Analytic Approximations of Scattering Effects on Beam Chromaticity in 21-cm Global Experiments

Alan E.E. Rogers, John P. Barrett, Judd D. Bowman, Rigel Cappallo, Colin J. Lonsdale, Nivedita Mahesh, Raul A. Monsalve, Steven G. Murray, Peter H. Sims

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


Scattering from objects near an antenna produce correlated signals from strong compact radio sources in a manner similar to those used by the “Sea Interferometer” to measure the radio source positions using the fine frequency structure in the total power spectrum of a single antenna. These fringes or ripples due to correlated signal interference are present at a low level in the spectrum of any single antenna and are a major source of systematics in systems used to measure the global redshifted 21-cm signal from the early universe. In the Sea Interferometer a single antenna on a cliff above the sea is used to add the signal from the direct path to the signal from the path reflected from the sea thereby forming an interferometer. This was used for mapping radio sources with a single antenna by Bolton and Slee in the 1950s. In this paper we derive analytic expressions to determine the level of these ripples and compare these results in a few simple cases with electromagnetic modeling software to verify that the analytic calculations are sufficient to obtain the magnitude of the scattering effects on the measurements of the global 21-cm signal. These analytic calculations are needed to evaluate the magnitude of the effects in cases that are either too complex or take too much time to be modeled using software.

Original languageEnglish (US)
Article numbere2022RS007558
JournalRadio Science
Issue number12
StatePublished - Dec 2022


  • 21-cm
  • Galactic center
  • beam chromaticity
  • radio sky spectrum
  • scattering effects

ASJC Scopus subject areas

  • Condensed Matter Physics
  • General Earth and Planetary Sciences
  • Electrical and Electronic Engineering


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