This work presents research in the development of medium frequency communications systems that the National Institute of Occupational Safety and Health (NIOSH) is performing in support of the 2006 MINER Act. In this work, a 280 m twin-lead transmission line (TL) is used to measure complex input impedance and line currents resulting from open and shorted terminations at medium frequencies (MF). The line is placed in free space and positioned close to a lossy earth surface such that two line configurations can be chosen in order to control the extent to which the ground's presence affects MF propagation. In this way, the effects of the earth return may be characterized and modeled. Further, full-wave computational models using moment methods are used to compare with the experimental measurements as a way to determine the material properties of the ground medium. In the future, these experiments will be conducted in underground mines in an effort to develop and improve MF communication systems for mining operations and emergency response.