The April 1999 issue of  the US "RF Design" magazine has an article
"Understanding range limitations for low-frequency unlicensed
transmissions" about  FCC "Part 15"  transmitters for local data links that
are so low power they don't need licensing. One of the bands they use is
160-190 kHz limited to a DC power INPUT to the amplifier of 1 watt and an
antenna no bigger than 15 metres. The author calculates a range of  12 kms
if everything is optimum (including zero ground loss!) but he also gets a
range of  375 kms for a one-tenth watt (DC input) transmitter on 1705 kHz
using a 3 metre antenna so I guess it's all a bit suspect. Anyway, he gives
the formulae and makes some pertinent points about transmission path and
background noise problems so it might be a handy summary for anyone new to