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, designed to keep terrain and conducting structures about 100 m away. We describe the current system design of EIGSEP, including the rotating antenna platform, a transmitter antenna to characterize 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 the measured response of the suspended and rotating antenna to an external transmitter. We also present an in situ measurement of the antenna reflection coefficient, whose delay spectrum shows no detectable terrestrial scattering at delays below 590 ns, the round-trip light travel time to the nearest canyon terrain, demonstrating the electromagnetic isolation of the antenna. Absolutely calibrated sky spectra and the characterization of the antenna beam are deferred to forthcoming work.