 ELF antennas must be tens to hundreds of kilometers long, and must be driven by powerful transmitters in the megawatt range, to produce even a few watts of ELF radiation.Fortunately, the attenuation of ELF waves with distance is so low (1 - 2 dB per 1000 km) that a few watts of radiated power is enough to communicate worldwide.A second problem stems from the required polarization of the waves.