[goodpoint]

I'm really surprised ELF are not commonly used for this. Even a simple single-frequency CW transmitter for emergencies.

I wonder if audio frequencies below 20KHz can be used to make a cheaper system.

[ttyprintk]

At the extremity of that band, fewer nations have constructed transmitters than have nuclear weaponry.

You can combine acoustic with radio in TARF (yes, really).

[goodpoint]

I meant acoustic frequencies in ELF, not using sound... but TARF looks amazing.

[ale42]

see e.g. https://www.mit.edu/~fadel/papers/TARF-paper.pdf for those who didn't hear about TARF (translational acoustic-RF communication)