The Electromagnetically Isolated Global Signal Estimation Platform (EIGSEP)

TL;DR

This work addresses the challenge of measuring the global 21-cm signal from Cosmic Dawn and the EoR by introducing EIGSEP, an electromagnetically isolated platform that suspends a bowtie antenna 100 m above ground to suppress ground reflections. The design constrains the instrument’s spectral response to a small set of modes, $N_{ ext{modes}} \sim N_{ ext{ant}} N_{ ext{fg}}$, and uses beam-modulation and platform rotations to enable independent measurements and robust foreground separation, while field-calibrated beam and impedance measurements validate simulations. The instrument comprises a three-element interferometer, a ground-transmitter beam-mapping scheme, and a comprehensive calibration plan including Dicke switching and interferometric cross-correlation, with deployments at Marjum Pass and initial field results. This approach improves electromagnetic isolation and calibration fidelity for global 21-cm experiments, enabling more reliable isolation of the cosmological monopole signal and paving the path for longer, absolutely calibrated observations.

Abstract

The Electromagnetically Isolated Global Signal Estimation Platform (EIGSEP) is a new instrument designed to measure the global 21-cm signal from Cosmic Dawn and the Epoch of Reionization, redshifted to frequencies below 250 MHz. To reduce spectral structure in the antenna beam associated with ground scattering, EIGSEP uses a shaped bowtie antenna suspended in a canyon 100 m above the ground. We describe the current system design of EIGSEP, including the rotating antenna platform, a transmitter antenna to characterise the beam of the bowtie antenna, and auxiliary ground antennas. We then discuss the EIGSEP calibration scheme, which incorporates traditional Dicke switching in the receiver, and novel approaches that include beam mapping, beam modulation, and interferometric cross-correlation. The instrument has been deployed near Marjum Pass, Utah, for testing and initial data collection. We discuss the site characteristics and present initial field measurements.

Figures (15)

• Figure 1: The EIGSEP concept: An antenna suspended in a canyon, with a 100 m zone free of electrical conductors. The inset shows a zoomed in view of the antenna. The conductor-free zone is shown in red, with the antenna in the middle of the canyon suspended from Kevlar ropes (black). The signal is transmitted to the backend electronics on the ground via non-conducting fibre-optic (pink).
• Figure 2: Delay–power spectra of simulated environmental scattering for the EIGSEP bowtie antenna suspended at different heights. The spectra show the environment-induced contribution to the antenna reflection coefficient. Environmental reflections appear at delays set by the two-way travel time of light between the antenna and the surrounding terrain, producing a height-dependent gap in delay space. As the height of the antenna increases, the gap in signal delay increases, due to increased light travel time to the environment. An antenna at 100 m suppresses reflections below 600 ns (brown). The curves are normalized by a common reference to highlight relative delay structure.
• Figure 3: A CAD model of the EIGSEP antenna (green) on top of the antenna platform. The drawing does not include the copper plates that shield the sides of the box. The two black arrows represent the two types of rotations possible with EIGSEP.
• Figure 4: High-level block diagram of the EIGSEP instrument. The signal path for sky measurements goes from the antenna and ground antennas to the analogue front-end (the custom EIGSEP front-end for the suspended antenna, see Fig. \ref{['fig:fe_block']}, and the HERA FEM in the case of the ground antennas), through RF over fibre to the PAMs. Coaxial cables connect the PAM to the SNAP, where the signals are digitized and correlated. The SNAP uses Ethernet to communicate with the single-board computer, which saves the data to an external solid-state drive.
• Figure 5: Magnitude of simulated antenna reflection coefficient for the bowtie antenna in free space as a function of frequency in dB.