500kHz WSPR with 20m spaced earth electrode antenna

The effectiveness of the 20m spaced earth electrode "antenna" (virtually nothing is in the air) continues to amaze me. Last night I copied PA0O on 500kHz WSPR and this morning I've got 4 reports of my sub-1mW ERP signal from M0BMU around -19dB S/N at a distance of 69km. This level suggests the (non) antenna is only around 6-8dB down on my best top loaded  short vertical used to get reports from OK2BVG 1232kms away last winter and the ERP is around 200-250uW.