After several days without success, I shall end my experiment later today. I have had success in the past, but not this time.
Tomorrow, if I use my 10m WSPR TX beacon, I shall turn the PA on again and run the full 500mW.
My 10mW ERP WSPR from the earth-electrode "antenna" in the ground was turned on at about 2114z. 7 stations have spotted me.