Earth Mode Communications
ULF has been used by the military for secure communications through the ground. NATO AGARD publications from the 1960s detailed many such systems, although one suspects the contents of the published papers left a lot unsaid about what actually was developed secretly for defense purposes. Communications through the ground using conduction fields is known as "Earth-Mode" communications and was first used in WWI. Radio amateurs and electronics hobbyists have used this mode for limited range communications using audio power amplifiers connected to widely spaced electrode pairs hammered into the soil. At the receiving end, the signal is detected as a weak electric current between a further pair of electrodes. Using weak signal reception methods with PC-based DSP filtering with extremely narrow bandwidths, it is possible to receive signals at a range of a few kilometers with a transmitting power of 10-100 W and electrode spacing of around 10-50 m.
Read more about this topic: Ultra-low Frequency
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