Since 1608gmt I have been WSPRing with an even shorter earth electrode 
"antenna" ( baseline about 15m) with BOTH ends connected to ground rods in the 
soil. Max connecting wire height still 1.5m. Yesterday I used the house pipes 
as one ground. 

Initial tests suggest reports are almost identical, suggesting the true " loop 
in the ground" hypothesis rather than propagation aided in any way by pipes. 
This is NOT the case at VLF. 

Most fascinating that this tiny "non antenna" works at all. 

73s
Roger G3XBM