Direct comparisons have been made of the ionospheric ELF radiation produced by the new (1 GW ERP) and old (250 MW ERP) antennas of the Troms¿ heater system, but no significant differences in the ELF signal strength have been detected. This initially surprising result is shown to require a value of unity for the index relating the received ELF signal strength to HF power input to the antenna. A series of experiments performed solely to derive more accurate values for this power index provided values ranging from 0.74 to 0.97, dependent on the ELF frequencies generated. It has been suggested that ELF radiation from the normal Troms¿ heater facility should be limited by saturation effects, even when operating well below the maximum HF power density (3 mW/m2 in the D-region). No evidence for such saturation effects has been found even at power densities greater than 10 mW/m2. ¿ American Geophysical Union 1991