The Electromagnetically Isolated Global Signal Estimation Platform (EIGSEP)

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.

💡 Research Summary

The paper presents the Electromagnetically Isolated Global Signal Estimation Platform (EIGSEP), a novel ground‑based experiment designed to detect the sky‑averaged 21‑cm signal from Cosmic Dawn (z ≈ 25) and the Epoch of Reionization (z ≈ 9). The authors begin by reviewing the scientific motivation for measuring the global 21‑cm line and summarizing the challenges that have plagued previous experiments such as EDGES, SARAS, and MIST. The dominant systematic is spectral structure introduced by antenna beam chromaticity and reflections from nearby conductive structures, which can generate ripples on frequency scales comparable to the expected cosmological signal.

To address these issues, EIGSEP adopts three design principles: (i) limit the number of spectral modes needed to describe the antenna response so that they minimally overlap with the cosmological signal modes; (ii) make the instrument configurable during or between observations to produce independent weightings of systematics; and (iii) enable direct, in‑field measurement of instrumental systematics. The core hardware innovation is a shaped bow‑tie antenna suspended 100 m above the ground in a canyon, thereby achieving electromagnetic isolation from any conductive object within a 100 m radius. Electromagnetic simulations using a digital elevation model of the canyon show that increasing the suspension height pushes reflection‑induced spectral ripples to longer delay times (τ ≈ 670 ns), corresponding to ≤5 MHz ripple periods—well below the 10–100 MHz scales of the global signal. The antenna is mounted on a rotating platform, allowing the beam to be steered across the sky without moving the transmitter. A separate ground‑based transmitter antenna provides a fixed reference for beam mapping; the rotation creates multiple beam‑weighted foreground realizations, which are used to construct a foreground covariance matrix. Eigen‑mode analysis of this matrix yields a small set of dominant modes (≈ N_ant × N_fg) that can be projected out, suppressing foregrounds to the sub‑mK level while preserving any residual cosmological signal.

The receiver chain incorporates traditional Dicke switching for real‑time gain and temperature calibration, but also adds two novel calibration layers: beam modulation (rapid rotation of the antenna to vary the sky weighting) and interferometric cross‑correlation with auxiliary ground antennas. The latter provides independent measurements of the antenna’s electromagnetic environment, enabling direct estimation of reflection coefficients and validation of the isolation model.

A field deployment was carried out near Marjum Pass, Utah. Site surveys indicated relatively low radio‑frequency interference in the 50–250 MHz band. Initial data, sampled at 1–10 kHz resolution, demonstrate the expected variation of beam‑weighted foregrounds with rotation angle and show good agreement (within ~10 %) between simulated and measured beam patterns obtained via the ground transmitter. The authors also present simulated reflection‑coefficient spectra for various suspension heights, confirming that the 100 m elevation pushes problematic spectral features to delays longer than the instrument’s intrinsic bandwidth.

In summary, EIGSEP combines (1) a physically isolated high‑altitude antenna to suppress ground‑induced spectral structure, (2) a simple geometrical bow‑tie design that can be modeled with a limited set of eigen‑modes, (3) a rotating platform that creates independent foreground weightings, and (4) a layered calibration scheme (Dicke switching, beam modulation, interferometric cross‑correlation). Together these innovations aim to reduce instrumental systematics to ≤10⁻⁴ of the foreground level, enabling robust extraction of the faint global 21‑cm signal. The authors outline a two‑year observing campaign targeting the absorption trough at z ≈ 25 and the early emission feature at z ≈ 9, and suggest that the EIGSEP architecture could serve as a template for future ultra‑quiet sites, including lunar far‑side deployments.