Super low frequency

However, SLF waves can penetrate seawater to a depth of hundreds of meters.

Therefore, in recent decades the U.S., Russian and Indian militaries have built huge radio transmitters using SLF frequencies to communicate with their submarines.

[1][2] The requirements for receivers at SLF frequencies are less stringent than for transmitters, because the signal strength (set by atmospheric noise) is far above the noise floor of the receiver, so small, inefficient antennas can be used.

Radio amateurs have received signals in this range using simple receivers built around personal computers, with coil or loop antennas connected to the PCs sound card.

Signals are analysed by a software fast Fourier transform algorithm and converted into audible sound.