The natural background levels of 50/60 Hz radio noise

Measurements of the natural background levels of 50- and 60-Hz radio noise by ELF/VLF (extremely low frequency/very low frequency) radiometers located at various sites around the world are reported. The measurements are made on the magnetic fields of the noise using loop antennas, and they cover the first three harmonics of the power line frequencies (frequencies in the range 50-180 Hz). Due to the likelihood of contamination from local electrical power systems, or even from nonlocal systems, the measurements at each power line frequency were made at adjacent frequencies and the desired noise amplitudes obtained by interpolation. It is shown that the magnetic spectral densities of the natural noise typically lie in the range 20-600 fT/√Hz or, taking the 50/60-Hz frequencies specifically, in the range 150-600 fT/√ Hz. These ranges should be typical for most locations on the Earth´s surface and for most seasons