If you Google LF antennas, numerous designs pop up. They seemed to be divided into the following:
- Amplified LF loops;
- Amplified short dipoles, typically the Datong AD370, but it is not spec’d below about 200kHz.
- Tuned loops (less convenient unless mounted at the operating position).
- Short amplified whips. But be careful to decouple the coax feed otherwise it will act as a noisy pick up. There used to be an excellent professional design from Surrey Electronics, but it cost £££s. There was a Marconi small wideband active whip developed for ICS3. I think it used valve circuitry to cope with the high RF on warships.
- Ferrite rods glued together. Described as “loopsticks” in US books. I think there are some pages in the ARRL Handbooks.