Hi.
Jim -KX4TD - have used a WSPR Desktop transmitter to investigate the propagation on the 40m band.
His explanation on what is going on is well thought out and has a lot of details, i highly recommend you view his video.
It contains a lot of information in pretty quick pace so you might want to pause sometimes to have time study the graphs.
Lots of great stuff and I found the discussion of the distance of spots versus the wave angle and the subsequent reflection and dampening in the various ionospheric layers as they shift over time especially interesting.