---
abstract: 'Preliminary results are presented from a simple, single-antenna experiment designed to measure the all-sky radio spectrum between 100 and 200 MHz. The system used an internal comparison-switching scheme to reduce non-smooth instrumental contaminants in the measured spectrum to 75 mK. From the observations, we place an initial upper limit of $450$ mK on the relative brightness temperature of the redshifted 21 cm contribution to the spectrum due to neutral hydrogen in the intergalactic medium (IGM) during the epoch of reionization, assuming a rapid transition to a fully ionized IGM at a redshift of 8. With refinement, this technique should be able to distinguish between slow and fast reionization scenarios. To constrain the duration of reionization to $\Delta z>2$, the systematic residuals in the measured spectrum must be reduced to 3 mK.'
author:
- 'Judd D. Bowman, Alan E. E. Rogers, and Jacqueline N. Hewitt'
title: Toward Empirical Constraints on the Global Redshifted 21 cm Brightness Temperature during the Epoch of Reionization
---

Introduction
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The transition period at the end of the cosmic “Dark Ages” is known as the epoch of reionization (EOR). During this epoch, radiation from the very first luminous sources—early stars, galaxies, and quasars—succeeded in 