Comparing two (40m) dipoles at different heights using the ham radio mode ‘WSPR’ (Weak Signal Propagation Reporter). This experiment is still ongoing! Find data at the below links.
WSPR data:
http://www.wsprnet.org
http://www.dxplorer.net
http://www.wspr.rocks
WSPR contests:
http://mardie4.100webspace.net/index.html
WSPR heat map:
https://maps.dj2ls.de