More 500kHz WSPRing with the earth electrodes "antenna"

This evening I've again been running my 500kHz QRP transverter into the earth electrodes separated by 20m of wire at about 1.5m above ground only. Reports from M0BMU (69km) and G0KTN (210km) confirm that this is only 6-8dB worse than my spiral top loaded 5m long vertical (i.e. ERP is now between 50-100uW). At the moment the only antennas I have up are a disconnected V2000 VHF/UHF vertical and this earth electrode antenna also used on 838Hz VLF, so the signal is definitely just what's coming from the loop formed in the ground.