So this is 1kHz radio wave (300km wave length), I didn't know they can get that low, what is theoretical limit? One of my favorite explanation of how the now-debunked em-drive supposed to work was that it produces radio waves with such a long wavelengths that it doesn't fit observable universe and that makes it push in one direction.
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."
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[ 2.6 ms ] story [ 29.7 ms ] threadhttps://www.youtube.com/user/VoyagerPWS/
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
"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...
https://www.subbrit.org.uk/sites/rugby-radio/
https://www.bas.ac.uk/project/sounds-of-space/