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> So this is 1kHz radio wave (300km wave length), I didn't know they can get that low, what is theoretical limit?

The theoretical limit is the size of the universe. For practical purposes, this is about the lowest: https://en.m.wikipedia.org/wiki/Extremely_low_frequency

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Interesting, clicked a bit further and found out that submarines use VLF communication, however it's one way only:

"Submarines carry only a VLF reception aerial and do not respond on such low frequencies, so a ground-to-submarine VLF broadcast is always a one-way broadcast, originating on the ground and received aboard the boat."

https://en.m.wikipedia.org/wiki/Communication_with_submarine...

Have a look at the photos of the Rugby VLF Transmitter to see why it is not practical to put one on a sub:

https://www.subbrit.org.uk/sites/rugby-radio/

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