The establishment of an LF/VLF/ELF transmitting facility at the Homer tunnel near Milford South in the South Island of New Zealand is described. The antenna consisted of a 3.3 km-long PVC-covered aluminium conductor passing through the tunnel and over the mountain above, forming a large vertical loop antenna. Measurements of the electric field radiated from the transmitter, recorded at both Lauder and Palmerston, 133 and 225 km from the transmitter site, respectively, are presented. The antenna radiation pattern derived from observations made onboard an Orion aircraft circling the antenna at a distance of 200 km is also presented. Theoretical models are derived which enable fits to the radiated field data to be made over a wide frequency range (0.5–40 kHz). With 1 kW of input power the antenna produced peak radiated fields at frequencies near 10 kHz equivalent to those of a vertical monopole source radiating 150 mW.