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.
The video is here:https://www.youtube.com/watch?v=WMHym3yL3VE&t=1s
It contains a lot of information in pretty quick pace so you might want to pause sometimes to have time study the graphs.
You can also download his paper he prepared that contains the graphs here: https://drive.google.com/file/d/1Wf5UpWvO-OKOZ_mi5yDnGodi3190RYj4/view *
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.
Great work Jim and thank you for sharing with us!
*The google doc comes with the following provision:"You are welcome to use this link or download the contents for personal or business use with attribution “Jim KX4TD”."