The normal radio frequencies that we mostly use (MF, HF, VHF, UHF) are not very good for communicating with submarines in salt water or cave explorers underground.
Instead much lower frequencies are used. These are around a few kilohertz. Antennas of a practical sized can't be very efficient due to the long wavelength. But they are still effective for their job which doesn't always require very long distances to be spanned (especially in caves). The latter may use induction loops or earth current systems.
Here are some interesting articles on techniques and antennas for these frequencies:
* Earth impedance model for through the earth communications
* Leaky feeder systems Also see this evaluation
* Leaky feeder systems Also see this evaluation
PS: Like something to read? Books by Peter Parker VK3YE are
read worldwide and have been favourably reviewed. Available in both electronic and paperback.
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