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."
https://www.youtube.com/user/VoyagerPWS/