 The U.S. military researched many different types of antenna for use at ELF frequencies.Cristofilos proposed applying currents to the Earth to create a vertical loop antenna, and it became clear that this was the most practical design.The feasibility of the ground dipole idea was tested in 1962 with a 42 km leased power line in Wyoming, and in 1963 with a 176 km prototype wire antenna extending from West Virginia to North Carolina.