Earth-electrode "antenna" in ground

As many know, this simple arrangement has given me great success on 1840kHz FT8 and 472kHz WSPR. I am sure a "proper" antenna and ground would be better, but for me, at 2 QTHs, it has worked.

On 160m QRP TX it has allowed me to be copied in North America! Although I used the mains earth at this QTH (you need to check if this is safe with your mains system), I used an external ground at my old QTH with the interconnecting wire on the ground with broadly similar results. My theory is this arrangement acts as a loop in the ground on VLF, LF and MF. At the old QTH I did not need the matching transformer.