On the west coast of Norway, two VLF transmitters, Omega Norway and JXN, are located within 6 km of each other. JXN has an output power of 150 kW at 16.4 kHz and may be expected to modify the ionospheric plasma in the region where the signals from Omega Norway also propagate. Nonlinear wave interactions have been observed between the signals from the two transmitters with clear signals being observed at Lycksele, Sweden, on 20.75 kHz (twice the frequency of the upper sideband of JXN-'unique' frequency of Omega Norway 12.1 kHz). Calibration results are presented which show the nonlinearity is a feature of the ionospheric plasma and not the receiving equipment. This nonlinear technique may be an effective means of generating ELF signals from suitably sited VLF transmitters of the appropriate frequency. ¿ American Geophysical Union 1996