RHINO: A large horn antenna for detecting the 21cm global signal

TL;DR

This work introduces RHINO, a global 21cm signal experiment that uses a large horn antenna to improve beam-characterisation and environmental shielding, paired with a continuous-wave calibration source to monitor gain without frequent switching. The paper details the antenna design, electromagnetic simulations, mechanical construction, and a complete system design including calibration loads, CW calibration, and RF front-end. Through basic simulations and forward modelling, it shows that a narrower 60–85 MHz bandwidth aids beam control but makes foreground separation more challenging, while a wider bandwidth improves parameter constraints albeit with more stringent calibration requirements. A scaled-down prototype at Jodrell Bank is under construction to test the hardware, calibration approach, and site-specific RFI conditions, aiming to provide a robust cross-check against existing global-signal experiments such as EDGES and SARAS.

Abstract

The sky-averaged brightness temperature of the 21cm line from neutral hydrogen provides a sensitive probe of the thermal state of the intergalactic medium, particularly before and during Cosmic Dawn and the Epoch of Reionisation. This `global signal' is faint, on the order of tens to hundreds of millikelvin, and spectrally relatively smooth, making it exceedingly difficult to disentangle from foreground radio emission and instrumental artefacts. In this paper, we introduce RHINO, an experiment based around a large horn antenna operating from 60-85 MHz. Horn antennas are highly characterisable and provide excellent shielding from their immediate environment, which are potentially decisive advantages when it comes to the beam measurement and modelling problems that are particularly challenging for this kind of experiment. The system also includes a novel continuous wave calibration source to control correlated gain fluctuations, allowing continuous monitoring of the overall gain level without needing to rapidly switch between the sky and a calibration source. Here, we describe the basic RHINO concept, including the antenna design, EM simulations, and receiver electronics. We use a basic simulation and analysis pipeline to study the impact of the limited bandwidth on recovery of physical 21cm global signal model parameters, and discuss a basic calibration scheme that incorporates the continuous wave signal. Finally, we report on the current state of a scaled-down prototype system under construction at Jodrell Bank Observatory.

Figures (17)

• Figure 1: (Left): Dimensions of the pyramidal horn that was selected as the reference design. The flare section has an aperture of $7.3 \times 6.0$m, an edge slant length of 5.099m, and an overall vertical height of 4.3m. The rectangular waveguide section has an aperture of $3.54 \times 2.00$m and a height of 2.997m. The horn will be static, pointing at zenith, and will have a welded mesh conductive surface. The location of the feed is marked on the waveguide. (Right): Detailed design drawing showing the multi-panel construction, consisting of $2.4 \times 1.2$ m mesh panels cut to size/shape and welded into steel angle frames. These are then bolted together to form a self-supporting structure.
• Figure 2: Slices through the normalised antenna pattern, as a function of frequency. The horizontal lines show: (dotted) the $-3$ dB (half-maximum) point; (dashed) the target sidelobe level; and (dot-dash) the target backlobe level. The shaded vertical regions show the target beam FWHM at the bottom and top of the design band.
• Figure 3: Normalised antenna patterns as a function of frequency, for angles above the horizon.
• Figure 4: Unnormalised antenna pattern as a function of frequency and zenith angle, for different slices in azimuth ($\phi)$.
• Figure 5: Return loss ($S_{11}$) of the antenna as a function of frequency. The design band is marked by vertical dashed lines, and the target return loss within this band is marked by a dotted line.