[Ellipsis753]

This site is a bit scarce on details. There's more details and some photos here: https://de.wikipedia.org/wiki/Cave-Link http://www.hoehlenverein-blaubeuren.de/index.php/cavelink-te... (Use Google Translate)

[dll]

I wonder how the licensing issue referenced in the Wikipedia article is handled in the UK. I imagine Ofcom will have had to approve and license these installations, or will they have made these exempt (I can't find any evidence of this)?

[gnfargbl]

I can't see these devices specifically mentioned in https://www.ofcom.org.uk/__data/assets/pdf_file/0028/84970/i..., though the frequency range they operate in (20-140kHz) is allowed to be used by induction loop type applications, at a suitably low power.

[ale42]

... but the cave link uses relatively high power levels (20-30 Watts if I remember correctly)

[lazide]

Inductive stove tops are also In the KW range

[detaro]

as far as I know similar general licensing exists for such purposes. Lots of inductive applications need it.

[meibo]

Something that is mentioned here and not on the main page: the devices form a mesh network and can relay messages between each other.

Seems strange, feels like a pretty big selling point.

[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)