More 500kHz weirdness

Today I experimented some more with 500kHz WSPR using earth electrode pair arrangements rather than conventional antennas. In the end I was still getting reports from 210km away when using around 100uW ERP (maybe less) from a loop consisting of an earth rod outside the back door connected to about 7m of wire back to the bedroom with the other connection going to the central heating radiator. Why this works I have no idea! My latest theory is that this small loop is acting as a coupling loop to local overhead phone wires. I can't see otherwise how this "non-antenna" can otherwise work.