325, pp. 796-798 (2010),
VHF-band RFI in Geographically Remote Areas, Proceedings of the RFI Mitigation Workshop. 29-31 March 2010. Groningen, the Netherlands (2010),
Spectral Index of the Diffuse Radio Background From 100 to 200 MHz, The Astronomical Journal, Volume 136, Issue 2, pp. 641-648 (2008),
Toward Empirical Constraints on the Global Redshifted 21 cm Brightness Temperature During the Epoch of Reionization, The Astrophysical Journal, Volume 676, Issue 1, pp. 1-9 (2008),