  The observation of the global 21 cm signal produced by neutral hydrogen gas
in the intergalactic medium (IGM) during the Dark Ages, Cosmic Dawn, and Epoch
of Reionization requires measurements with extremely well-calibrated wideband
radiometers. We describe the design and characterization of the Mapper of the
IGM Spin Temperature (MIST), which is a new ground-based, single-antenna,
global 21 cm experiment. The design of MIST was guided by the objectives of
avoiding systematics from an antenna ground plane and cables around the
antenna, as well as maximizing the instrument's on-sky efficiency and
portability for operations at remote sites. We have built two MIST instruments,
which observe in the range 25-105 MHz. For the 21 cm signal, this frequency
range approximately corresponds to redshifts 55.5 > z > 12.5, encompassing the
Dark Ages and Cosmic Dawn. The MIST antenna is a horizontal blade dipole of
2.42 m in length, 60 cm in width, and 52 cm in height above the ground. This
antenna operates without a metal ground plane. The instruments run on 12 V
batteries and have a maximum power consumption of 17 W. The batteries and
electronics are contained in a single receiver box located under the antenna.
We present the characterization of the instruments using electromagnetic
simulations and lab measurements. We also show sample sky measurements from
recent observations at remote sites in California, Nevada, and the Canadian
High Arctic. These measurements indicate that the instruments perform as
expected. Detailed analyses of the sky measurements are left for future work.
