The underground mining environment can affect radio signal propagation in various ways. Understanding these effects is especially critical in evaluating communications systems used during normal mining operations and during mine emergencies. One of these types of communications systems relies on Medium Frequency radio frequencies. This paper presents the simulation and measurement results of recent National Institute for Occupational Safety and Health (NIOSH) research aimed at investigating medium frequency coupling between a transmission line and a loop antenna in an underground coal mine. Two different types of measurements were completed: line current distribution and line to antenna coupling. Measurements were taken underground in an experimental coal mine and on a specially designed surface test area. The results of these tests are characterized by current along a transmission line and voltage induced in the loop from a line. The paper concludes with a discussion of issues for medium frequency transmission lines. These include magnetic fields at the ends of the transmission line, connection of the ends of the transmission line, the effect of other conductors underground, and the proximity of coal or earth. These results could help operators by providing examples of these challenges that may be experienced underground.