Probing the firn refractive index profile using antenna response

The Radio Neutrino Observatory-Greenland (RNO-G, at Summit Station) experiment comprises an extensive fat-dipole antenna array deployed into ice boreholes over an eventual area of approximately 35 km ² . Since the RNO-G experimental sensitivity depends on the radio-frequency properties of the firn, which are known to vary laterally on sub-km distance scales and vertically on sub-meter distance scales, a technique for quickly extracting information on firn ice properties with depth ( $n(z)$ ) during drilling and deployment is desirable. Given that a dipole’s resonant wavelength is fixed by geometry, the resonant frequency $f_{res}$ (measured as an S-parameter reflection coefficient [‘ $S_{11}$ ’] minimum) scales inversely with the local refractive index, allowing a translation of a depth-dependent $S_{11}$ (z) profile into $n(z)$ . $S_{11}$ (z) data were initially taken in August 2024 using a dipole lowered into a newly drilled 98 ± 1 mm diameter, 350 m deep borehole at Summit Station, Greenland, approximately 1 km from the site of the original GISP-2 core; improved measurements were subsequently made in May 2025. We conclude that $S_{11}$ (z) data can be used to estimate $n(z)$ , on 50 cm vertical scales, at the per cent level of accuracy required by experiments such as RN0-G.