In contrast, the signals required for communication purposes can be extraordinarily small and still be useful. By employing modern signal processing techniques, even signals buried deep in the background noise level can be recovered. The following chart compares the field strengths from a number of everyday sources with ELF signals that have been generated through ionospheric heating.
Consumer Reports [1] indicates that the magnetic field 50 feet away from a high voltage transmission line (230 kV, 85 feet high), would be approximately 20 milligauss. Levels for household appliances, power lines and typical household environments are given in [2]. Reference [3] summarizes the level of background noise which is produced by thunderstorm activity all over the earth. Nearby thunderstorm levels have been reported by Watt [4] and the level of ELF generation in the ionosphere that has been achieved was reported by McCarrick [5].
References:
[1] Consumer Reports, May 1994, pp 354- 359.
[2] ANSI Standard C95.1-1991, p70.
[3] "A Survey of Background Noise from Acoustic Frequencies to Optic
Frequencies," NRL Memo Report 3429, Dec 1976.
[4] Watt, A.D., VLF Radio Engineering, Pergamon Press:Oxford, 1967.
p - 451.
[5] McCarrick, M. J., et al, "Excitation of ELF waves in the Schumann resonance
range by modulated HF heating of the polar Electrojet," Radio Science, Vol 25,
Nr 6, pp 1291-1298, Nov - Dec 1990.
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